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RFC 4120 - The Kerberos Network Authentication Service (V5)


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Network Working Group                                          C. Neuman
Request for Comments: 4120                                       USC-ISI
Obsoletes: 1510                                                    T. Yu
Category: Standards Track                                     S. Hartman
                                                              K. Raeburn
                                                                     MIT
                                                               July 2005

            The Kerberos Network Authentication Service (V5)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document provides an overview and specification of Version 5 of
   the Kerberos protocol, and it obsoletes RFC 1510 to clarify aspects
   of the protocol and its intended use that require more detailed or
   clearer explanation than was provided in RFC 1510.  This document is
   intended to provide a detailed description of the protocol, suitable
   for implementation, together with descriptions of the appropriate use
   of protocol messages and fields within those messages.

Table of Contents

   1. Introduction ....................................................5
      1.1. The Kerberos Protocol ......................................6
      1.2. Cross-Realm Operation ......................................8
      1.3. Choosing a Principal with Which to Communicate .............9
      1.4. Authorization .............................................10
      1.5. Extending Kerberos without Breaking Interoperability ......11
           1.5.1. Compatibility with RFC 1510 ........................11
           1.5.2. Sending Extensible Messages ........................12
      1.6. Environmental Assumptions .................................12
      1.7. Glossary of Terms .........................................13
   2. Ticket Flag Uses and Requests ..................................16
      2.1. Initial, Pre-authenticated, and
           Hardware-Authenticated Tickets ............................17
      2.2. Invalid Tickets ...........................................17
      2.3. Renewable Tickets .........................................17
      2.4. Postdated Tickets .........................................18
      2.5. Proxiable and Proxy Tickets ...............................19
      2.6. Forwardable Tickets .......................................19
      2.7. Transited Policy Checking .................................20
      2.8. OK as Delegate ............................................21
      2.9. Other KDC Options .........................................21
           2.9.1. Renewable-OK .......................................21
           2.9.2. ENC-TKT-IN-SKEY ....................................22
           2.9.3. Passwordless Hardware Authentication ...............22
   3. Message Exchanges ..............................................22
      3.1. The Authentication Service Exchange .......................22
           3.1.1. Generation of KRB_AS_REQ Message ...................24
           3.1.2. Receipt of KRB_AS_REQ Message ......................24
           3.1.3. Generation of KRB_AS_REP Message ...................24
           3.1.4. Generation of KRB_ERROR Message ....................27
           3.1.5. Receipt of KRB_AS_REP Message ......................27
           3.1.6. Receipt of KRB_ERROR Message .......................28
      3.2. The Client/Server Authentication Exchange .................29
           3.2.1. The KRB_AP_REQ Message .............................29
           3.2.2. Generation of a KRB_AP_REQ Message .................29
           3.2.3. Receipt of KRB_AP_REQ Message ......................30
           3.2.4. Generation of a KRB_AP_REP Message .................33
           3.2.5. Receipt of KRB_AP_REP Message ......................33
           3.2.6. Using the Encryption Key ...........................33
      3.3. The Ticket-Granting Service (TGS) Exchange ................34
           3.3.1. Generation of KRB_TGS_REQ Message ..................35
           3.3.2. Receipt of KRB_TGS_REQ Message .....................37
           3.3.3. Generation of KRB_TGS_REP Message ..................38
           3.3.4. Receipt of KRB_TGS_REP Message .....................42

      3.4. The KRB_SAFE Exchange .....................................42
           3.4.1. Generation of a KRB_SAFE Message ...................42
           3.4.2. Receipt of KRB_SAFE Message ........................43
      3.5. The KRB_PRIV Exchange .....................................44
           3.5.1. Generation of a KRB_PRIV Message ...................44
           3.5.2. Receipt of KRB_PRIV Message ........................44
      3.6. The KRB_CRED Exchange .....................................45
           3.6.1. Generation of a KRB_CRED Message ...................45
           3.6.2. Receipt of KRB_CRED Message ........................46
      3.7. User-to-User Authentication Exchanges .....................47
   4. Encryption and Checksum Specifications .........................48
   5. Message Specifications .........................................50
      5.1. Specific Compatibility Notes on ASN.1 .....................51
           5.1.1. ASN.1 Distinguished Encoding Rules .................51
           5.1.2. Optional Integer Fields ............................52
           5.1.3. Empty SEQUENCE OF Types ............................52
           5.1.4. Unrecognized Tag Numbers ...........................52
           5.1.5. Tag Numbers Greater Than 30 ........................53
      5.2. Basic Kerberos Types ......................................53
           5.2.1. KerberosString .....................................53
           5.2.2. Realm and PrincipalName ............................55
           5.2.3. KerberosTime .......................................55
           5.2.4. Constrained Integer Types ..........................55
           5.2.5. HostAddress and HostAddresses ......................56
           5.2.6. AuthorizationData ..................................57
           5.2.7. PA-DATA ............................................60
           5.2.8. KerberosFlags ......................................64
           5.2.9. Cryptosystem-Related Types .........................65
      5.3. Tickets ...................................................66
      5.4. Specifications for the AS and TGS Exchanges ...............73
           5.4.1. KRB_KDC_REQ Definition .............................73
           5.4.2. KRB_KDC_REP Definition .............................81
      5.5. Client/Server (CS) Message Specifications .................84
           5.5.1. KRB_AP_REQ Definition ..............................84
           5.5.2. KRB_AP_REP Definition ..............................88
           5.5.3. Error Message Reply ................................89
      5.6. KRB_SAFE Message Specification ............................89
           5.6.1. KRB_SAFE definition ................................89
      5.7. KRB_PRIV Message Specification ............................91
           5.7.1. KRB_PRIV Definition ................................91
      5.8. KRB_CRED Message Specification ............................92
           5.8.1. KRB_CRED Definition ................................92
      5.9. Error Message Specification ...............................94
           5.9.1. KRB_ERROR Definition ...............................94
      5.10. Application Tag Numbers ..................................96

   6. Naming Constraints .............................................97
      6.1. Realm Names ...............................................97
      6.2. Principal Names .......................................... 99
           6.2.1. Name of Server Principals .........................100
   7. Constants and Other Defined Values ............................101
      7.1. Host Address Types .......................................101
      7.2. KDC Messaging: IP Transports .............................102
           7.2.1. UDP/IP transport ..................................102
           7.2.2. TCP/IP Transport ..................................103
           7.2.3. KDC Discovery on IP Networks ......................104
      7.3. Name of the TGS ..........................................105
      7.4. OID Arc for KerberosV5 ...................................106
      7.5. Protocol Constants and Associated Values .................106
           7.5.1. Key Usage Numbers .................................106
           7.5.2. PreAuthentication Data Types ......................108
           7.5.3. Address Types .....................................109
           7.5.4. Authorization Data Types ..........................109
           7.5.5. Transited Encoding Types ..........................109
           7.5.6. Protocol Version Number ...........................109
           7.5.7. Kerberos Message Types ............................110
           7.5.8. Name Types ........................................110
           7.5.9. Error Codes .......................................110
   8. Interoperability Requirements .................................113
      8.1. Specification 2 ..........................................113
      8.2. Recommended KDC Values ...................................116
   9. IANA Considerations ...........................................116
   10. Security Considerations ......................................117
   11. Acknowledgements .............................................121
   A. ASN.1 Module ..................................................123
   B. Changes since RFC 1510 ........................................131
   Normative References .............................................134
   Informative References ...........................................135

1.  Introduction

   This document describes the concepts and model upon which the
   Kerberos network authentication system is based.  It also specifies
   Version 5 of the Kerberos protocol.  The motivations, goals,
   assumptions, and rationale behind most design decisions are treated
   cursorily; they are more fully described in a paper available in IEEE
   communications [NT94] and earlier in the Kerberos portion of the
   Athena Technical Plan [MNSS87].

   This document is not intended to describe Kerberos to the end user,
   system administrator, or application developer.  Higher-level papers
   describing Version 5 of the Kerberos system [NT94] and documenting
   version 4 [SNS88] are available elsewhere.

   The Kerberos model is based in part on Needham and Schroeder's
   trusted third-party authentication protocol [NS78] and on
   modifications suggested by Denning and Sacco [DS81].  The original
   design and implementation of Kerberos Versions 1 through 4 was the
   work of two former Project Athena staff members, Steve Miller of
   Digital Equipment Corporation and Clifford Neuman (now at the
   Information Sciences Institute of the University of Southern
   California), along with Jerome Saltzer, Technical Director of Project
   Athena, and Jeffrey Schiller, MIT Campus Network Manager.  Many other
   members of Project Athena have also contributed to the work on
   Kerberos.

   Version 5 of the Kerberos protocol (described in this document) has
   evolved because of new requirements and desires for features not
   available in Version 4.  The design of Version 5 was led by Clifford
   Neuman and John Kohl with much input from the community.  The
   development of the MIT reference implementation was led at MIT by
   John Kohl and Theodore Ts'o, with help and contributed code from many
   others.  Since RFC 1510 was issued, many individuals have proposed
   extensions and revisions to the protocol.  This document reflects
   some of these proposals.  Where such changes involved significant
   effort, the document cites the contribution of the proposer.

   Reference implementations of both Version 4 and Version 5 of Kerberos
   are publicly available, and commercial implementations have been
   developed and are widely used.  Details on the differences between
   Versions 4 and 5 can be found in [KNT94].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

1.1.  The Kerberos Protocol

   Kerberos provides a means of verifying the identities of principals,
   (e.g., a workstation user or a network server) on an open
   (unprotected) network.  This is accomplished without relying on
   assertions by the host operating system, without basing trust on host
   addresses, without requiring physical security of all the hosts on
   the network, and under the assumption that packets traveling along
   the network can be read, modified, and inserted at will.  Kerberos
   performs authentication under these conditions as a trusted third-
   party authentication service by using conventional (shared secret
   key) cryptography.  Extensions to Kerberos (outside the scope of this
   document) can provide for the use of public key cryptography during
   certain phases of the authentication protocol.  Such extensions
   support Kerberos authentication for users registered with public key
   certification authorities and provide certain benefits of public key
   cryptography in situations where they are needed.

   The basic Kerberos authentication process proceeds as follows: A
   client sends a request to the authentication server (AS) for
   "credentials" for a given server.  The AS responds with these
   credentials, encrypted in the client's key.  The credentials consist
   of a "ticket" for the server and a temporary encryption key (often
   called a "session key").  The client transmits the ticket (which
   contains the client's identity and a copy of the session key, all
   encrypted in the server's key) to the server.  The session key (now
   shared by the client and server) is used to authenticate the client
   and may optionally be used to authenticate the server.  It may also
   be used to encrypt further communication between the two parties or
   to exchange a separate sub-session key to be used to encrypt further
   communication.  Note that many applications use Kerberos' functions
   only upon the initiation of a stream-based network connection.
   Unless an application performs encryption or integrity protection for
   the data stream, the identity verification applies only to the
   initiation of the connection, and it does not guarantee that
   subsequent messages on the connection originate from the same
   principal.

   Implementation of the basic protocol consists of one or more
   authentication servers running on physically secure hosts.  The
   authentication servers maintain a database of principals (i.e., users
   and servers) and their secret keys.  Code libraries provide
   encryption and implement the Kerberos protocol.  In order to add
   authentication to its transactions, a typical network application
   adds calls to the Kerberos library directly or through the Generic
   Security Services Application Programming Interface (GSS-API)
   described in a separate document [RFC4121].  These calls result in
   the transmission of the messages necessary to achieve authentication.

   The Kerberos protocol consists of several sub-protocols (or
   exchanges).  There are two basic methods by which a client can ask a
   Kerberos server for credentials.  In the first approach, the client
   sends a cleartext request for a ticket for the desired server to the
   AS.  The reply is sent encrypted in the client's secret key.  Usually
   this request is for a ticket-granting ticket (TGT), which can later
   be used with the ticket-granting server (TGS).  In the second method,
   the client sends a request to the TGS.  The client uses the TGT to
   authenticate itself to the TGS in the same manner as if it were
   contacting any other application server that requires Kerberos
   authentication.  The reply is encrypted in the session key from the
   TGT.  Though the protocol specification describes the AS and the TGS
   as separate servers, in practice they are implemented as different
   protocol entry points within a single Kerberos server.

   Once obtained, credentials may be used to verify the identity of the
   principals in a transaction, to ensure the integrity of messages
   exchanged between them, or to preserve privacy of the messages.  The
   application is free to choose whatever protection may be necessary.

   To verify the identities of the principals in a transaction, the
   client transmits the ticket to the application server.  Because the
   ticket is sent "in the clear" (parts of it are encrypted, but this
   encryption doesn't thwart replay) and might be intercepted and reused
   by an attacker, additional information is sent to prove that the
   message originated with the principal to whom the ticket was issued.
   This information (called the authenticator) is encrypted in the
   session key and includes a timestamp.  The timestamp proves that the
   message was recently generated and is not a replay.  Encrypting the
   authenticator in the session key proves that it was generated by a
   party possessing the session key.  Since no one except the requesting
   principal and the server know the session key (it is never sent over
   the network in the clear), this guarantees the identity of the
   client.

   The integrity of the messages exchanged between principals can also
   be guaranteed by using the session key (passed in the ticket and
   contained in the credentials).  This approach provides detection of
   both replay attacks and message stream modification attacks.  It is
   accomplished by generating and transmitting a collision-proof
   checksum (elsewhere called a hash or digest function) of the client's
   message, keyed with the session key.  Privacy and integrity of the
   messages exchanged between principals can be secured by encrypting
   the data to be passed by using the session key contained in the
   ticket or the sub-session key found in the authenticator.

   The authentication exchanges mentioned above require read-only access
   to the Kerberos database.  Sometimes, however, the entries in the
   database must be modified, such as when adding new principals or
   changing a principal's key.  This is done using a protocol between a
   client and a third Kerberos server, the Kerberos Administration
   Server (KADM).  There is also a protocol for maintaining multiple
   copies of the Kerberos database.  Neither of these protocols are
   described in this document.

1.2.  Cross-Realm Operation

   The Kerberos protocol is designed to operate across organizational
   boundaries.  A client in one organization can be authenticated to a
   server in another.  Each organization wishing to run a Kerberos
   server establishes its own "realm".  The name of the realm in which a
   client is registered is part of the client's name and can be used by
   the end-service to decide whether to honor a request.

   By establishing "inter-realm" keys, the administrators of two realms
   can allow a client authenticated in the local realm to prove its
   identity to servers in other realms.  The exchange of inter-realm
   keys (a separate key may be used for each direction) registers the
   ticket-granting service of each realm as a principal in the other
   realm.  A client is then able to obtain a TGT for the remote realm's
   ticket-granting service from its local realm.  When that TGT is used,
   the remote ticket-granting service uses the inter-realm key (which
   usually differs from its own normal TGS key) to decrypt the TGT; thus
   it is certain that the ticket was issued by the client's own TGS.
   Tickets issued by the remote ticket-granting service will indicate to
   the end-service that the client was authenticated from another realm.

   Without cross-realm operation, and with appropriate permission, the
   client can arrange registration of a separately-named principal in a
   remote realm and engage in normal exchanges with that realm's
   services.  However, for even small numbers of clients this becomes
   cumbersome, and more automatic methods as described here are
   necessary.

   A realm is said to communicate with another realm if the two realms
   share an inter-realm key, or if the local realm shares an inter-realm
   key with an intermediate realm that communicates with the remote
   realm.  An authentication path is the sequence of intermediate realms
   that are transited in communicating from one realm to another.

   Realms may be organized hierarchically.  Each realm shares a key with
   its parent and a different key with each child.  If an inter-realm
   key is not directly shared by two realms, the hierarchical
   organization allows an authentication path to be easily constructed.

   If a hierarchical organization is not used, it may be necessary to
   consult a database in order to construct an authentication path
   between realms.

   Although realms are typically hierarchical, intermediate realms may
   be bypassed to achieve cross-realm authentication through alternate
   authentication paths.  (These might be established to make
   communication between two realms more efficient.)  It is important
   for the end-service to know which realms were transited when deciding
   how much faith to place in the authentication process.  To facilitate
   this decision, a field in each ticket contains the names of the
   realms that were involved in authenticating the client.

   The application server is ultimately responsible for accepting or
   rejecting authentication and SHOULD check the transited field.  The
   application server may choose to rely on the Key Distribution Center
   (KDC) for the application server's realm to check the transited
   field.  The application server's KDC will set the
   TRANSITED-POLICY-CHECKED flag in this case.  The KDCs for
   intermediate realms may also check the transited field as they issue
   TGTs for other realms, but they are encouraged not to do so.  A
   client may request that the KDCs not check the transited field by
   setting the DISABLE-TRANSITED-CHECK flag.  KDCs SHOULD honor this
   flag.

1.3.  Choosing a Principal with Which to Communicate

   The Kerberos protocol provides the means for verifying (subject to
   the assumptions in Section 1.6) that the entity with which one
   communicates is the same entity that was registered with the KDC
   using the claimed identity (principal name).  It is still necessary
   to determine whether that identity corresponds to the entity with
   which one intends to communicate.

   When appropriate data has been exchanged in advance, the application
   may perform this determination syntactically based on the application
   protocol specification, information provided by the user, and
   configuration files.  For example, the server principal name
   (including realm) for a telnet server might be derived from the
   user-specified host name (from the telnet command line), the "host/"
   prefix specified in the application protocol specification, and a
   mapping to a Kerberos realm derived syntactically from the domain
   part of the specified hostname and information from the local
   Kerberos realms database.

   One can also rely on trusted third parties to make this
   determination, but only when the data obtained from the third party
   is suitably integrity-protected while resident on the third-party

   server and when transmitted.  Thus, for example, one should not rely
   on an unprotected DNS record to map a host alias to the primary name
   of a server, accepting the primary name as the party that one intends
   to contact, since an attacker can modify the mapping and impersonate
   the party.

   Implementations of Kerberos and protocols based on Kerberos MUST NOT
   use insecure DNS queries to canonicalize the hostname components of
   the service principal names (i.e., they MUST NOT use insecure DNS
   queries to map one name to another to determine the host part of the
   principal name with which one is to communicate).  In an environment
   without secure name service, application authors MAY append a
   statically configured domain name to unqualified hostnames before
   passing the name to the security mechanisms, but they should do no
   more than that.  Secure name service facilities, if available, might
   be trusted for hostname canonicalization, but such canonicalization
   by the client SHOULD NOT be required by KDC implementations.

   Implementation note: Many current implementations do some degree of
   canonicalization of the provided service name, often using DNS even
   though it creates security problems.  However, there is no
   consistency among implementations as to whether the service name is
   case folded to lowercase or whether reverse resolution is used.  To
   maximize interoperability and security, applications SHOULD provide
   security mechanisms with names that result from folding the user-
   entered name to lowercase without performing any other modifications
   or canonicalization.

1.4.  Authorization

   As an authentication service, Kerberos provides a means of verifying
   the identity of principals on a network.  Authentication is usually
   useful primarily as a first step in the process of authorization,
   determining whether a client may use a service, which objects the
   client is allowed to access, and the type of access allowed for each.
   Kerberos does not, by itself, provide authorization.  Possession of a
   client ticket for a service provides only for authentication of the
   client to that service, and in the absence of a separate
   authorization procedure, an application should not consider it to
   authorize the use of that service.

   Separate authorization methods MAY be implemented as application-
   specific access control functions and may utilize files on the
   application server, on separately issued authorization credentials
   such as those based on proxies [Neu93], or on other authorization
   services.  Separately authenticated authorization credentials MAY be
   embedded in a ticket's authorization data when encapsulated by the
   KDC-issued authorization data element.

   Applications should not accept the mere issuance of a service ticket
   by the Kerberos server (even by a modified Kerberos server) as
   granting authority to use the service, since such applications may
   become vulnerable to the bypass of this authorization check in an
   environment where other options for application authentication are
   provided, or if they interoperate with other KDCs.

1.5.  Extending Kerberos without Breaking Interoperability

   As the deployed base of Kerberos implementations grows, extending
   Kerberos becomes more important.  Unfortunately, some extensions to
   the existing Kerberos protocol create interoperability issues because
   of uncertainty regarding the treatment of certain extensibility
   options by some implementations.  This section includes guidelines
   that will enable future implementations to maintain interoperability.

   Kerberos provides a general mechanism for protocol extensibility.
   Some protocol messages contain typed holes -- sub-messages that
   contain an octet-string along with an integer that defines how to
   interpret the octet-string.  The integer types are registered
   centrally, but they can be used both for vendor extensions and for
   extensions standardized through the IETF.

   In this document, the word "extension" refers to extension by
   defining a new type to insert into an existing typed hole in a
   protocol message.  It does not refer to extension by addition of new
   fields to ASN.1 types, unless the text explicitly indicates
   otherwise.

1.5.1.  Compatibility with RFC 1510

   Note that existing Kerberos message formats cannot readily be
   extended by adding fields to the ASN.1 types.  Sending additional
   fields often results in the entire message being discarded without an
   error indication.  Future versions of this specification will provide
   guidelines to ensure that ASN.1 fields can be added without creating
   an interoperability problem.

   In the meantime, all new or modified implementations of Kerberos that
   receive an unknown message extension SHOULD preserve the encoding of
   the extension but otherwise ignore its presence.  Recipients MUST NOT
   decline a request simply because an extension is present.

   There is one exception to this rule.  If an unknown authorization
   data element type is received by a server other than the ticket-
   granting service either in an AP-REQ or in a ticket contained in an
   AP-REQ, then authentication MUST fail.  One of the primary uses of
   authorization data is to restrict the use of the ticket.  If the

   service cannot determine whether the restriction applies to that
   service, then a security weakness may result if the ticket can be
   used for that service.  Authorization elements that are optional
   SHOULD be enclosed in the AD-IF-RELEVANT element.

   The ticket-granting service MUST ignore but propagate to derivative
   tickets any unknown authorization data types, unless those data types
   are embedded in a MANDATORY-FOR-KDC element, in which case the
   request will be rejected.  This behavior is appropriate because
   requiring that the ticket-granting service understand unknown
   authorization data types would require that KDC software be upgraded
   to understand new application-level restrictions before applications
   used these restrictions, decreasing the utility of authorization data
   as a mechanism for restricting the use of tickets.  No security
   problem is created because services to which the tickets are issued
   will verify the authorization data.

   Implementation note: Many RFC 1510 implementations ignore unknown
   authorization data elements.  Depending on these implementations to
   honor authorization data restrictions may create a security weakness.

1.5.2.  Sending Extensible Messages

   Care must be taken to ensure that old implementations can understand
   messages sent to them, even if they do not understand an extension
   that is used.  Unless the sender knows that an extension is
   supported, the extension cannot change the semantics of the core
   message or previously defined extensions.

   For example, an extension including key information necessary to
   decrypt the encrypted part of a KDC-REP could only be used in
   situations where the recipient was known to support the extension.
   Thus when designing such extensions it is important to provide a way
   for the recipient to notify the sender of support for the extension.
   For example in the case of an extension that changes the KDC-REP
   reply key, the client could indicate support for the extension by
   including a padata element in the AS-REQ sequence.  The KDC should
   only use the extension if this padata element is present in the
   AS-REQ.  Even if policy requires the use of the extension, it is
   better to return an error indicating that the extension is required
   than to use the extension when the recipient may not support it.
   Debugging implementations that do not interoperate is easier when
   errors are returned.

1.6.  Environmental Assumptions

   Kerberos imposes a few assumptions on the environment in which it can
   properly function, including the following:

   *  "Denial of service" attacks are not solved with Kerberos.  There
      are places in the protocols where an intruder can prevent an
      application from participating in the proper authentication steps.
      Detection and solution of such attacks (some of which can appear
      to be not-uncommon "normal" failure modes for the system) are
      usually best left to the human administrators and users.

   *  Principals MUST keep their secret keys secret.  If an intruder
      somehow steals a principal's key, it will be able to masquerade as
      that principal or to impersonate any server to the legitimate
      principal.

   *  "Password guessing" attacks are not solved by Kerberos.  If a user
      chooses a poor password, it is possible for an attacker to
      successfully mount an offline dictionary attack by repeatedly
      attempting to decrypt, with successive entries from a dictionary,
      messages obtained which are encrypted under a key derived from the
      user's password.

   *  Each host on the network MUST have a clock which is "loosely
      synchronized" to the time of the other hosts; this synchronization
      is used to reduce the bookkeeping needs of application servers
      when they do replay detection.  The degree of "looseness" can be
      configured on a per-server basis, but it is typically on the order
      of 5 minutes.  If the clocks are synchronized over the network,
      the clock synchronization protocol MUST itself be secured from
      network attackers.

   *  Principal identifiers are not recycled on a short-term basis.  A
      typical mode of access control will use access control lists
      (ACLs) to grant permissions to particular principals.  If a stale
      ACL entry remains for a deleted principal and the principal
      identifier is reused, the new principal will inherit rights
      specified in the stale ACL entry.  By not re-using principal
      identifiers, the danger of inadvertent access is removed.

1.7.  Glossary of Terms

   Below is a list of terms used throughout this document.

   Authentication
      Verifying the claimed identity of a principal.

   Authentication header
      A record containing a Ticket and an Authenticator to be presented
      to a server as part of the authentication process.

   Authentication path
      A sequence of intermediate realms transited in the authentication
      process when communicating from one realm to another.

   Authenticator
      A record containing information that can be shown to have been
      recently generated using the session key known only by the client
      and server.

   Authorization
      The process of determining whether a client may use a service,
      which objects the client is allowed to access, and the type of
      access allowed for each.

   Capability
      A token that grants the bearer permission to access an object or
      service.  In Kerberos, this might be a ticket whose use is
      restricted by the contents of the authorization data field, but
      which lists no network addresses, together with the session key
      necessary to use the ticket.

   Ciphertext
      The output of an encryption function.  Encryption transforms
      plaintext into ciphertext.

   Client
      A process that makes use of a network service on behalf of a user.
      Note that in some cases a Server may itself be a client of some
      other server (e.g., a print server may be a client of a file
      server).

   Credentials
      A ticket plus the secret session key necessary to use that ticket
      successfully in an authentication exchange.

   Encryption Type (etype)
      When associated with encrypted data, an encryption type identifies
      the algorithm used to encrypt the data and is used to select the
      appropriate algorithm for decrypting the data.  Encryption type
      tags are communicated in other messages to enumerate algorithms
      that are desired, supported, preferred, or allowed to be used for
      encryption of data between parties.  This preference is combined
      with local information and policy to select an algorithm to be
      used.

   KDC
      Key Distribution Center.  A network service that supplies tickets
      and temporary session keys; or an instance of that service or the

      host on which it runs.  The KDC services both initial ticket and
      ticket-granting ticket requests.  The initial ticket portion is
      sometimes referred to as the Authentication Server (or service).
      The ticket-granting ticket portion is sometimes referred to as the
      ticket-granting server (or service).

   Kerberos
      The name given to the Project Athena's authentication service, the
      protocol used by that service, or the code used to implement the
      authentication service.  The name is adopted from the three-headed
      dog that guards Hades.

   Key Version Number (kvno)
      A tag associated with encrypted data identifies which key was used
      for encryption when a long-lived key associated with a principal
      changes over time.  It is used during the transition to a new key
      so that the party decrypting a message can tell whether the data
      was encrypted with the old or the new key.

   Plaintext
      The input to an encryption function or the output of a decryption
      function.  Decryption transforms ciphertext into plaintext.

   Principal
      A named client or server entity that participates in a network
      communication, with one name that is considered canonical.

   Principal identifier
      The canonical name used to identify each different principal
      uniquely.

   Seal
      To encipher a record containing several fields in such a way that
      the fields cannot be individually replaced without knowledge of
      the encryption key or leaving evidence of tampering.

   Secret key
      An encryption key shared by a principal and the KDC, distributed
      outside the bounds of the system, with a long lifetime.  In the
      case of a human user's principal, the secret key MAY be derived
      from a password.

   Server
      A particular Principal that provides a resource to network
      clients.  The server is sometimes referred to as the Application
      Server.

   Service
      A resource provided to network clients; often provided by more
      than one server (for example, remote file service).

   Session key
      A temporary encryption key used between two principals, with a
      lifetime limited to the duration of a single login "session".  In
      the Kerberos system, a session key is generated by the KDC.  The
      session key is distinct from the sub-session key, described next.

   Sub-session key
      A temporary encryption key used between two principals, selected
      and exchanged by the principals using the session key, and with a
      lifetime limited to the duration of a single association.  The
      sub-session key is also referred to as the subkey.

   Ticket
      A record that helps a client authenticate itself to a server; it
      contains the client's identity, a session key, a timestamp, and
      other information, all sealed using the server's secret key.  It
      only serves to authenticate a client when presented along with a
      fresh Authenticator.

2.  Ticket Flag Uses and Requests

   Each Kerberos ticket contains a set of flags that are used to
   indicate attributes of that ticket.  Most flags may be requested by a
   client when the ticket is obtained; some are automatically turned on
   and off by a Kerberos server as required.  The following sections
   explain what the various flags mean and give examples of reasons to
   use them.  With the exception of the INVALID flag, clients MUST
   ignore ticket flags that are not recognized.  KDCs MUST ignore KDC
   options that are not recognized.  Some implementations of RFC 1510
   are known to reject unknown KDC options, so clients may need to
   resend a request without new KDC options if the request was rejected
   when sent with options added since RFC 1510.  Because new KDCs will
   ignore unknown options, clients MUST confirm that the ticket returned
   by the KDC meets their needs.

   Note that it is not, in general, possible to determine whether an
   option was not honored because it was not understood or because it
   was rejected through either configuration or policy.  When adding a
   new option to the Kerberos protocol, designers should consider
   whether the distinction is important for their option.  If it is, a
   mechanism for the KDC to return an indication that the option was
   understood but rejected needs to be provided in the specification of
   the option.  Often in such cases, the mechanism needs to be broad
   enough to permit an error or reason to be returned.

2.1.  Initial, Pre-authenticated, and Hardware-Authenticated Tickets

   The INITIAL flag indicates that a ticket was issued using the AS
   protocol, rather than issued based on a TGT.  Application servers
   that want to require the demonstrated knowledge of a client's secret
   key (e.g., a password-changing program) can insist that this flag be
   set in any tickets they accept, and can thus be assured that the
   client's key was recently presented to the authentication server.

   The PRE-AUTHENT and HW-AUTHENT flags provide additional information
   about the initial authentication, regardless of whether the current
   ticket was issued directly (in which case INITIAL will also be set)
   or issued on the basis of a TGT (in which case the INITIAL flag is
   clear, but the PRE-AUTHENT and HW-AUTHENT flags are carried forward
   from the TGT).

2.2.  Invalid Tickets

   The INVALID flag indicates that a ticket is invalid.  Application
   servers MUST reject tickets that have this flag set.  A postdated
   ticket will be issued in this form.  Invalid tickets MUST be
   validated by the KDC before use, by being presented to the KDC in a
   TGS request with the VALIDATE option specified.  The KDC will only
   validate tickets after their starttime has passed.  The validation is
   required so that postdated tickets that have been stolen before their
   starttime can be rendered permanently invalid (through a hot-list
   mechanism) (see Section 3.3.3.1).

2.3.  Renewable Tickets

   Applications may desire to hold tickets that can be valid for long
   periods of time.  However, this can expose their credentials to
   potential theft for equally long periods, and those stolen
   credentials would be valid until the expiration time of the
   ticket(s).  Simply using short-lived tickets and obtaining new ones
   periodically would require the client to have long-term access to its
   secret key, an even greater risk.  Renewable tickets can be used to
   mitigate the consequences of theft.  Renewable tickets have two
   "expiration times": the first is when the current instance of the
   ticket expires, and the second is the latest permissible value for an
   individual expiration time.  An application client must periodically
   (i.e., before it expires) present a renewable ticket to the KDC, with
   the RENEW option set in the KDC request.  The KDC will issue a new
   ticket with a new session key and a later expiration time.  All other
   fields of the ticket are left unmodified by the renewal process.
   When the latest permissible expiration time arrives, the ticket
   expires permanently.  At each renewal, the KDC MAY consult a hot-list
   to determine whether the ticket had been reported stolen since its

   last renewal; it will refuse to renew stolen tickets, and thus the
   usable lifetime of stolen tickets is reduced.

   The RENEWABLE flag in a ticket is normally only interpreted by the
   ticket-granting service (discussed below in Section 3.3).  It can
   usually be ignored by application servers.  However, some
   particularly careful application servers MAY disallow renewable
   tickets.

   If a renewable ticket is not renewed by its expiration time, the KDC
   will not renew the ticket.  The RENEWABLE flag is reset by default,
   but a client MAY request it be set by setting the RENEWABLE option in
   the KRB_AS_REQ message.  If it is set, then the renew-till field in
   the ticket contains the time after which the ticket may not be
   renewed.

2.4.  Postdated Tickets

   Applications may occasionally need to obtain tickets for use much
   later; e.g., a batch submission system would need tickets to be valid
   at the time the batch job is serviced.  However, it is dangerous to
   hold valid tickets in a batch queue, since they will be on-line
   longer and more prone to theft.  Postdated tickets provide a way to
   obtain these tickets from the KDC at job submission time, but to
   leave them "dormant" until they are activated and validated by a
   further request of the KDC.  If a ticket theft were reported in the
   interim, the KDC would refuse to validate the ticket, and the thief
   would be foiled.

   The MAY-POSTDATE flag in a ticket is normally only interpreted by the
   ticket-granting service.  It can be ignored by application servers.
   This flag MUST be set in a TGT in order to issue a postdated ticket
   based on the presented ticket.  It is reset by default; a client MAY
   request it by setting the ALLOW-POSTDATE option in the KRB_AS_REQ
   message.  This flag does not allow a client to obtain a postdated
   TGT; postdated TGTs can only be obtained by requesting the postdating
   in the KRB_AS_REQ message.  The life (endtime-starttime) of a
   postdated ticket will be the remaining life of the TGT at the time of
   the request, unless the RENEWABLE option is also set, in which case
   it can be the full life (endtime-starttime) of the TGT.  The KDC MAY
   limit how far in the future a ticket may be postdated.

   The POSTDATED flag indicates that a ticket has been postdated.  The
   application server can check the authtime field in the ticket to see
   when the original authentication occurred.  Some services MAY choose
   to reject postdated tickets, or they may only accept them within a
   certain period after the original authentication.  When the KDC
   issues a POSTDATED ticket, it will also be marked as INVALID, so that

   the application client MUST present the ticket to the KDC to be
   validated before use.

2.5.  Proxiable and Proxy Tickets

   At times it may be necessary for a principal to allow a service to
   perform an operation on its behalf.  The service must be able to take
   on the identity of the client, but only for a particular purpose.  A
   principal can allow a service to do this by granting it a proxy.

   The process of granting a proxy by using the proxy and proxiable
   flags is used to provide credentials for use with specific services.
   Though conceptually also a proxy, users wishing to delegate their
   identity in a form usable for all purposes MUST use the ticket
   forwarding mechanism described in the next section to forward a TGT.

   The PROXIABLE flag in a ticket is normally only interpreted by the
   ticket-granting service.  It can be ignored by application servers.
   When set, this flag tells the ticket-granting server that it is OK to
   issue a new ticket (but not a TGT) with a different network address
   based on this ticket.  This flag is set if requested by the client on
   initial authentication.  By default, the client will request that it
   be set when requesting a TGT, and that it be reset when requesting
   any other ticket.

   This flag allows a client to pass a proxy to a server to perform a
   remote request on its behalf (e.g., a print service client can give
   the print server a proxy to access the client's files on a particular
   file server in order to satisfy a print request).

   In order to complicate the use of stolen credentials, Kerberos
   tickets are often valid only from those network addresses
   specifically included in the ticket, but it is permissible as a
   policy option to allow requests and to issue tickets with no network
   addresses specified.  When granting a proxy, the client MUST specify
   the new network address from which the proxy is to be used or
   indicate that the proxy is to be issued for use from any address.

   The PROXY flag is set in a ticket by the TGS when it issues a proxy
   ticket.  Application servers MAY check this flag; and at their option
   they MAY require additional authentication from the agent presenting
   the proxy in order to provide an audit trail.

2.6.  Forwardable Tickets

   Authentication forwarding is an instance of a proxy where the service
   that is granted is complete use of the client's identity.  An example
   of where it might be used is when a user logs in to a remote system

   and wants authentication to work from that system as if the login
   were local.

   The FORWARDABLE flag in a ticket is normally only interpreted by the
   ticket-granting service.  It can be ignored by application servers.
   The FORWARDABLE flag has an interpretation similar to that of the
   PROXIABLE flag, except TGTs may also be issued with different network
   addresses.  This flag is reset by default, but users MAY request that
   it be set by setting the FORWARDABLE option in the AS request when
   they request their initial TGT.

   This flag allows for authentication forwarding without requiring the
   user to enter a password again.  If the flag is not set, then
   authentication forwarding is not permitted, but the same result can
   still be achieved if the user engages in the AS exchange, specifies
   the requested network addresses, and supplies a password.

   The FORWARDED flag is set by the TGS when a client presents a ticket
   with the FORWARDABLE flag set and requests a forwarded ticket by
   specifying the FORWARDED KDC option and supplying a set of addresses
   for the new ticket.  It is also set in all tickets issued based on
   tickets with the FORWARDED flag set.  Application servers may choose
   to process FORWARDED tickets differently than non-FORWARDED tickets.

   If addressless tickets are forwarded from one system to another,
   clients SHOULD still use this option to obtain a new TGT in order to
   have different session keys on the different systems.

2.7.  Transited Policy Checking

   In Kerberos, the application server is ultimately responsible for
   accepting or rejecting authentication, and it SHOULD check that only
   suitably trusted KDCs are relied upon to authenticate a principal.
   The transited field in the ticket identifies which realms (and thus
   which KDCs) were involved in the authentication process, and an
   application server would normally check this field.  If any of these
   are untrusted to authenticate the indicated client principal
   (probably determined by a realm-based policy), the authentication
   attempt MUST be rejected.  The presence of trusted KDCs in this list
   does not provide any guarantee; an untrusted KDC may have fabricated
   the list.

   Although the end server ultimately decides whether authentication is
   valid, the KDC for the end server's realm MAY apply a realm-specific
   policy for validating the transited field and accepting credentials
   for cross-realm authentication.  When the KDC applies such checks and
   accepts such cross-realm authentication, it will set the
   TRANSITED-POLICY-CHECKED flag in the service tickets it issues based

   on the cross-realm TGT.  A client MAY request that the KDCs not check
   the transited field by setting the DISABLE-TRANSITED-CHECK flag.
   KDCs are encouraged but not required to honor this flag.

   Application servers MUST either do the transited-realm checks
   themselves or reject cross-realm tickets without
   TRANSITED-POLICY-CHECKED set.

2.8.  OK as Delegate

   For some applications, a client may need to delegate authority to a
   server to act on its behalf in contacting other services.  This
   requires that the client forward credentials to an intermediate
   server.  The ability for a client to obtain a service ticket to a
   server conveys no information to the client about whether the server
   should be trusted to accept delegated credentials.  The
   OK-AS-DELEGATE provides a way for a KDC to communicate local realm
   policy to a client regarding whether an intermediate server is
   trusted to accept such credentials.

   The copy of the ticket flags in the encrypted part of the KDC reply
   may have the OK-AS-DELEGATE flag set to indicate to the client that
   the server specified in the ticket has been determined by the policy
   of the realm to be a suitable recipient of delegation.  A client can
   use the presence of this flag to help it decide whether to delegate
   credentials (grant either a proxy or a forwarded TGT) to this server.
   It is acceptable to ignore the value of this flag.  When setting this
   flag, an administrator should consider the security and placement of
   the server on which the service will run, as well as whether the
   service requires the use of delegated credentials.

2.9.  Other KDC Options

   There are three additional options that MAY be set in a client's
   request of the KDC.

2.9.1.  Renewable-OK

   The RENEWABLE-OK option indicates that the client will accept a
   renewable ticket if a ticket with the requested life cannot otherwise
   be provided.  If a ticket with the requested life cannot be provided,
   then the KDC MAY issue a renewable ticket with a renew-till equal to
   the requested endtime.  The value of the renew-till field MAY still
   be adjusted by site-determined limits or limits imposed by the
   individual principal or server.

2.9.2.  ENC-TKT-IN-SKEY

   In its basic form, the Kerberos protocol supports authentication in a
   client-server setting and is not well suited to authentication in a
   peer-to-peer environment because the long-term key of the user does
   not remain on the workstation after initial login.  Authentication of
   such peers may be supported by Kerberos in its user-to-user variant.
   The ENC-TKT-IN-SKEY option supports user-to-user authentication by
   allowing the KDC to issue a service ticket encrypted using the
   session key from another TGT issued to another user.  The
   ENC-TKT-IN-SKEY option is honored only by the ticket-granting
   service.  It indicates that the ticket to be issued for the end
   server is to be encrypted in the session key from the additional
   second TGT provided with the request.  See Section 3.3.3 for specific
   details.

2.9.3.  Passwordless Hardware Authentication

   The OPT-HARDWARE-AUTH option indicates that the client wishes to use
   some form of hardware authentication instead of or in addition to the
   client's password or other long-lived encryption key.
   OPT-HARDWARE-AUTH is honored only by the authentication service.  If
   supported and allowed by policy, the KDC will return an error code of
   KDC_ERR_PREAUTH_REQUIRED and include the required METHOD-DATA to
   perform such authentication.

3.  Message Exchanges

   The following sections describe the interactions between network
   clients and servers and the messages involved in those exchanges.

3.1.  The Authentication Service Exchange

                             Summary

         Message direction       Message type    Section
         1. Client to Kerberos   KRB_AS_REQ      5.4.1
         2. Kerberos to client   KRB_AS_REP or   5.4.2
                                 KRB_ERROR       5.9.1

   The Authentication Service (AS) Exchange between the client and the
   Kerberos Authentication Server is initiated by a client when it
   wishes to obtain authentication credentials for a given server but
   currently holds no credentials.  In its basic form, the client's
   secret key is used for encryption and decryption.  This exchange is
   typically used at the initiation of a login session to obtain
   credentials for a Ticket-Granting Server, which will subsequently be
   used to obtain credentials for other servers (see Section 3.3)

   without requiring further use of the client's secret key.  This
   exchange is also used to request credentials for services that must
   not be mediated through the Ticket-Granting Service, but rather
   require knowledge of a principal's secret key, such as the password-
   changing service (the password-changing service denies requests
   unless the requester can demonstrate knowledge of the user's old
   password; requiring this knowledge prevents unauthorized password
   changes by someone walking up to an unattended session).

   This exchange does not by itself provide any assurance of the
   identity of the user.  To authenticate a user logging on to a local
   system, the credentials obtained in the AS exchange may first be used
   in a TGS exchange to obtain credentials for a local server; those
   credentials must then be verified by a local server through
   successful completion of the Client/Server exchange.

   The AS exchange consists of two messages: KRB_AS_REQ from the client
   to Kerberos, and KRB_AS_REP or KRB_ERROR in reply.  The formats for
   these messages are described in Sections 5.4.1, 5.4.2, and 5.9.1.

   In the request, the client sends (in cleartext) its own identity and
   the identity of the server for which it is requesting credentials,
   other information about the credentials it is requesting, and a
   randomly generated nonce, which can be used to detect replays and to
   associate replies with the matching requests.  This nonce MUST be
   generated randomly by the client and remembered for checking against
   the nonce in the expected reply.  The response, KRB_AS_REP, contains
   a ticket for the client to present to the server, and a session key
   that will be shared by the client and the server.  The session key
   and additional information are encrypted in the client's secret key.
   The encrypted part of the KRB_AS_REP message also contains the nonce
   that MUST be matched with the nonce from the KRB_AS_REQ message.

   Without pre-authentication, the authentication server does not know
   whether the client is actually the principal named in the request.
   It simply sends a reply without knowing or caring whether they are
   the same.  This is acceptable because nobody but the principal whose
   identity was given in the request will be able to use the reply.  Its
   critical information is encrypted in that principal's key.  However,
   an attacker can send a KRB_AS_REQ message to get known plaintext in
   order to attack the principal's key.  Especially if the key is based
   on a password, this may create a security exposure.  So the initial
   request supports an optional field that can be used to pass
   additional information that might be needed for the initial exchange.
   This field SHOULD be used for pre-authentication as described in
   sections 3.1.1 and 5.2.7.

   Various errors can occur; these are indicated by an error response
   (KRB_ERROR) instead of the KRB_AS_REP response.  The error message is
   not encrypted.  The KRB_ERROR message contains information that can
   be used to associate it with the message to which it replies.  The
   contents of the KRB_ERROR message are not integrity-protected.  As
   such, the client cannot detect replays, fabrications, or
   modifications.  A solution to this problem will be included in a
   future version of the protocol.

3.1.1.  Generation of KRB_AS_REQ Message

   The client may specify a number of options in the initial request.
   Among these options are whether pre-authentication is to be
   performed; whether the requested ticket is to be renewable,
   proxiable, or forwardable; whether it should be postdated or allow
   postdating of derivative tickets; and whether a renewable ticket will
   be accepted in lieu of a non-renewable ticket if the requested ticket
   expiration date cannot be satisfied by a non-renewable ticket (due to
   configuration constraints).

   The client prepares the KRB_AS_REQ message and sends it to the KDC.

3.1.2.  Receipt of KRB_AS_REQ Message

   If all goes well, processing the KRB_AS_REQ message will result in
   the creation of a ticket for the client to present to the server.
   The format for the ticket is described in Section 5.3.

   Because Kerberos can run over unreliable transports such as UDP, the
   KDC MUST be prepared to retransmit responses in case they are lost.
   If a KDC receives a request identical to one it has recently
   processed successfully, the KDC MUST respond with a KRB_AS_REP
   message rather than a replay error.  In order to reduce ciphertext
   given to a potential attacker, KDCs MAY send the same response
   generated when the request was first handled.  KDCs MUST obey this
   replay behavior even if the actual transport in use is reliable.

3.1.3.  Generation of KRB_AS_REP Message

   The authentication server looks up the client and server principals
   named in the KRB_AS_REQ in its database, extracting their respective
   keys.  If the requested client principal named in the request is
   unknown because it doesn't exist in the KDC's principal database,
   then an error message with a KDC_ERR_C_PRINCIPAL_UNKNOWN is returned.

   If required to do so, the server pre-authenticates the request, and
   if the pre-authentication check fails, an error message with the code
   KDC_ERR_PREAUTH_FAILED is returned.  If pre-authentication is

   required, but was not present in the request, an error message with
   the code KDC_ERR_PREAUTH_REQUIRED is returned, and a METHOD-DATA
   object will be stored in the e-data field of the KRB-ERROR message to
   specify which pre-authentication mechanisms are acceptable.  Usually
   this will include PA-ETYPE-INFO and/or PA-ETYPE-INFO2 elements as
   described below.  If the server cannot accommodate any encryption
   type requested by the client, an error message with code
   KDC_ERR_ETYPE_NOSUPP is returned.  Otherwise, the KDC generates a
   'random' session key, meaning that, among other things, it should be
   impossible to guess the next session key based on knowledge of past
   session keys.  Although this can be achieved in a pseudo-random
   number generator if it is based on cryptographic principles, it is
   more desirable to use a truly random number generator, such as one
   based on measurements of random physical phenomena.  See [RFC4086]
   for an in-depth discussion of randomness.

   In response to an AS request, if there are multiple encryption keys
   registered for a client in the Kerberos database, then the etype
   field from the AS request is used by the KDC to select the encryption
   method to be used to protect the encrypted part of the KRB_AS_REP
   message that is sent to the client.  If there is more than one
   supported strong encryption type in the etype list, the KDC SHOULD
   use the first valid strong etype for which an encryption key is
   available.

   When the user's key is generated from a password or pass phrase, the
   string-to-key function for the particular encryption key type is
   used, as specified in [RFC3961].  The salt value and additional
   parameters for the string-to-key function have default values
   (specified by Section 4 and by the encryption mechanism
   specification, respectively) that may be overridden by
   pre-authentication data (PA-PW-SALT, PA-AFS3-SALT, PA-ETYPE-INFO,
   PA-ETYPE-INFO2, etc).  Since the KDC is presumed to store a copy of
   the resulting key only, these values should not be changed for
   password-based keys except when changing the principal's key.

   When the AS server is to include pre-authentication data in a
   KRB-ERROR or in an AS-REP, it MUST use PA-ETYPE-INFO2, not PA-ETYPE-
   INFO, if the etype field of the client's AS-REQ lists at least one
   "newer" encryption type.  Otherwise (when the etype field of the
   client's AS-REQ does not list any "newer" encryption types), it MUST
   send both PA-ETYPE-INFO2 and PA-ETYPE-INFO (both with an entry for
   each enctype).  A "newer" enctype is any enctype first officially
   specified concurrently with or subsequent to the issue of this RFC.
   The enctypes DES, 3DES, or RC4 and any defined in [RFC1510] are not
   "newer" enctypes.

   It is not possible to generate a user's key reliably given a pass
   phrase without contacting the KDC, since it will not be known whether
   alternate salt or parameter values are required.

   The KDC will attempt to assign the type of the random session key
   from the list of methods in the etype field.  The KDC will select the
   appropriate type using the list of methods provided and information
   from the Kerberos database indicating acceptable encryption methods
   for the application server.  The KDC will not issue tickets with a
   weak session key encryption type.

   If the requested starttime is absent, indicates a time in the past,
   or is within the window of acceptable clock skew for the KDC and the
   POSTDATE option has not been specified, then the starttime of the
   ticket is set to the authentication server's current time.  If it
   indicates a time in the future beyond the acceptable clock skew, but
   the POSTDATED option has not been specified, then the error
   KDC_ERR_CANNOT_POSTDATE is returned.  Otherwise the requested
   starttime is checked against the policy of the local realm (the
   administrator might decide to prohibit certain types or ranges of
   postdated tickets), and if the ticket's starttime is acceptable, it
   is set as requested, and the INVALID flag is set in the new ticket.
   The postdated ticket MUST be validated before use by presenting it to
   the KDC after the starttime has been reached.

   The expiration time of the ticket will be set to the earlier of the
   requested endtime and a time determined by local policy, possibly by
   using realm- or principal-specific factors.  For example, the
   expiration time MAY be set to the earliest of the following:

      *  The expiration time (endtime) requested in the KRB_AS_REQ
         message.

      *  The ticket's starttime plus the maximum allowable lifetime
         associated with the client principal from the authentication
         server's database.

      *  The ticket's starttime plus the maximum allowable lifetime
         associated with the server principal.

      *  The ticket's starttime plus the maximum lifetime set by the
         policy of the local realm.

   If the requested expiration time minus the starttime (as determined
   above) is less than a site-determined minimum lifetime, an error
   message with code KDC_ERR_NEVER_VALID is returned.  If the requested
   expiration time for the ticket exceeds what was determined as above,
   and if the 'RENEWABLE-OK' option was requested, then the 'RENEWABLE'

   flag is set in the new ticket, and the renew-till value is set as if
   the 'RENEWABLE' option were requested (the field and option names are
   described fully in Section 5.4.1).

   If the RENEWABLE option has been requested or if the RENEWABLE-OK
   option has been set and a renewable ticket is to be issued, then the
   renew-till field MAY be set to the earliest of:

      *  Its requested value.

      *  The starttime of the ticket plus the minimum of the two maximum
         renewable lifetimes associated with the principals' database
         entries.

      *  The starttime of the ticket plus the maximum renewable lifetime
         set by the policy of the local realm.

   The flags field of the new ticket will have the following options set
   if they have been requested and if the policy of the local realm
   allows:  FORWARDABLE, MAY-POSTDATE, POSTDATED, PROXIABLE, RENEWABLE.
   If the new ticket is postdated (the starttime is in the future), its
   INVALID flag will also be set.

   If all of the above succeed, the server will encrypt the ciphertext
   part of the ticket using the encryption key extracted from the server
   principal's record in the Kerberos database using the encryption type
   associated with the server principal's key.  (This choice is NOT
   affected by the etype field in the request.)  It then formats a
   KRB_AS_REP message (see Section 5.4.2), copying the addresses in the
   request into the caddr of the response, placing any required pre-
   authentication data into the padata of the response, and encrypts the
   ciphertext part in the client's key using an acceptable encryption
   method requested in the etype field of the request, or in some key
   specified by pre-authentication mechanisms being used.

3.1.4.  Generation of KRB_ERROR Message

   Several errors can occur, and the Authentication Server responds by
   returning an error message, KRB_ERROR, to the client, with the
   error-code and e-text fields set to appropriate values.  The error
   message contents and details are described in Section 5.9.1.

3.1.5.  Receipt of KRB_AS_REP Message

   If the reply message type is KRB_AS_REP, then the client verifies
   that the cname and crealm fields in the cleartext portion of the
   reply match what it requested.  If any padata fields are present,
   they may be used to derive the proper secret key to decrypt the

   message.  The client decrypts the encrypted part of the response
   using its secret key and verifies that the nonce in the encrypted
   part matches the nonce it supplied in its request (to detect
   replays).  It also verifies that the sname and srealm in the response
   match those in the request (or are otherwise expected values), and
   that the host address field is also correct.  It then stores the
   ticket, session key, start and expiration times, and other
   information for later use.  The last-req field (and the deprecated
   key-expiration field) from the encrypted part of the response MAY be
   checked to notify the user of impending key expiration.  This enables
   the client program to suggest remedial action, such as a password
   change.

   Upon validation of the KRB_AS_REP message (by checking the returned
   nonce against that sent in the KRB_AS_REQ message), the client knows
   that the current time on the KDC is that read from the authtime field
   of the encrypted part of the reply.  The client can optionally use
   this value for clock synchronization in subsequent messages by
   recording with the ticket the difference (offset) between the
   authtime value and the local clock.  This offset can then be used by
   the same user to adjust the time read from the system clock when
   generating messages [DGT96].

   This technique MUST be used when adjusting for clock skew instead of
   directly changing the system clock, because the KDC reply is only
   authenticated to the user whose secret key was used, but not to the
   system or workstation.  If the clock were adjusted, an attacker
   colluding with a user logging into a workstation could agree on a
   password, resulting in a KDC reply that would be correctly validated
   even though it did not originate from a KDC trusted by the
   workstation.

   Proper decryption of the KRB_AS_REP message is not sufficient for the
   host to verify the identity of the user; the user and an attacker
   could cooperate to generate a KRB_AS_REP format message that decrypts
   properly but is not from the proper KDC.  If the host wishes to
   verify the identity of the user, it MUST require the user to present
   application credentials that can be verified using a securely-stored
   secret key for the host.  If those credentials can be verified, then
   the identity of the user can be assured.

3.1.6.  Receipt of KRB_ERROR Message

   If the reply message type is KRB_ERROR, then the client interprets it
   as an error and performs whatever application-specific tasks are
   necessary for recovery.

3.2.  The Client/Server Authentication Exchange

                                Summary

   Message direction                         Message type    Section
   Client to Application server              KRB_AP_REQ      5.5.1
   [optional] Application server to client   KRB_AP_REP or   5.5.2
                                             KRB_ERROR       5.9.1

   The client/server authentication (CS) exchange is used by network
   applications to authenticate the client to the server and vice versa.
   The client MUST have already acquired credentials for the server
   using the AS or TGS exchange.

3.2.1.  The KRB_AP_REQ Message

   The KRB_AP_REQ contains authentication information that SHOULD be
   part of the first message in an authenticated transaction.  It
   contains a ticket, an authenticator, and some additional bookkeeping
   information (see Section 5.5.1 for the exact format).  The ticket by
   itself is insufficient to authenticate a client, since tickets are
   passed across the network in cleartext (tickets contain both an
   encrypted and unencrypted portion, so cleartext here refers to the
   entire unit, which can be copied from one message and replayed in
   another without any cryptographic skill).  The authenticator is used
   to prevent invalid replay of tickets by proving to the server that
   the client knows the session key of the ticket and thus is entitled
   to use the ticket.  The KRB_AP_REQ message is referred to elsewhere
   as the 'authentication header'.

3.2.2.  Generation of a KRB_AP_REQ Message

   When a client wishes to initiate authentication to a server, it
   obtains (either through a credentials cache, the AS exchange, or the
   TGS exchange) a ticket and session key for the desired service.  The
   client MAY re-use any tickets it holds until they expire.  To use a
   ticket, the client constructs a new Authenticator from the system
   time and its name, and optionally from an application-specific
   checksum, an initial sequence number to be used in KRB_SAFE or
   KRB_PRIV messages, and/or a session subkey to be used in negotiations
   for a session key unique to this particular session.  Authenticators
   MUST NOT be re-used and SHOULD be rejected if replayed to a server.
   Note that this can make applications based on unreliable transports
   difficult to code correctly.  If the transport might deliver
   duplicated messages, either a new authenticator MUST be generated for
   each retry, or the application server MUST match requests and replies
   and replay the first reply in response to a detected duplicate.

   If a sequence number is to be included, it SHOULD be randomly chosen
   so that even after many messages have been exchanged it is not likely
   to collide with other sequence numbers in use.

   The client MAY indicate a requirement of mutual authentication or the
   use of a session-key based ticket (for user-to-user authentication,
   see section 3.7) by setting the appropriate flag(s) in the ap-options
   field of the message.

   The Authenticator is encrypted in the session key and combined with
   the ticket to form the KRB_AP_REQ message, which is then sent to the
   end server along with any additional application-specific
   information.

3.2.3.  Receipt of KRB_AP_REQ Message

   Authentication is based on the server's current time of day (clocks
   MUST be loosely synchronized), the authenticator, and the ticket.
   Several errors are possible.  If an error occurs, the server is
   expected to reply to the client with a KRB_ERROR message.  This
   message MAY be encapsulated in the application protocol if its raw
   form is not acceptable to the protocol.  The format of error messages
   is described in Section 5.9.1.

   The algorithm for verifying authentication information is as follows.
   If the message type is not KRB_AP_REQ, the server returns the
   KRB_AP_ERR_MSG_TYPE error.  If the key version indicated by the
   Ticket in the KRB_AP_REQ is not one the server can use (e.g., it
   indicates an old key, and the server no longer possesses a copy of
   the old key), the KRB_AP_ERR_BADKEYVER error is returned.  If the
   USE-SESSION-KEY flag is set in the ap-options field, it indicates to
   the server that user-to-user authentication is in use, and that the
   ticket is encrypted in the session key from the server's TGT rather
   than in the server's secret key.  See Section 3.7 for a more complete
   description of the effect of user-to-user authentication on all
   messages in the Kerberos protocol.

   Because it is possible for the server to be registered in multiple
   realms, with different keys in each, the srealm field in the
   unencrypted portion of the ticket in the KRB_AP_REQ is used to
   specify which secret key the server should use to decrypt that
   ticket.  The KRB_AP_ERR_NOKEY error code is returned if the server
   doesn't have the proper key to decipher the ticket.

   The ticket is decrypted using the version of the server's key
   specified by the ticket.  If the decryption routines detect a
   modification of the ticket (each encryption system MUST provide
   safeguards to detect modified ciphertext), the

   KRB_AP_ERR_BAD_INTEGRITY error is returned (chances are good that
   different keys were used to encrypt and decrypt).

   The authenticator is decrypted using the session key extracted from
   the decrypted ticket.  If decryption shows that is has been modified,
   the KRB_AP_ERR_BAD_INTEGRITY error is returned.  The name and realm
   of the client from the ticket are compared against the same fields in
   the authenticator.  If they don't match, the KRB_AP_ERR_BADMATCH
   error is returned; normally this is caused by a client error or an
   attempted attack.  The addresses in the ticket (if any) are then
   searched for an address matching the operating-system reported
   address of the client.  If no match is found or the server insists on
   ticket addresses but none are present in the ticket, the
   KRB_AP_ERR_BADADDR error is returned.  If the local (server) time and
   the client time in the authenticator differ by more than the
   allowable clock skew (e.g., 5 minutes), the KRB_AP_ERR_SKEW error is
   returned.

   Unless the application server provides its own suitable means to
   protect against replay (for example, a challenge-response sequence
   initiated by the server after authentication, or use of a server-
   generated encryption subkey), the server MUST utilize a replay cache
   to remember any authenticator presented within the allowable clock
   skew.  Careful analysis of the application protocol and
   implementation is recommended before eliminating this cache.  The
   replay cache will store at least the server name, along with the
   client name, time, and microsecond fields from the recently-seen
   authenticators, and if a matching tuple is found, the
   KRB_AP_ERR_REPEAT error is returned.  Note that the rejection here is
   restricted to authenticators from the same principal to the same
   server.  Other client principals communicating with the same server
   principal should not have their authenticators rejected if the time
   and microsecond fields happen to match some other client's
   authenticator.

   If a server loses track of authenticators presented within the
   allowable clock skew, it MUST reject all requests until the clock
   skew interval has passed, providing assurance that any lost or
   replayed authenticators will fall outside the allowable clock skew
   and can no longer be successfully replayed.  If this were not done,
   an attacker could subvert the authentication by recording the ticket
   and authenticator sent over the network to a server and replaying
   them following an event that caused the server to lose track of
   recently seen authenticators.

   Implementation note: If a client generates multiple requests to the
   KDC with the same timestamp, including the microsecond field, all but
   the first of the requests received will be rejected as replays.  This

   might happen, for example, if the resolution of the client's clock is
   too coarse.  Client implementations SHOULD ensure that the timestamps
   are not reused, possibly by incrementing the microseconds field in
   the time stamp when the clock returns the same time for multiple
   requests.

   If multiple servers (for example, different services on one machine,
   or a single service implemented on multiple machines) share a service
   principal (a practice that we do not recommend in general, but that
   we acknowledge will be used in some cases), either they MUST share
   this replay cache, or the application protocol MUST be designed so as
   to eliminate the need for it.  Note that this applies to all of the
   services.  If any of the application protocols does not have replay
   protection built in, an authenticator used with such a service could
   later be replayed to a different service with the same service
   principal but no replay protection, if the former doesn't record the
   authenticator information in the common replay cache.

   If a sequence number is provided in the authenticator, the server
   saves it for later use in processing KRB_SAFE and/or KRB_PRIV
   messages.  If a subkey is present, the server either saves it for
   later use or uses it to help generate its own choice for a subkey to
   be returned in a KRB_AP_REP message.

   The server computes the age of the ticket: local (server) time minus
   the starttime inside the Ticket.  If the starttime is later than the
   current time by more than the allowable clock skew, or if the INVALID
   flag is set in the ticket, the KRB_AP_ERR_TKT_NYV error is returned.
   Otherwise, if the current time is later than end time by more than
   the allowable clock skew, the KRB_AP_ERR_TKT_EXPIRED error is
   returned.

   If all these checks succeed without an error, the server is assured
   that the client possesses the credentials of the principal named in
   the ticket, and thus, that the client has been authenticated to the
   server.

   Passing these checks provides only authentication of the named
   principal; it does not imply authorization to use the named service.
   Applications MUST make a separate authorization decision based upon
   the authenticated name of the user, the requested operation, local
   access control information such as that contained in a .k5login or
   .k5users file, and possibly a separate distributed authorization
   service.

3.2.4.  Generation of a KRB_AP_REP Message

   Typically, a client's request will include both the authentication
   information and its initial request in the same message, and the
   server need not explicitly reply to the KRB_AP_REQ.  However, if
   mutual authentication (authenticating not only the client to the
   server, but also the server to the client) is being performed, the
   KRB_AP_REQ message will have MUTUAL-REQUIRED set in its ap-options
   field, and a KRB_AP_REP message is required in response.  As with the
   error message, this message MAY be encapsulated in the application
   protocol if its "raw" form is not acceptable to the application's
   protocol.  The timestamp and microsecond field used in the reply MUST
   be the client's timestamp and microsecond field (as provided in the
   authenticator).  If a sequence number is to be included, it SHOULD be
   randomly chosen as described above for the authenticator.  A subkey
   MAY be included if the server desires to negotiate a different
   subkey.  The KRB_AP_REP message is encrypted in the session key
   extracted from the ticket.

   Note that in the Kerberos Version 4 protocol, the timestamp in the
   reply was the client's timestamp plus one.  This is not necessary in
   Version 5 because Version 5 messages are formatted in such a way that
   it is not possible to create the reply by judicious message surgery
   (even in encrypted form) without knowledge of the appropriate
   encryption keys.

3.2.5.  Receipt of KRB_AP_REP Message

   If a KRB_AP_REP message is returned, the client uses the session key
   from the credentials obtained for the server to decrypt the message
   and verifies that the timestamp and microsecond fields match those in
   the Authenticator it sent to the server.  If they match, then the
   client is assured that the server is genuine.  The sequence number
   and subkey (if present) are retained for later use.  (Note that for
   encrypting the KRB_AP_REP message, the sub-session key is not used,
   even if it is present in the Authentication.)

3.2.6.  Using the Encryption Key

   After the KRB_AP_REQ/KRB_AP_REP exchange has occurred, the client and
   server share an encryption key that can be used by the application.
   In some cases, the use of this session key will be implicit in the
   protocol; in others the method of use must be chosen from several
   alternatives.  The application MAY choose the actual encryption key
   to be used for KRB_PRIV, KRB_SAFE, or other application-specific uses
   based on the session key from the ticket and subkeys in the
   KRB_AP_REP message and the authenticator.  Implementations of the
   protocol MAY provide routines to choose subkeys based on session keys

   and random numbers and to generate a negotiated key to be returned in
   the KRB_AP_REP message.

   To mitigate the effect of failures in random number generation on the
   client, it is strongly encouraged that any key derived by an
   application for subsequent use include the full key entropy derived
   from the KDC-generated session key carried in the ticket.  We leave
   the protocol negotiations of how to use the key (e.g., for selecting
   an encryption or checksum type) to the application programmer.  The
   Kerberos protocol does not constrain the implementation options, but
   an example of how this might be done follows.

   One way that an application may choose to negotiate a key to be used
   for subsequent integrity and privacy protection is for the client to
   propose a key in the subkey field of the authenticator.  The server
   can then choose a key using the key proposed by the client as input,
   returning the new subkey in the subkey field of the application
   reply.  This key could then be used for subsequent communication.

   With both the one-way and mutual authentication exchanges, the peers
   should take care not to send sensitive information to each other
   without proper assurances.  In particular, applications that require
   privacy or integrity SHOULD use the KRB_AP_REP response from the
   server to the client to assure both client and server of their peer's
   identity.  If an application protocol requires privacy of its
   messages, it can use the KRB_PRIV message (section 3.5).  The
   KRB_SAFE message (Section 3.4) can be used to ensure integrity.

3.3.  The Ticket-Granting Service (TGS) Exchange

                             Summary

         Message direction       Message type     Section
         1. Client to Kerberos   KRB_TGS_REQ      5.4.1
         2. Kerberos to client   KRB_TGS_REP or   5.4.2
                                 KRB_ERROR        5.9.1

   The TGS exchange between a client and the Kerberos TGS is initiated
   by a client when it seeks to obtain authentication credentials for a
   given server (which might be registered in a remote realm), when it
   seeks to renew or validate an existing ticket, or when it seeks to
   obtain a proxy ticket.  In the first case, the client must already
   have acquired a ticket for the Ticket-Granting Service using the AS
   exchange (the TGT is usually obtained when a client initially
   authenticates to the system, such as when a user logs in).  The
   message format for the TGS exchange is almost identical to that for
   the AS exchange.  The primary difference is that encryption and
   decryption in the TGS exchange does not take place under the client's

   key.  Instead, the session key from the TGT or renewable ticket, or
   sub-session key from an Authenticator is used.  As is the case for
   all application servers, expired tickets are not accepted by the TGS,
   so once a renewable or TGT expires, the client must use a separate
   exchange to obtain valid tickets.

   The TGS exchange consists of two messages: a request (KRB_TGS_REQ)
   from the client to the Kerberos Ticket-Granting Server, and a reply
   (KRB_TGS_REP or KRB_ERROR).  The KRB_TGS_REQ message includes
   information authenticating the client plus a request for credentials.
   The authentication information consists of the authentication header
   (KRB_AP_REQ), which includes the client's previously obtained
   ticket-granting, renewable, or invalid ticket.  In the TGT and proxy
   cases, the request MAY include one or more of the following: a list
   of network addresses, a collection of typed authorization data to be
   sealed in the ticket for authorization use by the application server,
   or additional tickets (the use of which are described later).  The
   TGS reply (KRB_TGS_REP) contains the requested credentials, encrypted
   in the session key from the TGT or renewable ticket, or, if present,
   in the sub-session key from the Authenticator (part of the
   authentication header).  The KRB_ERROR message contains an error code
   and text explaining what went wrong.  The KRB_ERROR message is not
   encrypted.  The KRB_TGS_REP message contains information that can be
   used to detect replays, and to associate it with the message to which
   it replies.  The KRB_ERROR message also contains information that can
   be used to associate it with the message to which it replies.  The
   same comments about integrity protection of KRB_ERROR messages
   mentioned in Section 3.1 apply to the TGS exchange.

3.3.1.  Generation of KRB_TGS_REQ Message

   Before sending a request to the ticket-granting service, the client
   MUST determine in which realm the application server is believed to
   be registered.  This can be accomplished in several ways.  It might
   be known beforehand (since the realm is part of the principal
   identifier), it might be stored in a nameserver, or it might be
   obtained from a configuration file.  If the realm to be used is
   obtained from a nameserver, there is a danger of being spoofed if the
   nameservice providing the realm name is not authenticated.  This
   might result in the use of a realm that has been compromised, which
   would result in an attacker's ability to compromise the
   authentication of the application server to the client.

   If the client knows the service principal name and realm and it does
   not already possess a TGT for the appropriate realm, then one must be
   obtained.  This is first attempted by requesting a TGT for the
   destination realm from a Kerberos server for which the client
   possesses a TGT (by using the KRB_TGS_REQ message recursively).  The

   Kerberos server MAY return a TGT for the desired realm, in which case
   one can proceed.  Alternatively, the Kerberos server MAY return a TGT
   for a realm that is 'closer' to the desired realm (further along the
   standard hierarchical path between the client's realm and the
   requested realm server's realm).  Note that in this case
   misconfiguration of the Kerberos servers may cause loops in the
   resulting authentication path, which the client should be careful to
   detect and avoid.

   If the Kerberos server returns a TGT for a realm 'closer' than the
   desired realm, the client MAY use local policy configuration to
   verify that the authentication path used is an acceptable one.
   Alternatively, a client MAY choose its own authentication path,
   rather than rely on the Kerberos server to select one.  In either
   case, any policy or configuration information used to choose or
   validate authentication paths, whether by the Kerberos server or by
   the client, MUST be obtained from a trusted source.

   When a client obtains a TGT that is 'closer' to the destination
   realm, the client MAY cache this ticket and reuse it in future
   KRB-TGS exchanges with services in the 'closer' realm.  However, if
   the client were to obtain a TGT for the 'closer' realm by starting at
   the initial KDC rather than as part of obtaining another ticket, then
   a shorter path to the 'closer' realm might be used.  This shorter
   path may be desirable because fewer intermediate KDCs would know the
   session key of the ticket involved.  For this reason, clients SHOULD
   evaluate whether they trust the realms transited in obtaining the
   'closer' ticket when making a decision to use the ticket in future.

   Once the client obtains a TGT for the appropriate realm, it
   determines which Kerberos servers serve that realm and contacts one
   of them.  The list might be obtained through a configuration file or
   network service, or it MAY be generated from the name of the realm.
   As long as the secret keys exchanged by realms are kept secret, only
   denial of service results from using a false Kerberos server.

   As in the AS exchange, the client MAY specify a number of options in
   the KRB_TGS_REQ message.  One of these options is the ENC-TKT-IN-SKEY
   option used for user-to-user authentication.  An overview of user-
   to-user authentication can be found in Section 3.7.  When generating
   the KRB_TGS_REQ message, this option indicates that the client is
   including a TGT obtained from the application server in the
   additional tickets field of the request and that the KDC SHOULD
   encrypt the ticket for the application server using the session key
   from this additional ticket, instead of a server key from the
   principal database.

   The client prepares the KRB_TGS_REQ message, providing an
   authentication header as an element of the padata field, and
   including the same fields as used in the KRB_AS_REQ message along
   with several optional fields: the enc-authorizatfion-data field for
   application server use and additional tickets required by some
   options.

   In preparing the authentication header, the client can select a sub-
   session key under which the response from the Kerberos server will be
   encrypted.  If the client selects a sub-session key, care must be
   taken to ensure the randomness of the selected sub-session key.

   If the sub-session key is not specified, the session key from the TGT
   will be used.  If the enc-authorization-data is present, it MUST be
   encrypted in the sub-session key, if present, from the authenticator
   portion of the authentication header, or, if not present, by using
   the session key from the TGT.

   Once prepared, the message is sent to a Kerberos server for the
   destination realm.

3.3.2.  Receipt of KRB_TGS_REQ Message

   The KRB_TGS_REQ message is processed in a manner similar to the
   KRB_AS_REQ message, but there are many additional checks to be
   performed.  First, the Kerberos server MUST determine which server
   the accompanying ticket is for, and it MUST select the appropriate
   key to decrypt it.  For a normal KRB_TGS_REQ message, it will be for
   the ticket-granting service, and the TGS's key will be used.  If the
   TGT was issued by another realm, then the appropriate inter-realm key
   MUST be used.  If (a) the accompanying ticket is not a TGT for the
   current realm, but is for an application server in the current realm,
   (b) the RENEW, VALIDATE, or PROXY options are specified in the
   request, and (c) the server for which a ticket is requested is the
   server named in the accompanying ticket, then the KDC will decrypt
   the ticket in the authentication header using the key of the server
   for which it was issued.  If no ticket can be found in the padata
   field, the KDC_ERR_PADATA_TYPE_NOSUPP error is returned.

   Once the accompanying ticket has been decrypted, the user-supplied
   checksum in the Authenticator MUST be verified against the contents
   of the request, and the message MUST be rejected if the checksums do
   not match (with an error code of KRB_AP_ERR_MODIFIED) or if the
   checksum is not collision-proof (with an error code of
   KRB_AP_ERR_INAPP_CKSUM).  If the checksum type is not supported, the
   KDC_ERR_SUMTYPE_NOSUPP error is returned.  If the authorization-data
   are present, they are decrypted using the sub-session key from the
   Authenticator.

   If any of the decryptions indicate failed integrity checks, the
   KRB_AP_ERR_BAD_INTEGRITY error is returned.

   As discussed in Section 3.1.2, the KDC MUST send a valid KRB_TGS_REP
   message if it receives a KRB_TGS_REQ message identical to one it has
   recently processed.  However, if the authenticator is a replay, but
   the rest of the request is not identical, then the KDC SHOULD return
   KRB_AP_ERR_REPEAT.

3.3.3.  Generation of KRB_TGS_REP Message

   The KRB_TGS_REP message shares its format with the KRB_AS_REP
   (KRB_KDC_REP), but with its type field set to KRB_TGS_REP.  The
   detailed specification is in Section 5.4.2.

   The response will include a ticket for the requested server or for a
   ticket granting server of an intermediate KDC to be contacted to
   obtain the requested ticket.  The Kerberos database is queried to
   retrieve the record for the appropriate server (including the key
   with which the ticket will be encrypted).  If the request is for a
   TGT for a remote realm, and if no key is shared with the requested
   realm, then the Kerberos server will select the realm 'closest' to
   the requested realm with which it does share a key and use that realm
   instead.  This is the only case where the response for the KDC will
   be for a different server than that requested by the client.

   By default, the address field, the client's name and realm, the list
   of transited realms, the time of initial authentication, the
   expiration time, and the authorization data of the newly-issued
   ticket will be copied from the TGT or renewable ticket.  If the
   transited field needs to be updated, but the transited type is not
   supported, the KDC_ERR_TRTYPE_NOSUPP error is returned.

   If the request specifies an endtime, then the endtime of the new
   ticket is set to the minimum of (a) that request, (b) the endtime
   from the TGT, and (c) the starttime of the TGT plus the minimum of
   the maximum life for the application server and the maximum life for
   the local realm (the maximum life for the requesting principal was
   already applied when the TGT was issued).  If the new ticket is to be
   a renewal, then the endtime above is replaced by the minimum of (a)
   the value of the renew_till field of the ticket and (b) the starttime
   for the new ticket plus the life (endtime-starttime) of the old
   ticket.

   If the FORWARDED option has been requested, then the resulting ticket
   will contain the addresses specified by the client.  This option will
   only be honored if the FORWARDABLE flag is set in the TGT.  The PROXY
   option is similar; the resulting ticket will contain the addresses

   specified by the client.  It will be honored only if the PROXIABLE
   flag in the TGT is set.  The PROXY option will not be honored on
   requests for additional TGTs.

   If the requested starttime is absent, indicates a time in the past,
   or is within the window of acceptable clock skew for the KDC and the
   POSTDATE option has not been specified, then the starttime of the
   ticket is set to the authentication server's current time.  If it
   indicates a time in the future beyond the acceptable clock skew, but
   the POSTDATED option has not been specified or the MAY-POSTDATE flag
   is not set in the TGT, then the error KDC_ERR_CANNOT_POSTDATE is
   returned.  Otherwise, if the TGT has the MAY-POSTDATE flag set, then
   the resulting ticket will be postdated, and the requested starttime
   is checked against the policy of the local realm.  If acceptable, the
   ticket's starttime is set as requested, and the INVALID flag is set.
   The postdated ticket MUST be validated before use by presenting it to
   the KDC after the starttime has been reached.  However, in no case
   may the starttime, endtime, or renew-till time of a newly-issued
   postdated ticket extend beyond the renew-till time of the TGT.

   If the ENC-TKT-IN-SKEY option has been specified and an additional
   ticket has been included in the request, it indicates that the client
   is using user-to-user authentication to prove its identity to a
   server that does not have access to a persistent key.  Section 3.7
   describes the effect of this option on the entire Kerberos protocol.
   When generating the KRB_TGS_REP message, this option in the
   KRB_TGS_REQ message tells the KDC to decrypt the additional ticket
   using the key for the server to which the additional ticket was
   issued and to verify that it is a TGT.  If the name of the requested
   server is missing from the request, the name of the client in the
   additional ticket will be used.  Otherwise, the name of the requested
   server will be compared to the name of the client in the additional
   ticket.  If it is different, the request will be rejected.  If the
   request succeeds, the session key from the additional ticket will be
   used to encrypt the new ticket that is issued instead of using the
   key of the server for which the new ticket will be used.

   If (a) the name of the server in the ticket that is presented to the
   KDC as part of the authentication header is not that of the TGS
   itself, (b) the server is registered in the realm of the KDC, and (c)
   the RENEW option is requested, then the KDC will verify that the
   RENEWABLE flag is set in the ticket, that the INVALID flag is not set
   in the ticket, and that the renew_till time is still in the future.
   If the VALIDATE option is requested, the KDC will check that the
   starttime has passed and that the INVALID flag is set.  If the PROXY
   option is requested, then the KDC will check that the PROXIABLE flag

   is set in the ticket.  If the tests succeed and the ticket passes the
   hotlist check described in the next section, the KDC will issue the
   appropriate new ticket.

   The ciphertext part of the response in the KRB_TGS_REP message is
   encrypted in the sub-session key from the Authenticator, if present,
   or in the session key from the TGT.  It is not encrypted using the
   client's secret key.  Furthermore, the client's key's expiration date
   and the key version number fields are left out since these values are
   stored along with the client's database record, and that record is
   not needed to satisfy a request based on a TGT.

3.3.3.1.  Checking for Revoked Tickets

   Whenever a request is made to the ticket-granting server, the
   presented ticket(s) is (are) checked against a hot-list of tickets
   that have been canceled.  This hot-list might be implemented by
   storing a range of issue timestamps for 'suspect tickets'; if a
   presented ticket had an authtime in that range, it would be rejected.
   In this way, a stolen TGT or renewable ticket cannot be used to gain
   additional tickets (renewals or otherwise) once the theft has been
   reported to the KDC for the realm in which the server resides.  Any
   normal ticket obtained before it was reported stolen will still be
   valid (because tickets require no interaction with the KDC), but only
   until its normal expiration time.  If TGTs have been issued for
   cross-realm authentication, use of the cross-realm TGT will not be
   affected unless the hot-list is propagated to the KDCs for the realms
   for which such cross-realm tickets were issued.

3.3.3.2.  Encoding the Transited Field

   If the identity of the server in the TGT that is presented to the KDC
   as part of the authentication header is that of the ticket-granting
   service, but the TGT was issued from another realm, the KDC will look
   up the inter-realm key shared with that realm and use that key to
   decrypt the ticket.  If the ticket is valid, then the KDC will honor
   the request, subject to the constraints outlined above in the section
   describing the AS exchange.  The realm part of the client's identity
   will be taken from the TGT.  The name of the realm that issued the
   TGT, if it is not the realm of the client principal, will be added to
   the transited field of the ticket to be issued.  This is accomplished
   by reading the transited field from the TGT (which is treated as an
   unordered set of realm names), adding the new realm to the set, and
   then constructing and writing out its encoded (shorthand) form (this
   may involve a rearrangement of the existing encoding).

   Note that the ticket-granting service does not add the name of its
   own realm.  Instead, its responsibility is to add the name of the

   previous realm.  This prevents a malicious Kerberos server from
   intentionally leaving out its own name (it could, however, omit other
   realms' names).

   The names of neither the local realm nor the principal's realm are to
   be included in the transited field.  They appear elsewhere in the
   ticket and both are known to have taken part in authenticating the
   principal.  Because the endpoints are not included, both local and
   single-hop inter-realm authentication result in a transited field
   that is empty.

   Because this field has the name of each transited realm added to it,
   it might potentially be very long.  To decrease the length of this
   field, its contents are encoded.  The initially supported encoding is
   optimized for the normal case of inter-realm communication: a
   hierarchical arrangement of realms using either domain or X.500 style
   realm names.  This encoding (called DOMAIN-X500-COMPRESS) is now
   described.

   Realm names in the transited field are separated by a ",".  The ",",
   "\", trailing "."s, and leading spaces (" ") are special characters,
   and if they are part of a realm name, they MUST be quoted in the
   transited field by preceding them with a "\".

   A realm name ending with a "." is interpreted as being prepended to
   the previous realm.  For example, we can encode traversal of EDU,
   MIT.EDU, ATHENA.MIT.EDU, WASHINGTON.EDU, and CS.WASHINGTON.EDU as:

      "EDU,MIT.,ATHENA.,WASHINGTON.EDU,CS.".

   Note that if either ATHENA.MIT.EDU, or CS.WASHINGTON.EDU were
   endpoints, they would not be included in this field, and we would
   have:

      "EDU,MIT.,WASHINGTON.EDU"

   A realm name beginning with a "/" is interpreted as being appended to
   the previous realm.  For the purpose of appending, the realm
   preceding the first listed realm is considered the null realm ("").
   If a realm name beginning with a "/" is to stand by itself, then it
   SHOULD be preceded by a space (" ").  For example, we can encode
   traversal of /COM/HP/APOLLO, /COM/HP, /COM, and /COM/DEC as:

      "/COM,/HP,/APOLLO, /COM/DEC".

   As in the example above, if /COM/HP/APOLLO and /COM/DEC were
   endpoints, they would not be included in this field, and we would
   have:

      "/COM,/HP"

   A null subfield preceding or following a "," indicates that all
   realms between the previous realm and the next realm have been
   traversed.  For the purpose of interpreting null subfields, the
   client's realm is considered to precede those in the transited field,
   and the server's realm is considered to follow them.  Thus, "," means
   that all realms along the path between the client and the server have
   been traversed.  ",EDU, /COM," means that all realms from the
   client's realm up to EDU (in a domain style hierarchy) have been
   traversed, and that everything from /COM down to the server's realm
   in an X.500 style has also been traversed.  This could occur if the
   EDU realm in one hierarchy shares an inter-realm key directly with
   the /COM realm in another hierarchy.

3.3.4.  Receipt of KRB_TGS_REP Message

   When the KRB_TGS_REP is received by the client, it is processed in
   the same manner as the KRB_AS_REP processing described above.  The
   primary difference is that the ciphertext part of the response must
   be decrypted using the sub-session key from the Authenticator, if it
   was specified in the request, or the session key from the TGT, rather
   than the client's secret key.  The server name returned in the reply
   is the true principal name of the service.

3.4.  The KRB_SAFE Exchange

   The KRB_SAFE message MAY be used by clients requiring the ability to
   detect modifications of messages they exchange.  It achieves this by
   including a keyed collision-proof checksum of the user data and some
   control information.  The checksum is keyed with an encryption key
   (usually the last key negotiated via subkeys, or the session key if
   no negotiation has occurred).

3.4.1.  Generation of a KRB_SAFE Message

   When an application wishes to send a KRB_SAFE message, it collects
   its data and the appropriate control information and computes a
   checksum over them.  The checksum algorithm should be the keyed
   checksum mandated to be implemented along with the crypto system used
   for the sub-session or session key.  The checksum is generated using
   the sub-session key, if present, or the session key.  Some
   implementations use a different checksum algorithm for the KRB_SAFE
   messages, but doing so in an interoperable manner is not always
   possible.

   The control information for the KRB_SAFE message includes both a
   timestamp and a sequence number.  The designer of an application

   using the KRB_SAFE message MUST choose at least one of the two
   mechanisms.  This choice SHOULD be based on the needs of the
   application protocol.

   Sequence numbers are useful when all messages sent will be received
   by one's peer.  Connection state is presently required to maintain
   the session key, so maintaining the next sequence number should not
   present an additional problem.

   If the application protocol is expected to tolerate lost messages
   without their being resent, the use of the timestamp is the
   appropriate replay detection mechanism.  Using timestamps is also the
   appropriate mechanism for multi-cast protocols in which all of one's
   peers share a common sub-session key, but some messages will be sent
   to a subset of one's peers.

   After computing the checksum, the client then transmits the
   information and checksum to the recipient in the message format
   specified in Section 5.6.1.

3.4.2.  Receipt of KRB_SAFE Message

   When an application receives a KRB_SAFE message, it verifies it as
   follows.  If any error occurs, an error code is reported for use by
   the application.

   The message is first checked by verifying that the protocol version
   and type fields match the current version and KRB_SAFE, respectively.
   A mismatch generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE
   error.  The application verifies that the checksum used is a
   collision-proof keyed checksum that uses keys compatible with the
   sub-session or session key as appropriate (or with the application
   key derived from the session or sub-session keys).  If it is not, a
   KRB_AP_ERR_INAPP_CKSUM error is generated.  The sender's address MUST
   be included in the control information; the recipient verifies that
   the operating system's report of the sender's address matches the
   sender's address in the message, and (if a recipient address is
   specified or the recipient requires an address) that one of the
   recipient's addresses appears as the recipient's address in the
   message.  To work with network address translation, senders MAY use
   the directional address type specified in Section 8.1 for the sender
   address and not include recipient addresses.  A failed match for
   either case generates a KRB_AP_ERR_BADADDR error.  Then the timestamp
   and usec and/or the sequence number fields are checked.  If timestamp
   and usec are expected and not present, or if they are present but not
   current, the KRB_AP_ERR_SKEW error is generated.  Timestamps are not
   required to be strictly ordered; they are only required to be in the
   skew window.  If the server name, along with the client name, time,

   and microsecond fields from the Authenticator match any recently-seen
   (sent or received) such tuples, the KRB_AP_ERR_REPEAT error is
   generated.  If an incorrect sequence number is included, or if a
   sequence number is expected but not present, the KRB_AP_ERR_BADORDER
   error is generated.  If neither a time-stamp and usec nor a sequence
   number is present, a KRB_AP_ERR_MODIFIED error is generated.
   Finally, the checksum is computed over the data and control
   information, and if it doesn't match the received checksum, a
   KRB_AP_ERR_MODIFIED error is generated.

   If all the checks succeed, the application is assured that the
   message was generated by its peer and was not modified in transit.

   Implementations SHOULD accept any checksum algorithm they implement
   that has both adequate security and keys compatible with the sub-
   session or session key.  Unkeyed or non-collision-proof checksums are
   not suitable for this use.

3.5.  The KRB_PRIV Exchange

   The KRB_PRIV message MAY be used by clients requiring confidentiality
   and the ability to detect modifications of exchanged messages.  It
   achieves this by encrypting the messages and adding control
   information.

3.5.1.  Generation of a KRB_PRIV Message

   When an application wishes to send a KRB_PRIV message, it collects
   its data and the appropriate control information (specified in
   Section 5.7.1) and encrypts them under an encryption key (usually the
   last key negotiated via subkeys, or the session key if no negotiation
   has occurred).  As part of the control information, the client MUST
   choose to use either a timestamp or a sequence number (or both); see
   the discussion in Section 3.4.1 for guidelines on which to use.
   After the user data and control information are encrypted, the client
   transmits the ciphertext and some 'envelope' information to the
   recipient.

3.5.2.  Receipt of KRB_PRIV Message

   When an application receives a KRB_PRIV message, it verifies it as
   follows.  If any error occurs, an error code is reported for use by
   the application.

   The message is first checked by verifying that the protocol version
   and type fields match the current version and KRB_PRIV, respectively.
   A mismatch generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE
   error.  The application then decrypts the ciphertext and processes

   the resultant plaintext.  If decryption shows that the data has been
   modified, a KRB_AP_ERR_BAD_INTEGRITY error is generated.

   The sender's address MUST be included in the control information; the
   recipient verifies that the operating system's report of the sender's
   address matches the sender's address in the message.  If a recipient
   address is specified or the recipient requires an address, then one
   of the recipient's addresses MUST also appear as the recipient's
   address in the message.  Where a sender's or receiver's address might
   not otherwise match the address in a message because of network
   address translation, an application MAY be written to use addresses
   of the directional address type in place of the actual network
   address.

   A failed match for either case generates a KRB_AP_ERR_BADADDR error.
   To work with network address translation, implementations MAY use the
   directional address type defined in Section 7.1 for the sender
   address and include no recipient address.

   Next the timestamp and usec and/or the sequence number fields are
   checked.  If timestamp and usec are expected and not present, or if
   they are present but not current, the KRB_AP_ERR_SKEW error is
   generated.  If the server name, along with the client name, time, and
   microsecond fields from the Authenticator match any such recently-
   seen tuples, the KRB_AP_ERR_REPEAT error is generated.  If an
   incorrect sequence number is included, or if a sequence number is
   expected but not present, the KRB_AP_ERR_BADORDER error is generated.
   If neither a time-stamp and usec nor a sequence number is present, a
   KRB_AP_ERR_MODIFIED error is generated.

   If all the checks succeed, the application can assume the message was
   generated by its peer and was securely transmitted (without intruders
   seeing the unencrypted contents).

3.6.  The KRB_CRED Exchange

   The KRB_CRED message MAY be used by clients requiring the ability to
   send Kerberos credentials from one host to another.  It achieves this
   by sending the tickets together with encrypted data containing the
   session keys and other information associated with the tickets.

3.6.1.  Generation of a KRB_CRED Message

   When an application wishes to send a KRB_CRED message, it first
   (using the KRB_TGS exchange) obtains credentials to be sent to the
   remote host.  It then constructs a KRB_CRED message using the ticket
   or tickets so obtained, placing the session key needed to use each

   ticket in the key field of the corresponding KrbCredInfo sequence of
   the encrypted part of the KRB_CRED message.

   Other information associated with each ticket and obtained during the
   KRB_TGS exchange is also placed in the corresponding KrbCredInfo
   sequence in the encrypted part of the KRB_CRED message.  The current
   time and, if they are specifically required by the application, the
   nonce, s-address, and r-address fields are placed in the encrypted
   part of the KRB_CRED message, which is then encrypted under an
   encryption key previously exchanged in the KRB_AP exchange (usually
   the last key negotiated via subkeys, or the session key if no
   negotiation has occurred).

   Implementation note: When constructing a KRB_CRED message for
   inclusion in a GSSAPI initial context token, the MIT implementation
   of Kerberos will not encrypt the KRB_CRED message if the session key
   is a DES or triple DES key.  For interoperability with MIT, the
   Microsoft implementation will not encrypt the KRB_CRED in a GSSAPI
   token if it is using a DES session key.  Starting at version 1.2.5,
   MIT Kerberos can receive and decode either encrypted or unencrypted
   KRB_CRED tokens in the GSSAPI exchange.  The Heimdal implementation
   of Kerberos can also accept either encrypted or unencrypted KRB_CRED
   messages.  Since the KRB_CRED message in a GSSAPI token is encrypted
   in the authenticator, the MIT behavior does not present a security
   problem, although it is a violation of the Kerberos specification.

3.6.2.  Receipt of KRB_CRED Message

   When an application receives a KRB_CRED message, it verifies it.  If
   any error occurs, an error code is reported for use by the
   application.  The message is verified by checking that the protocol
   version and type fields match the current version and KRB_CRED,
   respectively.  A mismatch generates a KRB_AP_ERR_BADVERSION or
   KRB_AP_ERR_MSG_TYPE error.  The application then decrypts the
   ciphertext and processes the resultant plaintext.  If decryption
   shows the data to have been modified, a KRB_AP_ERR_BAD_INTEGRITY
   error is generated.

   If present or required, the recipient MAY verify that the operating
   system's report of the sender's address matches the sender's address
   in the message, and that one of the recipient's addresses appears as
   the recipient's address in the message.  The address check does not
   provide any added security, since the address, if present, has
   already been checked in the KRB_AP_REQ message and there is not any
   benefit to be gained by an attacker in reflecting a KRB_CRED message
   back to its originator.  Thus, the recipient MAY ignore the address
   even if it is present in order to work better in Network Address
   Translation (NAT) environments.  A failed match for either case

   generates a KRB_AP_ERR_BADADDR error.  Recipients MAY skip the
   address check, as the KRB_CRED message cannot generally be reflected
   back to the originator.  The timestamp and usec fields (and the nonce
   field, if required) are checked next.  If the timestamp and usec are
   not present, or if they are present but not current, the
   KRB_AP_ERR_SKEW error is generated.

   If all the checks succeed, the application stores each of the new
   tickets in its credentials cache together with the session key and
   other information in the corresponding KrbCredInfo sequence from the
   encrypted part of the KRB_CRED message.

3.7.  User-to-User Authentication Exchanges

   User-to-User authentication provides a method to perform
   authentication when the verifier does not have a access to long-term
   service key.  This might be the case when running a server (for
   example, a window server) as a user on a workstation.  In such cases,
   the server may have access to the TGT obtained when the user logged
   in to the workstation, but because the server is running as an
   unprivileged user, it might not have access to system keys.  Similar
   situations may arise when running peer-to-peer applications.

                             Summary

       Message direction                    Message type     Sections
       0. Message from application server   Not specified
       1. Client to Kerberos                KRB_TGS_REQ      3.3 & 5.4.1
       2. Kerberos to client                KRB_TGS_REP or   3.3 & 5.4.2
                                            KRB_ERROR        5.9.1
       3. Client to application server      KRB_AP_REQ       3.2 & 5.5.1

   To address this problem, the Kerberos protocol allows the client to
   request that the ticket issued by the KDC be encrypted using a
   session key from a TGT issued to the party that will verify the
   authentication.  This TGT must be obtained from the verifier by means
   of an exchange external to the Kerberos protocol, usually as part of
   the application protocol.  This message is shown in the summary above
   as message 0.  Note that because the TGT is encrypted in the KDC's
   secret key, it cannot be used for authentication without possession
   of the corresponding secret key.  Furthermore, because the verifier
   does not reveal the corresponding secret key, providing a copy of the
   verifier's TGT does not allow impersonation of the verifier.

   Message 0 in the table above represents an application-specific
   negotiation between the client and server, at the end of which both
   have determined that they will use user-to-user authentication, and
   the client has obtained the server's TGT.

   Next, the client includes the server's TGT as an additional ticket in
   its KRB_TGS_REQ request to the KDC (message 1 in the table above) and
   specifies the ENC-TKT-IN-SKEY option in its request.

   If validated according to the instructions in Section 3.3.3, the
   application ticket returned to the client (message 2 in the table
   above) will be encrypted using the session key from the additional
   ticket and the client will note this when it uses or stores the
   application ticket.

   When contacting the server using a ticket obtained for user-to-user
   authentication (message 3 in the table above), the client MUST
   specify the USE-SESSION-KEY flag in the ap-options field.  This tells
   the application server to use the session key associated with its TGT
   to decrypt the server ticket provided in the application request.

4.  Encryption and Checksum Specifications

   The Kerberos protocols described in this document are designed to
   encrypt messages of arbitrary sizes, using stream or block encryption
   ciphers.  Encryption is used to prove the identities of the network
   entities participating in message exchanges.  The Key Distribution
   Center for each realm is trusted by all principals registered in that
   realm to store a secret key in confidence.  Proof of knowledge of
   this secret key is used to verify the authenticity of a principal.

   The KDC uses the principal's secret key (in the AS exchange) or a
   shared session key (in the TGS exchange) to encrypt responses to
   ticket requests; the ability to obtain the secret key or session key
   implies the knowledge of the appropriate keys and the identity of the
   KDC.  The ability of a principal to decrypt the KDC response and to
   present a Ticket and a properly formed Authenticator (generated with
   the session key from the KDC response) to a service verifies the
   identity of the principal; likewise the ability of the service to
   extract the session key from the Ticket and to prove its knowledge
   thereof in a response verifies the identity of the service.

   [RFC3961] defines a framework for defining encryption and checksum
   mechanisms for use with Kerberos.  It also defines several such
   mechanisms, and more may be added in future updates to that document.

   The string-to-key operation provided by [RFC3961] is used to produce
   a long-term key for a principal (generally for a user).  The default
   salt string, if none is provided via pre-authentication data, is the
   concatenation of the principal's realm and name components, in order,
   with no separators.  Unless it is indicated otherwise, the default
   string-to-key opaque parameter set as defined in [RFC3961] is used.

   Encrypted data, keys, and checksums are transmitted using the
   EncryptedData, EncryptionKey, and Checksum data objects defined in
   Section 5.2.9.  The encryption, decryption, and checksum operations
   described in this document use the corresponding encryption,
   decryption, and get_mic operations described in [RFC3961], with
   implicit "specific key" generation using the "key usage" values
   specified in the description of each EncryptedData or Checksum object
   to vary the key for each operation.  Note that in some cases, the
   value to be used is dependent on the method of choosing the key or
   the context of the message.

   Key usages are unsigned 32-bit integers; zero is not permitted.  The
   key usage values for encrypting or checksumming Kerberos messages are
   indicated in Section 5 along with the message definitions.  The key
   usage values 512-1023 are reserved for uses internal to a Kerberos
   implementation.  (For example, seeding a pseudo-random number
   generator with a value produced by encrypting something with a
   session key and a key usage value not used for any other purpose.)
   Key usage values between 1024 and 2047 (inclusive) are reserved for
   application use; applications SHOULD use even values for encryption
   and odd values for checksums within this range.  Key usage values are
   also summarized in a table in Section 7.5.1.

   There might exist other documents that define protocols in terms of
   the RFC 1510 encryption types or checksum types.  These documents
   would not know about key usages.  In order that these specifications
   continue to be meaningful until they are updated, if no key usage
   values are specified, then key usages 1024 and 1025 must be used to
   derive keys for encryption and checksums, respectively.  (This does
   not apply to protocols that do their own encryption independent of
   this framework, by directly using the key resulting from the Kerberos
   authentication exchange.)  New protocols defined in terms of the
   Kerberos encryption and checksum types SHOULD use their own key usage
   values.

   Unless it is indicated otherwise, no cipher state chaining is done
   from one encryption operation to another.

   Implementation note: Although it is not recommended, some application
   protocols will continue to use the key data directly, even if only in
   currently existing protocol specifications.  An implementation
   intended to support general Kerberos applications may therefore need
   to make key data available, as well as the attributes and operations
   described in [RFC3961].  One of the more common reasons for directly
   performing encryption is direct control over negotiation and
   selection of a "sufficiently strong" encryption algorithm (in the
   context of a given application).  Although Kerberos does not directly
   provide a facility for negotiating encryption types between the

   application client and server, there are approaches for using
   Kerberos to facilitate this negotiation.  For example, a client may
   request only "sufficiently strong" session key types from the KDC and
   expect that any type returned by the KDC will be understood and
   supported by the application server.

5.  Message Specifications

   The ASN.1 collected here should be identical to the contents of
   Appendix A.  In the case of a conflict, the contents of Appendix A
   shall take precedence.

   The Kerberos protocol is defined here in terms of Abstract Syntax
   Notation One (ASN.1) [X680], which provides a syntax for specifying
   both the abstract layout of protocol messages as well as their
   encodings.  Implementors not utilizing an existing ASN.1 compiler or
   support library are cautioned to understand the actual ASN.1
   specification thoroughly in order to ensure correct implementation
   behavior.  There is more complexity in the notation than is
   immediately obvious, and some tutorials and guides to ASN.1 are
   misleading or erroneous.

   Note that in several places, changes to abstract types from RFC 1510
   have been made.  This is in part to address widespread assumptions
   that various implementors have made, in some cases resulting in
   unintentional violations of the ASN.1 standard.  These are clearly
   flagged where they occur.  The differences between the abstract types
   in RFC 1510 and abstract types in this document can cause
   incompatible encodings to be emitted when certain encoding rules,
   e.g., the Packed Encoding Rules (PER), are used.  This theoretical
   incompatibility should not be relevant for Kerberos, since Kerberos
   explicitly specifies the use of the Distinguished Encoding Rules
   (DER).  It might be an issue for protocols seeking to use Kerberos
   types with other encoding rules.  (This practice is not recommended.)
   With very few exceptions (most notably the usages of BIT STRING), the
   encodings resulting from using the DER remain identical between the
   types defined in RFC 1510 and the types defined in this document.

   The type definitions in this section assume an ASN.1 module
   definition of the following form:

   KerberosV5Spec2 {
           iso(1) identified-organization(3) dod(6) internet(1)
           security(5) kerberosV5(2) modules(4) krb5spec2(2)
   } DEFINITIONS EXPLICIT TAGS ::= BEGIN

   -- rest of definitions here

   END

   This specifies that the tagging context for the module will be
   explicit and non-automatic.

   Note that in some other publications (such as [RFC1510] and
   [RFC1964]), the "dod" portion of the object identifier is erroneously
   specified as having the value "5".  In the case of RFC 1964, use of
   the "correct" OID value would result in a change in the wire
   protocol; therefore, it remains unchanged for now.

   Note that elsewhere in this document, nomenclature for various
   message types is inconsistent, but it largely follows C language
   conventions, including use of underscore (_) characters and all-caps
   spelling of names intended to be numeric constants.  Also, in some
   places, identifiers (especially those referring to constants) are
   written in all-caps in order to distinguish them from surrounding
   explanatory text.

   The ASN.1 notation does not permit underscores in identifiers, so in
   actual ASN.1 definitions, underscores are replaced with hyphens (-).
   Additionally, structure member names and defined values in ASN.1 MUST
   begin with a lowercase letter, whereas type names MUST begin with an
   uppercase letter.

5.1.  Specific Compatibility Notes on ASN.1

   For compatibility purposes, implementors should heed the following
   specific notes regarding the use of ASN.1 in Kerberos.  These notes
   do not describe deviations from standard usage of ASN.1.  The purpose
   of these notes is instead to describe some historical quirks and
   non-compliance of various implementations, as well as historical
   ambiguities, which, although they are valid ASN.1, can lead to
   confusion during implementation.

5.1.1.  ASN.1 Distinguished Encoding Rules

   The encoding of Kerberos protocol messages shall obey the
   Distinguished Encoding Rules (DER) of ASN.1 as described in [X690].
   Some implementations (believed primarily to be those derived from DCE
   1.1 and earlier) are known to use the more general Basic Encoding

   Rules (BER); in particular, these implementations send indefinite
   encodings of lengths.  Implementations MAY accept such encodings in
   the interest of backward compatibility, though implementors are
   warned that decoding fully-general BER is fraught with peril.

5.1.2.  Optional Integer Fields

   Some implementations do not internally distinguish between an omitted
   optional integer value and a transmitted value of zero.  The places
   in the protocol where this is relevant include various microseconds
   fields, nonces, and sequence numbers.  Implementations SHOULD treat
   omitted optional integer values as having been transmitted with a
   value of zero, if the application is expecting this.

5.1.3.  Empty SEQUENCE OF Types

   There are places in the protocol where a message contains a SEQUENCE
   OF type as an optional member.  This can result in an encoding that
   contains an empty SEQUENCE OF encoding.  The Kerberos protocol does
   not semantically distinguish between an absent optional SEQUENCE OF
   type and a present optional but empty SEQUENCE OF type.
   Implementations SHOULD NOT send empty SEQUENCE OF encodings that are
   marked OPTIONAL, but SHOULD accept them as being equivalent to an
   omitted OPTIONAL type.  In the ASN.1 syntax describing Kerberos
   messages, instances of these problematic optional SEQUENCE OF types
   are indicated with a comment.

5.1.4.  Unrecognized Tag Numbers

   Future revisions to this protocol may include new message types with
   different APPLICATION class tag numbers.  Such revisions should
   protect older implementations by only sending the message types to
   parties that are known to understand them; e.g., by means of a flag
   bit set by the receiver in a preceding request.  In the interest of
   robust error handling, implementations SHOULD gracefully handle
   receiving a message with an unrecognized tag anyway, and return an
   error message, if appropriate.

   In particular, KDCs SHOULD return KRB_AP_ERR_MSG_TYPE if the
   incorrect tag is sent over a TCP transport.  The KDCs SHOULD NOT
   respond to messages received with an unknown tag over UDP transport
   in order to avoid denial of service attacks.  For non-KDC
   applications, the Kerberos implementation typically indicates an
   error to the application which takes appropriate steps based on the
   application protocol.

5.1.5.  Tag Numbers Greater Than 30

   A naive implementation of a DER ASN.1 decoder may experience problems
   with ASN.1 tag numbers greater than 30, due to such tag numbers being
   encoded using more than one byte.  Future revisions of this protocol
   may utilize tag numbers greater than 30, and implementations SHOULD
   be prepared to gracefully return an error, if appropriate, when they
   do not recognize the tag.

5.2.  Basic Kerberos Types

   This section defines a number of basic types that are potentially
   used in multiple Kerberos protocol messages.

5.2.1.  KerberosString

   The original specification of the Kerberos protocol in RFC 1510 uses
   GeneralString in numerous places for human-readable string data.
   Historical implementations of Kerberos cannot utilize the full power
   of GeneralString.  This ASN.1 type requires the use of designation
   and invocation escape sequences as specified in ISO-2022/ECMA-35
   [ISO-2022/ECMA-35] to switch character sets, and the default
   character set that is designated as G0 is the ISO-646/ECMA-6
   [ISO-646/ECMA-6] International Reference Version (IRV) (a.k.a. U.S.
   ASCII), which mostly works.

   ISO-2022/ECMA-35 defines four character-set code elements (G0..G3)
   and two Control-function code elements (C0..C1).  DER prohibits the
   designation of character sets as any but the G0 and C0 sets.
   Unfortunately, this seems to have the side effect of prohibiting the
   use of ISO-8859 (ISO Latin) [ISO-8859] character sets or any other
   character sets that utilize a 96-character set, as ISO-2022/ECMA-35
   prohibits designating them as the G0 code element.  This side effect
   is being investigated in the ASN.1 standards community.

   In practice, many implementations treat GeneralStrings as if they
   were 8-bit strings of whichever character set the implementation
   defaults to, without regard to correct usage of character-set
   designation escape sequences.  The default character set is often
   determined by the current user's operating system-dependent locale.
   At least one major implementation places unescaped UTF-8 encoded
   Unicode characters in the GeneralString.  This failure to adhere to
   the GeneralString specifications results in interoperability issues
   when conflicting character encodings are utilized by the Kerberos
   clients, services, and KDC.

   This unfortunate situation is the result of improper documentation of
   the restrictions of the ASN.1 GeneralString type in prior Kerberos
   specifications.

   The new (post-RFC 1510) type KerberosString, defined below, is a
   GeneralString that is constrained to contain only characters in
   IA5String.

      KerberosString  ::= GeneralString (IA5String)

   In general, US-ASCII control characters should not be used in
   KerberosString.  Control characters SHOULD NOT be used in principal
   names or realm names.

   For compatibility, implementations MAY choose to accept GeneralString
   values that contain characters other than those permitted by
   IA5String, but they should be aware that character set designation
   codes will likely be absent, and that the encoding should probably be
   treated as locale-specific in almost every way.  Implementations MAY
   also choose to emit GeneralString values that are beyond those
   permitted by IA5String, but they should be aware that doing so is
   extraordinarily risky from an interoperability perspective.

   Some existing implementations use GeneralString to encode unescaped
   locale-specific characters.  This is a violation of the ASN.1
   standard.  Most of these implementations encode US-ASCII in the
   left-hand half, so as long as the implementation transmits only
   US-ASCII, the ASN.1 standard is not violated in this regard.  As soon
   as such an implementation encodes unescaped locale-specific
   characters with the high bit set, it violates the ASN.1 standard.

   Other implementations have been known to use GeneralString to contain
   a UTF-8 encoding.  This also violates the ASN.1 standard, since UTF-8
   is a different encoding, not a 94 or 96 character "G" set as defined
   by ISO 2022.  It is believed that these implementations do not even
   use the ISO 2022 escape sequence to change the character encoding.
   Even if implementations were to announce the encoding change by using
   that escape sequence, the ASN.1 standard prohibits the use of any
   escape sequences other than those used to designate/invoke "G" or "C"
   sets allowed by GeneralString.

   Future revisions to this protocol will almost certainly allow for a
   more interoperable representation of principal names, probably
   including UTF8String.

   Note that applying a new constraint to a previously unconstrained
   type constitutes creation of a new ASN.1 type.  In this particular
   case, the change does not result in a changed encoding under DER.

5.2.2.  Realm and PrincipalName

   Realm           ::= KerberosString

   PrincipalName   ::= SEQUENCE {
           name-type       [0] Int32,
           name-string     [1] SEQUENCE OF KerberosString
   }

   Kerberos realm names are encoded as KerberosStrings.  Realms shall
   not contain a character with the code 0 (the US-ASCII NUL).  Most
   realms will usually consist of several components separated by
   periods (.), in the style of Internet Domain Names, or separated by
   slashes (/), in the style of X.500 names.  Acceptable forms for realm
   names are specified in Section 6.1.  A PrincipalName is a typed
   sequence of components consisting of the following subfields:

   name-type
      This field specifies the type of name that follows.  Pre-defined
      values for this field are specified in Section 6.2.  The name-type
      SHOULD be treated as a hint.  Ignoring the name type, no two names
      can be the same (i.e., at least one of the components, or the
      realm, must be different).

   name-string
      This field encodes a sequence of components that form a name, each
      component encoded as a KerberosString.  Taken together, a
      PrincipalName and a Realm form a principal identifier.  Most
      PrincipalNames will have only a few components (typically one or
      two).

5.2.3.  KerberosTime

   KerberosTime    ::= GeneralizedTime -- with no fractional seconds

   The timestamps used in Kerberos are encoded as GeneralizedTimes.  A
   KerberosTime value shall not include any fractional portions of the
   seconds.  As required by the DER, it further shall not include any
   separators, and it shall specify the UTC time zone (Z).  Example: The
   only valid format for UTC time 6 minutes, 27 seconds after 9 pm on 6
   November 1985 is 19851106210627Z.

5.2.4.  Constrained Integer Types

   Some integer members of types SHOULD be constrained to values
   representable in 32 bits, for compatibility with reasonable
   implementation limits.

   Int32           ::= INTEGER (-2147483648..2147483647)
                       -- signed values representable in 32 bits

   UInt32          ::= INTEGER (0..4294967295)
                       -- unsigned 32 bit values

   Microseconds    ::= INTEGER (0..999999)
                       -- microseconds

   Although this results in changes to the abstract types from the RFC
   1510 version, the encoding in DER should be unaltered.  Historical
   implementations were typically limited to 32-bit integer values
   anyway, and assigned numbers SHOULD fall in the space of integer
   values representable in 32 bits in order to promote interoperability
   anyway.

   Several integer fields in messages are constrained to fixed values.

   pvno
      also TKT-VNO or AUTHENTICATOR-VNO, this recurring field is always
      the constant integer 5.  There is no easy way to make this field
      into a useful protocol version number, so its value is fixed.

   msg-type
      this integer field is usually identical to the application tag
      number of the containing message type.

5.2.5.  HostAddress and HostAddresses

   HostAddress     ::= SEQUENCE  {
           addr-type       [0] Int32,
           address         [1] OCTET STRING
   }

   -- NOTE: HostAddresses is always used as an OPTIONAL field and
   -- should not be empty.
   HostAddresses   -- NOTE: subtly different from rfc1510,
                   -- but has a value mapping and encodes the same
           ::= SEQUENCE OF HostAddress

   The host address encodings consist of two fields:

   addr-type
      This field specifies the type of address that follows.  Pre-
      defined values for this field are specified in Section 7.5.3.

   address
      This field encodes a single address of type addr-type.

5.2.6.  AuthorizationData

      -- NOTE: AuthorizationData is always used as an OPTIONAL field and
      -- should not be empty.
      AuthorizationData       ::= SEQUENCE OF SEQUENCE {
              ad-type         [0] Int32,
              ad-data         [1] OCTET STRING
      }

   ad-data
      This field contains authorization data to be interpreted according
      to the value of the corresponding ad-type field.

   ad-type
      This field specifies the format for the ad-data subfield.  All
      negative values are reserved for local use.  Non-negative values
      are reserved for registered use.

   Each sequence of type and data is referred to as an authorization
   element.  Elements MAY be application specific; however, there is a
   common set of recursive elements that should be understood by all
   implementations.  These elements contain other elements embedded
   within them, and the interpretation of the encapsulating element
   determines which of the embedded elements must be interpreted, and
   which may be ignored.

   These common authorization data elements are recursively defined,
   meaning that the ad-data for these types will itself contain a
   sequence of authorization data whose interpretation is affected by
   the encapsulating element.  Depending on the meaning of the
   encapsulating element, the encapsulated elements may be ignored,
   might be interpreted as issued directly by the KDC, or might be
   stored in a separate plaintext part of the ticket.  The types of the
   encapsulating elements are specified as part of the Kerberos
   specification because the behavior based on these values should be
   understood across implementations, whereas other elements need only
   be understood by the applications that they affect.

   Authorization data elements are considered critical if present in a
   ticket or authenticator.  If an unknown authorization data element
   type is received by a server either in an AP-REQ or in a ticket
   contained in an AP-REQ, then, unless it is encapsulated in a known
   authorization data element amending the criticality of the elements
   it contains, authentication MUST fail.  Authorization data is
   intended to restrict the use of a ticket.  If the service cannot
   determine whether the restriction applies to that service, then a

   security weakness may result if the ticket can be used for that
   service.  Authorization elements that are optional can be enclosed in
   an AD-IF-RELEVANT element.

   In the definitions that follow, the value of the ad-type for the
   element will be specified as the least significant part of the
   subsection number, and the value of the ad-data will be as shown in
   the ASN.1 structure that follows the subsection heading.

   Contents of ad-data                ad-type

   DER encoding of AD-IF-RELEVANT        1

   DER encoding of AD-KDCIssued          4

   DER encoding of AD-AND-OR             5

   DER encoding of AD-MANDATORY-FOR-KDC  8

5.2.6.1.  IF-RELEVANT

   AD-IF-RELEVANT          ::= AuthorizationData

   AD elements encapsulated within the if-relevant element are intended
   for interpretation only by application servers that understand the
   particular ad-type of the embedded element.  Application servers that
   do not understand the type of an element embedded within the
   if-relevant element MAY ignore the uninterpretable element.  This
   element promotes interoperability across implementations that may
   have local extensions for authorization.  The ad-type for
   AD-IF-RELEVANT is (1).

5.2.6.2.  KDCIssued

   AD-KDCIssued            ::= SEQUENCE {
           ad-checksum     [0] Checksum,
           i-realm         [1] Realm OPTIONAL,
           i-sname         [2] PrincipalName OPTIONAL,
           elements        [3] AuthorizationData
   }

   ad-checksum
      A cryptographic checksum computed over the DER encoding of the
      AuthorizationData in the "elements" field, keyed with the session
      key.  Its checksumtype is the mandatory checksum type for the
      encryption type of the session key, and its key usage value is 19.

   i-realm, i-sname
      The name of the issuing principal if different from that of the
      KDC itself.  This field would be used when the KDC can verify the
      authenticity of elements signed by the issuing principal, and it
      allows this KDC to notify the application server of the validity
      of those elements.

   elements
      A sequence of authorization data elements issued by the KDC.

   The KDC-issued ad-data field is intended to provide a means for
   Kerberos principal credentials to embed within themselves privilege
   attributes and other mechanisms for positive authorization,
   amplifying the privileges of the principal beyond what can be done
   using credentials without such an a-data element.

   The above means cannot be provided without this element because the
   definition of the authorization-data field allows elements to be
   added at will by the bearer of a TGT at the time when they request
   service tickets, and elements may also be added to a delegated ticket
   by inclusion in the authenticator.

   For KDC-issued elements, this is prevented because the elements are
   signed by the KDC by including a checksum encrypted using the
   server's key (the same key used to encrypt the ticket or a key
   derived from that key).  Elements encapsulated with in the KDC-issued
   element MUST be ignored by the application server if this "signature"
   is not present.  Further, elements encapsulated within this element
   from a TGT MAY be interpreted by the KDC, and used as a basis
   according to policy for including new signed elements within
   derivative tickets, but they will not be copied to a derivative
   ticket directly.  If they are copied directly to a derivative ticket
   by a KDC that is not aware of this element, the signature will not be
   correct for the application ticket elements, and the field will be
   ignored by the application server.

   This element and the elements it encapsulates MAY safely be ignored
   by applications, application servers, and KDCs that do not implement
   this element.

   The ad-type for AD-KDC-ISSUED is (4).

5.2.6.3.  AND-OR

   AD-AND-OR               ::= SEQUENCE {
           condition-count [0] Int32,
           elements        [1] AuthorizationData
   }

   When restrictive AD elements are encapsulated within the and-or
   element, the and-or element is considered satisfied if and only if at
   least the number of encapsulated elements specified in condition-
   count are satisfied.  Therefore, this element MAY be used to
   implement an "or" operation by setting the condition-count field to
   1, and it MAY specify an "and" operation by setting the condition
   count to the number of embedded elements.  Application servers that
   do not implement this element MUST reject tickets that contain
   authorization data elements of this type.

   The ad-type for AD-AND-OR is (5).

5.2.6.4.  MANDATORY-FOR-KDC

   AD-MANDATORY-FOR-KDC    ::= AuthorizationData

   AD elements encapsulated within the mandatory-for-kdc element are to
   be interpreted by the KDC.  KDCs that do not understand the type of
   an element embedded within the mandatory-for-kdc element MUST reject
   the request.

   The ad-type for AD-MANDATORY-FOR-KDC is (8).

5.2.7.  PA-DATA

   Historically, PA-DATA have been known as "pre-authentication data",
   meaning that they were used to augment the initial authentication
   with the KDC.  Since that time, they have also been used as a typed
   hole with which to extend protocol exchanges with the KDC.

   PA-DATA         ::= SEQUENCE {
           -- NOTE: first tag is [1], not [0]
           padata-type     [1] Int32,
           padata-value    [2] OCTET STRING -- might be encoded AP-REQ
   }

   padata-type
      Indicates the way that the padata-value element is to be
      interpreted.  Negative values of padata-type are reserved for
      unregistered use; non-negative values are used for a registered
      interpretation of the element type.

   padata-value
      Usually contains the DER encoding of another type; the padata-type
      field identifies which type is encoded here.

      padata-type  Name             Contents of padata-value

      1            pa-tgs-req       DER encoding of AP-REQ

      2            pa-enc-timestamp DER encoding of PA-ENC-TIMESTAMP

      3            pa-pw-salt       salt (not ASN.1 encoded)

      11           pa-etype-info    DER encoding of ETYPE-INFO

      19           pa-etype-info2   DER encoding of ETYPE-INFO2

      This field MAY also contain information needed by certain
      extensions to the Kerberos protocol.  For example, it might be
      used to verify the identity of a client initially before any
      response is returned.

      The padata field can also contain information needed to help the
      KDC or the client select the key needed for generating or
      decrypting the response.  This form of the padata is useful for
      supporting the use of certain token cards with Kerberos.  The
      details of such extensions are specified in separate documents.
      See [Pat92] for additional uses of this field.

5.2.7.1.  PA-TGS-REQ

   In the case of requests for additional tickets (KRB_TGS_REQ),
   padata-value will contain an encoded AP-REQ.  The checksum in the
   authenticator (which MUST be collision-proof) is to be computed over
   the KDC-REQ-BODY encoding.

5.2.7.2.  Encrypted Timestamp Pre-authentication

   There are pre-authentication types that may be used to pre-
   authenticate a client by means of an encrypted timestamp.

   PA-ENC-TIMESTAMP        ::= EncryptedData -- PA-ENC-TS-ENC

   PA-ENC-TS-ENC           ::= SEQUENCE {
           patimestamp     [0] KerberosTime -- client's time --,
           pausec          [1] Microseconds OPTIONAL
   }

   Patimestamp contains the client's time, and pausec contains the
   microseconds, which MAY be omitted if a client will not generate more
   than one request per second.  The ciphertext (padata-value) consists
   of the PA-ENC-TS-ENC encoding, encrypted using the client's secret
   key and a key usage value of 1.

   This pre-authentication type was not present in RFC 1510, but many
   implementations support it.

5.2.7.3.  PA-PW-SALT

   The padata-value for this pre-authentication type contains the salt
   for the string-to-key to be used by the client to obtain the key for
   decrypting the encrypted part of an AS-REP message.  Unfortunately,
   for historical reasons, the character set to be used is unspecified
   and probably locale-specific.

   This pre-authentication type was not present in RFC 1510, but many
   implementations support it.  It is necessary in any case where the
   salt for the string-to-key algorithm is not the default.

   In the trivial example, a zero-length salt string is very commonplace
   for realms that have converted their principal databases from
   Kerberos Version 4.

   A KDC SHOULD NOT send PA-PW-SALT when issuing a KRB-ERROR message
   that requests additional pre-authentication.  Implementation note:
   Some KDC implementations issue an erroneous PA-PW-SALT when issuing a
   KRB-ERROR message that requests additional pre-authentication.
   Therefore, clients SHOULD ignore a PA-PW-SALT accompanying a
   KRB-ERROR message that requests additional pre-authentication.  As
   noted in section 3.1.3, a KDC MUST NOT send PA-PW-SALT when the
   client's AS-REQ includes at least one "newer" etype.

5.2.7.4.  PA-ETYPE-INFO

   The ETYPE-INFO pre-authentication type is sent by the KDC in a
   KRB-ERROR indicating a requirement for additional pre-authentication.
   It is usually used to notify a client of which key to use for the
   encryption of an encrypted timestamp for the purposes of sending a
   PA-ENC-TIMESTAMP pre-authentication value.  It MAY also be sent in an
   AS-REP to provide information to the client about which key salt to
   use for the string-to-key to be used by the client to obtain the key
   for decrypting the encrypted part the AS-REP.

   ETYPE-INFO-ENTRY        ::= SEQUENCE {
           etype           [0] Int32,
           salt            [1] OCTET STRING OPTIONAL
   }

   ETYPE-INFO              ::= SEQUENCE OF ETYPE-INFO-ENTRY

   The salt, like that of PA-PW-SALT, is also completely unspecified
   with respect to character set and is probably locale-specific.

   If ETYPE-INFO is sent in an AS-REP, there shall be exactly one
   ETYPE-INFO-ENTRY, and its etype shall match that of the enc-part in
   the AS-REP.

   This pre-authentication type was not present in RFC 1510, but many
   implementations that support encrypted timestamps for pre-
   authentication need to support ETYPE-INFO as well.  As noted in
   Section 3.1.3, a KDC MUST NOT send PA-ETYPE-INFO when the client's
   AS-REQ includes at least one "newer" etype.

5.2.7.5.  PA-ETYPE-INFO2

   The ETYPE-INFO2 pre-authentication type is sent by the KDC in a
   KRB-ERROR indicating a requirement for additional pre-authentication.
   It is usually used to notify a client of which key to use for the
   encryption of an encrypted timestamp for the purposes of sending a
   PA-ENC-TIMESTAMP pre-authentication value.  It MAY also be sent in an
   AS-REP to provide information to the client about which key salt to
   use for the string-to-key to be used by the client to obtain the key
   for decrypting the encrypted part the AS-REP.

ETYPE-INFO2-ENTRY       ::= SEQUENCE {
        etype           [0] Int32,
        salt            [1] KerberosString OPTIONAL,
        s2kparams       [2] OCTET STRING OPTIONAL
}

ETYPE-INFO2              ::= SEQUENCE SIZE (1..MAX) OF ETYPE-INFO2-ENTRY

   The type of the salt is KerberosString, but existing installations
   might have locale-specific characters stored in salt strings, and
   implementors MAY choose to handle them.

   The interpretation of s2kparams is specified in the cryptosystem
   description associated with the etype.  Each cryptosystem has a
   default interpretation of s2kparams that will hold if that element is
   omitted from the encoding of ETYPE-INFO2-ENTRY.

   If ETYPE-INFO2 is sent in an AS-REP, there shall be exactly one
   ETYPE-INFO2-ENTRY, and its etype shall match that of the enc-part in
   the AS-REP.

   The preferred ordering of the "hint" pre-authentication data that
   affect client key selection is: ETYPE-INFO2, followed by ETYPE-INFO,
   followed by PW-SALT.  As noted in Section 3.1.3, a KDC MUST NOT send
   ETYPE-INFO or PW-SALT when the client's AS-REQ includes at least one
   "newer" etype.

   The ETYPE-INFO2 pre-authentication type was not present in RFC 1510.

5.2.8.  KerberosFlags

   For several message types, a specific constrained bit string type,
   KerberosFlags, is used.

   KerberosFlags   ::= BIT STRING (SIZE (32..MAX))
                       -- minimum number of bits shall be sent,
                       -- but no fewer than 32

   Compatibility note: The following paragraphs describe a change from
   the RFC 1510 description of bit strings that would result in
   incompatility in the case of an implementation that strictly
   conformed to ASN.1 DER and RFC 1510.

   ASN.1 bit strings have multiple uses.  The simplest use of a bit
   string is to contain a vector of bits, with no particular meaning
   attached to individual bits.  This vector of bits is not necessarily
   a multiple of eight bits long.  The use in Kerberos of a bit string
   as a compact boolean vector wherein each element has a distinct
   meaning poses some problems.  The natural notation for a compact
   boolean vector is the ASN.1 "NamedBit" notation, and the DER require
   that encodings of a bit string using "NamedBit" notation exclude any
   trailing zero bits.  This truncation is easy to neglect, especially
   given C language implementations that naturally choose to store
   boolean vectors as 32-bit integers.

   For example, if the notation for KDCOptions were to include the
   "NamedBit" notation, as in RFC 1510, and a KDCOptions value to be
   encoded had only the "forwardable" (bit number one) bit set, the DER
   encoding MUST include only two bits: the first reserved bit
   ("reserved", bit number zero, value zero) and the one-valued bit (bit
   number one) for "forwardable".

   Most existing implementations of Kerberos unconditionally send 32
   bits on the wire when encoding bit strings used as boolean vectors.
   This behavior violates the ASN.1 syntax used for flag values in RFC
   1510, but it occurs on such a widely installed base that the protocol
   description is being modified to accommodate it.

   Consequently, this document removes the "NamedBit" notations for
   individual bits, relegating them to comments.  The size constraint on
   the KerberosFlags type requires that at least 32 bits be encoded at
   all times, though a lenient implementation MAY choose to accept fewer
   than 32 bits and to treat the missing bits as set to zero.

   Currently, no uses of KerberosFlags specify more than 32 bits' worth
   of flags, although future revisions of this document may do so.  When
   more than 32 bits are to be transmitted in a KerberosFlags value,
   future revisions to this document will likely specify that the
   smallest number of bits needed to encode the highest-numbered one-
   valued bit should be sent.  This is somewhat similar to the DER
   encoding of a bit string that is declared with the "NamedBit"
   notation.

5.2.9.  Cryptosystem-Related Types

   Many Kerberos protocol messages contain an EncryptedData as a
   container for arbitrary encrypted data, which is often the encrypted
   encoding of another data type.  Fields within EncryptedData assist
   the recipient in selecting a key with which to decrypt the enclosed
   data.

   EncryptedData   ::= SEQUENCE {
           etype   [0] Int32 -- EncryptionType --,
           kvno    [1] UInt32 OPTIONAL,
           cipher  [2] OCTET STRING -- ciphertext
   }

   etype
      This field identifies which encryption algorithm was used to
      encipher the cipher.

   kvno
      This field contains the version number of the key under which data
      is encrypted.  It is only present in messages encrypted under long
      lasting keys, such as principals' secret keys.

   cipher
      This field contains the enciphered text, encoded as an OCTET
      STRING.  (Note that the encryption mechanisms defined in [RFC3961]
      MUST incorporate integrity protection as well, so no additional
      checksum is required.)

   The EncryptionKey type is the means by which cryptographic keys used
   for encryption are transferred.

   EncryptionKey   ::= SEQUENCE {
           keytype         [0] Int32 -- actually encryption type --,
           keyvalue        [1] OCTET STRING
   }

   keytype
      This field specifies the encryption type of the encryption key
      that follows in the keyvalue field.  Although its name is
      "keytype", it actually specifies an encryption type.  Previously,
      multiple cryptosystems that performed encryption differently but
      were capable of using keys with the same characteristics were
      permitted to share an assigned number to designate the type of
      key; this usage is now deprecated.

   keyvalue
      This field contains the key itself, encoded as an octet string.

   Messages containing cleartext data to be authenticated will usually
   do so by using a member of type Checksum.  Most instances of Checksum
   use a keyed hash, though exceptions will be noted.

   Checksum        ::= SEQUENCE {
           cksumtype       [0] Int32,
           checksum        [1] OCTET STRING
   }

   cksumtype
      This field indicates the algorithm used to generate the
      accompanying checksum.

   checksum
      This field contains the checksum itself, encoded as an octet
      string.

   See Section 4 for a brief description of the use of encryption and
   checksums in Kerberos.

5.3.  Tickets

   This section describes the format and encryption parameters for
   tickets and authenticators.  When a ticket or authenticator is
   included in a protocol message, it is treated as an opaque object.  A
   ticket is a record that helps a client authenticate to a service.  A
   Ticket contains the following information:

   Ticket          ::= [APPLICATION 1] SEQUENCE {
           tkt-vno         [0] INTEGER (5),
           realm           [1] Realm,
           sname           [2] PrincipalName,
           enc-part        [3] EncryptedData -- EncTicketPart
   }

   -- Encrypted part of ticket

   EncTicketPart   ::= [APPLICATION 3] SEQUENCE {
           flags                   [0] TicketFlags,
           key                     [1] EncryptionKey,
           crealm                  [2] Realm,
           cname                   [3] PrincipalName,
           transited               [4] TransitedEncoding,
           authtime                [5] KerberosTime,
           starttime               [6] KerberosTime OPTIONAL,
           endtime                 [7] KerberosTime,
           renew-till              [8] KerberosTime OPTIONAL,
           caddr                   [9] HostAddresses OPTIONAL,
           authorization-data      [10] AuthorizationData OPTIONAL
   }

   -- encoded Transited field
   TransitedEncoding       ::= SEQUENCE {
           tr-type         [0] Int32 -- must be registered --,
           contents        [1] OCTET STRING
   }

   TicketFlags     ::= KerberosFlags
           -- reserved(0),
           -- forwardable(1),
           -- forwarded(2),
           -- proxiable(3),
           -- proxy(4),
           -- may-postdate(5),
           -- postdated(6),
           -- invalid(7),
           -- renewable(8),
           -- initial(9),
           -- pre-authent(10),
           -- hw-authent(11),
   -- the following are new since 1510
           -- transited-policy-checked(12),
           -- ok-as-delegate(13)

   tkt-vno
      This field specifies the version number for the ticket format.
      This document describes version number 5.

   realm
      This field specifies the realm that issued a ticket.  It also
      serves to identify the realm part of the server's principal
      identifier.  Since a Kerberos server can only issue tickets for
      servers within its realm, the two will always be identical.

   sname
      This field specifies all components of the name part of the
      server's identity, including those parts that identify a specific
      instance of a service.

   enc-part
      This field holds the encrypted encoding of the EncTicketPart
      sequence.  It is encrypted in the key shared by Kerberos and the
      end server (the server's secret key), using a key usage value of
      2.

   flags
      This field indicates which of various options were used or
      requested when the ticket was issued.  The meanings of the flags
      are as follows:

   Bit(s)  Name             Description

   0       reserved         Reserved for future expansion of this field.

   1       forwardable      The FORWARDABLE flag is normally only
                            interpreted by the TGS, and can be ignored
                            by end servers.  When set, this flag tells
                            the ticket-granting server that it is OK to
                            issue a new TGT with a different network
                            address based on the presented ticket.

   2       forwarded        When set, this flag indicates that the
                            ticket has either been forwarded or was
                            issued based on authentication involving a
                            forwarded TGT.

   3       proxiable        The PROXIABLE flag is normally only
                            interpreted by the TGS, and can be ignored
                            by end servers.  The PROXIABLE flag has an
                            interpretation identical to that of the
                            FORWARDABLE flag, except that the PROXIABLE
                            flag tells the ticket-granting server that
                            only non-TGTs may be issued with different
                            network addresses.

   4       proxy            When set, this flag indicates that a ticket
                            is a proxy.

   5       may-postdate     The MAY-POSTDATE flag is normally only
                            interpreted by the TGS, and can be ignored
                            by end servers.  This flag tells the

                            ticket-granting server that a post-dated
                            ticket MAY be issued based on this TGT.

   6       postdated        This flag indicates that this ticket has
                            been postdated.  The end-service can check
                            the authtime field to see when the original
                            authentication occurred.

   7       invalid          This flag indicates that a ticket is
                            invalid, and it must be validated by the KDC
                            before use.  Application servers must reject
                            tickets which have this flag set.

   8       renewable        The RENEWABLE flag is normally only
                            interpreted by the TGS, and can usually be
                            ignored by end servers (some particularly
                            careful servers MAY disallow renewable
                            tickets).  A renewable ticket can be used to
                            obtain a replacement ticket that expires at
                            a later date.

   9       initial          This flag indicates that this ticket was
                            issued using the AS protocol, and not issued
                            based on a TGT.

   10      pre-authent      This flag indicates that during initial
                            authentication, the client was authenticated
                            by the KDC before a ticket was issued.  The
                            strength of the pre-authentication method is
                            not indicated, but is acceptable to the KDC.

   11      hw-authent       This flag indicates that the protocol
                            employed for initial authentication required
                            the use of hardware expected to be possessed
                            solely by the named client.  The hardware
                            authentication method is selected by the KDC
                            and the strength of the method is not
                            indicated.

   12      transited-       This flag indicates that the KDC for
           policy-checked   the realm has checked the transited field
                            against a realm-defined policy for trusted
                            certifiers.  If this flag is reset (0), then
                            the application server must check the
                            transited field itself, and if unable to do
                            so, it must reject the authentication.  If
                            the flag is set (1), then the application
                            server MAY skip its own validation of the

                            transited field, relying on the validation
                            performed by the KDC.  At its option the
                            application server MAY still apply its own
                            validation based on a separate policy for
                            acceptance.

                            This flag is new since RFC 1510.

   13      ok-as-delegate   This flag indicates that the server (not the
                            client) specified in the ticket has been
                            determined by policy of the realm to be a
                            suitable recipient of delegation.  A client
                            can use the presence of this flag to help it
                            decide whether to delegate credentials
                            (either grant a proxy or a forwarded TGT) to
                            this server.  The client is free to ignore
                            the value of this flag.  When setting this
                            flag, an administrator should consider the
                            security and placement of the server on
                            which the service will run, as well as
                            whether the service requires the use of
                            delegated credentials.

                            This flag is new since RFC 1510.

   14-31   reserved         Reserved for future use.

   key
      This field exists in the ticket and the KDC response and is used
      to pass the session key from Kerberos to the application server
      and the client.

   crealm
      This field contains the name of the realm in which the client is
      registered and in which initial authentication took place.

   cname
      This field contains the name part of the client's principal
      identifier.

   transited
      This field lists the names of the Kerberos realms that took part
      in authenticating the user to whom this ticket was issued.  It
      does not specify the order in which the realms were transited.
      See Section 3.3.3.2 for details on how this field encodes the
      traversed realms.  When the names of CAs are to be embedded in the
      transited field (as specified for some extensions to the

      protocol), the X.500 names of the CAs SHOULD be mapped into items
      in the transited field using the mapping defined by RFC 2253.

   authtime
      This field indicates the time of initial authentication for the
      named principal.  It is the time of issue for the original ticket
      on which this ticket is based.  It is included in the ticket to
      provide additional information to the end service, and to provide
      the necessary information for implementation of a "hot list"
      service at the KDC.  An end service that is particularly paranoid
      could refuse to accept tickets for which the initial
      authentication occurred "too far" in the past.  This field is also
      returned as part of the response from the KDC.  When it is
      returned as part of the response to initial authentication
      (KRB_AS_REP), this is the current time on the Kerberos server.  It
      is NOT recommended that this time value be used to adjust the
      workstation's clock, as the workstation cannot reliably determine
      that such a KRB_AS_REP actually came from the proper KDC in a
      timely manner.

   starttime
      This field in the ticket specifies the time after which the ticket
      is valid.  Together with endtime, this field specifies the life of
      the ticket.  If the starttime field is absent from the ticket,
      then the authtime field SHOULD be used in its place to determine
      the life of the ticket.

   endtime
      This field contains the time after which the ticket will not be
      honored (its expiration time).  Note that individual services MAY
      place their own limits on the life of a ticket and MAY reject
      tickets which have not yet expired.  As such, this is really an
      upper bound on the expiration time for the ticket.

   renew-till
      This field is only present in tickets that have the RENEWABLE flag
      set in the flags field.  It indicates the maximum endtime that may
      be included in a renewal.  It can be thought of as the absolute
      expiration time for the ticket, including all renewals.

   caddr
      This field in a ticket contains zero (if omitted) or more (if
      present) host addresses.  These are the addresses from which the
      ticket can be used.  If there are no addresses, the ticket can be
      used from any location.  The decision by the KDC to issue or by
      the end server to accept addressless tickets is a policy decision
      and is left to the Kerberos and end-service administrators; they
      MAY refuse to issue or accept such tickets.  Because of the wide

      deployment of network address translation, it is recommended that
      policy allow the issue and acceptance of such tickets.

      Network addresses are included in the ticket to make it harder for
      an attacker to use stolen credentials.  Because the session key is
      not sent over the network in cleartext, credentials can't be
      stolen simply by listening to the network; an attacker has to gain
      access to the session key (perhaps through operating system
      security breaches or a careless user's unattended session) to make
      use of stolen tickets.

      Note that the network address from which a connection is received
      cannot be reliably determined.  Even if it could be, an attacker
      who has compromised the client's workstation could use the
      credentials from there.  Including the network addresses only
      makes it more difficult, not impossible, for an attacker to walk
      off with stolen credentials and then to use them from a "safe"
      location.

   authorization-data
      The authorization-data field is used to pass authorization data
      from the principal on whose behalf a ticket was issued to the
      application service.  If no authorization data is included, this
      field will be left out.  Experience has shown that the name of
      this field is confusing, and that a better name would be
      "restrictions".  Unfortunately, it is not possible to change the
      name at this time.

      This field contains restrictions on any authority obtained on the
      basis of authentication using the ticket.  It is possible for any
      principal in possession of credentials to add entries to the
      authorization data field since these entries further restrict what
      can be done with the ticket.  Such additions can be made by
      specifying the additional entries when a new ticket is obtained
      during the TGS exchange, or they MAY be added during chained
      delegation using the authorization data field of the
      authenticator.

      Because entries may be added to this field by the holder of
      credentials, except when an entry is separately authenticated by
      encapsulation in the KDC-issued element, it is not allowable for
      the presence of an entry in the authorization data field of a
      ticket to amplify the privileges one would obtain from using a
      ticket.

      The data in this field may be specific to the end service; the
      field will contain the names of service specific objects, and the
      rights to those objects.  The format for this field is described

      in Section 5.2.6.  Although Kerberos is not concerned with the
      format of the contents of the subfields, it does carry type
      information (ad-type).

      By using the authorization_data field, a principal is able to
      issue a proxy that is valid for a specific purpose.  For example,
      a client wishing to print a file can obtain a file server proxy to
      be passed to the print server.  By specifying the name of the file
      in the authorization_data field, the file server knows that the
      print server can only use the client's rights when accessing the
      particular file to be printed.

      A separate service providing authorization or certifying group
      membership may be built using the authorization-data field.  In
      this case, the entity granting authorization (not the authorized
      entity) may obtain a ticket in its own name (e.g., the ticket is
      issued in the name of a privilege server), and this entity adds
      restrictions on its own authority and delegates the restricted
      authority through a proxy to the client.  The client would then
      present this authorization credential to the application server
      separately from the authentication exchange.  Alternatively, such
      authorization credentials MAY be embedded in the ticket
      authenticating the authorized entity, when the authorization is
      separately authenticated using the KDC-issued authorization data
      element (see 5.2.6.2).

      Similarly, if one specifies the authorization-data field of a
      proxy and leaves the host addresses blank, the resulting ticket
      and session key can be treated as a capability.  See [Neu93] for
      some suggested uses of this field.

      The authorization-data field is optional and does not have to be
      included in a ticket.

5.4.  Specifications for the AS and TGS Exchanges

   This section specifies the format of the messages used in the
   exchange between the client and the Kerberos server.  The format of
   possible error messages appears in Section 5.9.1.

5.4.1.  KRB_KDC_REQ Definition

   The KRB_KDC_REQ message has no application tag number of its own.
   Instead, it is incorporated into either KRB_AS_REQ or KRB_TGS_REQ,
   each of which has an application tag, depending on whether the
   request is for an initial ticket or an additional ticket.  In either
   case, the message is sent from the client to the KDC to request
   credentials for a service.

   The message fields are as follows:

AS-REQ          ::= [APPLICATION 10] KDC-REQ

TGS-REQ         ::= [APPLICATION 12] KDC-REQ

KDC-REQ         ::= SEQUENCE {
        -- NOTE: first tag is [1], not [0]
        pvno            [1] INTEGER (5) ,
        msg-type        [2] INTEGER (10 -- AS -- | 12 -- TGS --),
        padata          [3] SEQUENCE OF PA-DATA OPTIONAL
                            -- NOTE: not empty --,
        req-body        [4] KDC-REQ-BODY
}

KDC-REQ-BODY    ::= SEQUENCE {
        kdc-options             [0] KDCOptions,
        cname                   [1] PrincipalName OPTIONAL
                                    -- Used only in AS-REQ --,
        realm                   [2] Realm
                                    -- Server's realm
                                    -- Also client's in AS-REQ --,
        sname                   [3] PrincipalName OPTIONAL,
        from                    [4] KerberosTime OPTIONAL,
        till                    [5] KerberosTime,
        rtime                   [6] KerberosTime OPTIONAL,
        nonce                   [7] UInt32,
        etype                   [8] SEQUENCE OF Int32 -- EncryptionType
                                    -- in preference order --,
        addresses               [9] HostAddresses OPTIONAL,
        enc-authorization-data  [10] EncryptedData OPTIONAL
                                    -- AuthorizationData --,
        additional-tickets      [11] SEQUENCE OF Ticket OPTIONAL
                                       -- NOTE: not empty
}

KDCOptions      ::= KerberosFlags
        -- reserved(0),
        -- forwardable(1),
        -- forwarded(2),
        -- proxiable(3),
        -- proxy(4),
        -- allow-postdate(5),
        -- postdated(6),
        -- unused7(7),
        -- renewable(8),
        -- unused9(9),
        -- unused10(10),

        -- opt-hardware-auth(11),
        -- unused12(12),
        -- unused13(13),
-- 15 is reserved for canonicalize
        -- unused15(15),
-- 26 was unused in 1510
        -- disable-transited-check(26),
--
        -- renewable-ok(27),
        -- enc-tkt-in-skey(28),
        -- renew(30),
        -- validate(31)

   The fields in this message are as follows:

   pvno
      This field is included in each message, and specifies the protocol
      version number.  This document specifies protocol version 5.

   msg-type
      This field indicates the type of a protocol message.  It will
      almost always be the same as the application identifier associated
      with a message.  It is included to make the identifier more
      readily accessible to the application.  For the KDC-REQ message,
      this type will be KRB_AS_REQ or KRB_TGS_REQ.

   padata
      Contains pre-authentication data.  Requests for additional tickets
      (KRB_TGS_REQ) MUST contain a padata of PA-TGS-REQ.

      The padata (pre-authentication data) field contains a sequence of
      authentication information that may be needed before credentials
      can be issued or decrypted.

   req-body
      This field is a placeholder delimiting the extent of the remaining
      fields.  If a checksum is to be calculated over the request, it is
      calculated over an encoding of the KDC-REQ-BODY sequence which is
      enclosed within the req-body field.

   kdc-options
      This field appears in the KRB_AS_REQ and KRB_TGS_REQ requests to
      the KDC and indicates the flags that the client wants set on the
      tickets as well as other information that is to modify the
      behavior of the KDC.  Where appropriate, the name of an option may
      be the same as the flag that is set by that option.  Although in
      most cases, the bit in the options field will be the same as that
      in the flags field, this is not guaranteed, so it is not

      acceptable simply to copy the options field to the flags field.
      There are various checks that must be made before an option is
      honored anyway.

      The kdc_options field is a bit-field, where the selected options
      are indicated by the bit being set (1), and the unselected options
      and reserved fields being reset (0).  The encoding of the bits is
      specified in Section 5.2.  The options are described in more
      detail above in Section 2.  The meanings of the options are as
      follows:

   Bits    Name                     Description

   0       RESERVED                 Reserved for future expansion of
                                    this field.

   1       FORWARDABLE              The FORWARDABLE option indicates
                                    that the ticket to be issued is to
                                    have its forwardable flag set.  It
                                    may only be set on the initial
                                    request, or in a subsequent request
                                    if the TGT on which it is based is
                                    also forwardable.

   2       FORWARDED                The FORWARDED option is only
                                    specified in a request to the
                                    ticket-granting server and will only
                                    be honored if the TGT in the request
                                    has its FORWARDABLE bit set.  This
                                    option indicates that this is a
                                    request for forwarding.  The
                                    address(es) of the host from which
                                    the resulting ticket is to be valid
                                    are included in the addresses field
                                    of the request.

   3       PROXIABLE                The PROXIABLE option indicates that
                                    the ticket to be issued is to have
                                    its proxiable flag set.  It may only
                                    be set on the initial request, or a
                                    subsequent request if the TGT on
                                    which it is based is also proxiable.

   4       PROXY                    The PROXY option indicates that this
                                    is a request for a proxy.  This
                                    option will only be honored if the
                                    TGT in the request has its PROXIABLE
                                    bit set.  The address(es) of the

                                    host from which the resulting ticket
                                    is to be valid are included in the
                                    addresses field of the request.

   5       ALLOW-POSTDATE           The ALLOW-POSTDATE option indicates
                                    that the ticket to be issued is to
                                    have its MAY-POSTDATE flag set.  It
                                    may only be set on the initial
                                    request, or in a subsequent request
                                    if the TGT on which it is based also
                                    has its MAY-POSTDATE flag set.

   6       POSTDATED                The POSTDATED option indicates that
                                    this is a request for a postdated
                                    ticket.  This option will only be
                                    honored if the TGT on which it is
                                    based has its MAY-POSTDATE flag set.
                                    The resulting ticket will also have
                                    its INVALID flag set, and that flag
                                    may be reset by a subsequent request
                                    to the KDC after the starttime in
                                    the ticket has been reached.

   7       RESERVED                 This option is presently unused.

   8       RENEWABLE                The RENEWABLE option indicates that
                                    the ticket to be issued is to have
                                    its RENEWABLE flag set.  It may only
                                    be set on the initial request, or
                                    when the TGT on which the request is
                                    based is also renewable.  If this
                                    option is requested, then the rtime
                                    field in the request contains the
                                    desired absolute expiration time for
                                    the ticket.

   9       RESERVED                 Reserved for PK-Cross.

   10      RESERVED                 Reserved for future use.

   11      RESERVED                 Reserved for opt-hardware-auth.

   12-25   RESERVED                 Reserved for future use.

   26      DISABLE-TRANSITED-CHECK  By default the KDC will check the
                                    transited field of a TGT against the
                                    policy of the local realm before it
                                    will issue derivative tickets based

                                    on the TGT.  If this flag is set in
                                    the request, checking of the
                                    transited field is disabled.
                                    Tickets issued without the
                                    performance of this check will be
                                    noted by the reset (0) value of the
                                    TRANSITED-POLICY-CHECKED flag,
                                    indicating to the application server
                                    that the transited field must be
                                    checked locally.  KDCs are
                                    encouraged but not required to honor
                                    the DISABLE-TRANSITED-CHECK option.

                                    This flag is new since RFC 1510.

   27      RENEWABLE-OK             The RENEWABLE-OK option indicates
                                    that a renewable ticket will be
                                    acceptable if a ticket with the
                                    requested life cannot otherwise be
                                    provided, in which case a renewable
                                    ticket may be issued with a renew-
                                    till equal to the requested endtime.
                                    The value of the renew-till field
                                    may still be limited by local
                                    limits, or limits selected by the
                                    individual principal or server.

   28      ENC-TKT-IN-SKEY          This option is used only by the
                                    ticket-granting service.  The ENC-
                                    TKT-IN-SKEY option indicates that
                                    the ticket for the end server is to
                                    be encrypted in the session key from
                                    the additional TGT provided.

   29      RESERVED                 Reserved for future use.

   30      RENEW                    This option is used only by the
                                    ticket-granting service.  The RENEW
                                    option indicates that the present
                                    request is for a renewal.  The
                                    ticket provided is encrypted in the
                                    secret key for the server on which
                                    it is valid.  This option will only
                                    be honored if the ticket to be
                                    renewed has its RENEWABLE flag set
                                    and if the time in its renew-till
                                    field has not passed.  The ticket to
                                    be renewed is passed in the padata

                                    field as part of the authentication
                                    header.

   31      VALIDATE                 This option is used only by the
                                    ticket-granting service.  The
                                    VALIDATE option indicates that the
                                    request is to validate a postdated
                                    ticket.  It will only be honored if
                                    the ticket presented is postdated,
                                    presently has its INVALID flag set,
                                    and would otherwise be usable at
                                    this time.  A ticket cannot be
                                    validated before its starttime.  The
                                    ticket presented for validation is
                                    encrypted in the key of the server
                                    for which it is valid and is passed
                                    in the padata field as part of the
                                    authentication header.

   cname and sname
      These fields are the same as those described for the ticket in
      section 5.3.  The sname may only be absent when the ENC-TKT-IN-
      SKEY option is specified.  If the sname is absent, the name of the
      server is taken from the name of the client in the ticket passed
      as additional-tickets.

   enc-authorization-data
      The enc-authorization-data, if present (and it can only be present
      in the TGS_REQ form), is an encoding of the desired
      authorization-data encrypted under the sub-session key if present
      in the Authenticator, or alternatively from the session key in the
      TGT (both the Authenticator and TGT come from the padata field in
      the KRB_TGS_REQ).  The key usage value used when encrypting is 5
      if a sub-session key is used, or 4 if the session key is used.

   realm
      This field specifies the realm part of the server's principal
      identifier.  In the AS exchange, this is also the realm part of
      the client's principal identifier.

   from
      This field is included in the KRB_AS_REQ and KRB_TGS_REQ ticket
      requests when the requested ticket is to be postdated.  It
      specifies the desired starttime for the requested ticket.  If this
      field is omitted, then the KDC SHOULD use the current time
      instead.

   till
      This field contains the expiration date requested by the client in
      a ticket request.  It is not optional, but if the requested
      endtime is "19700101000000Z", the requested ticket is to have the
      maximum endtime permitted according to KDC policy.  Implementation
      note: This special timestamp corresponds to a UNIX time_t value of
      zero on most systems.

   rtime
      This field is the requested renew-till time sent from a client to
      the KDC in a ticket request.  It is optional.

   nonce
      This field is part of the KDC request and response.  It is
      intended to hold a random number generated by the client.  If the
      same number is included in the encrypted response from the KDC, it
      provides evidence that the response is fresh and has not been
      replayed by an attacker.  Nonces MUST NEVER be reused.

   etype
      This field specifies the desired encryption algorithm to be used
      in the response.

   addresses
      This field is included in the initial request for tickets, and it
      is optionally included in requests for additional tickets from the
      ticket-granting server.  It specifies the addresses from which the
      requested ticket is to be valid.  Normally it includes the
      addresses for the client's host.  If a proxy is requested, this
      field will contain other addresses.  The contents of this field
      are usually copied by the KDC into the caddr field of the
      resulting ticket.

   additional-tickets
      Additional tickets MAY be optionally included in a request to the
      ticket-granting server.  If the ENC-TKT-IN-SKEY option has been
      specified, then the session key from the additional ticket will be
      used in place of the server's key to encrypt the new ticket.  When
      the ENC-TKT-IN-SKEY option is used for user-to-user
      authentication, this additional ticket MAY be a TGT issued by the
      local realm or an inter-realm TGT issued for the current KDC's
      realm by a remote KDC.  If more than one option that requires
      additional tickets has been specified, then the additional tickets
      are used in the order specified by the ordering of the options
      bits (see kdc-options, above).

   The application tag number will be either ten (10) or twelve (12)
   depending on whether the request is for an initial ticket (AS-REQ) or
   for an additional ticket (TGS-REQ).

   The optional fields (addresses, authorization-data, and additional-
   tickets) are only included if necessary to perform the operation
   specified in the kdc-options field.

   Note that in KRB_TGS_REQ, the protocol version number appears twice
   and two different message types appear: the KRB_TGS_REQ message
   contains these fields as does the authentication header (KRB_AP_REQ)
   that is passed in the padata field.

5.4.2.  KRB_KDC_REP Definition

   The KRB_KDC_REP message format is used for the reply from the KDC for
   either an initial (AS) request or a subsequent (TGS) request.  There
   is no message type for KRB_KDC_REP.  Instead, the type will be either
   KRB_AS_REP or KRB_TGS_REP.  The key used to encrypt the ciphertext
   part of the reply depends on the message type.  For KRB_AS_REP, the
   ciphertext is encrypted in the client's secret key, and the client's
   key version number is included in the key version number for the
   encrypted data.  For KRB_TGS_REP, the ciphertext is encrypted in the
   sub-session key from the Authenticator; if it is absent, the
   ciphertext is encrypted in the session key from the TGT used in the
   request.  In that case, no version number will be present in the
   EncryptedData sequence.

   The KRB_KDC_REP message contains the following fields:

   AS-REP          ::= [APPLICATION 11] KDC-REP

   TGS-REP         ::= [APPLICATION 13] KDC-REP

   KDC-REP         ::= SEQUENCE {
           pvno            [0] INTEGER (5),
           msg-type        [1] INTEGER (11 -- AS -- | 13 -- TGS --),
           padata          [2] SEQUENCE OF PA-DATA OPTIONAL
                                   -- NOTE: not empty --,
           crealm          [3] Realm,
           cname           [4] PrincipalName,
           ticket          [5] Ticket,
           enc-part        [6] EncryptedData
                                   -- EncASRepPart or EncTGSRepPart,
                                   -- as appropriate
   }

   EncASRepPart    ::= [APPLICATION 25] EncKDCRepPart

   EncTGSRepPart   ::= [APPLICATION 26] EncKDCRepPart

   EncKDCRepPart   ::= SEQUENCE {
           key             [0] EncryptionKey,
           last-req        [1] LastReq,
           nonce           [2] UInt32,
           key-expiration  [3] KerberosTime OPTIONAL,
           flags           [4] TicketFlags,
           authtime        [5] KerberosTime,
           starttime       [6] KerberosTime OPTIONAL,
           endtime         [7] KerberosTime,
           renew-till      [8] KerberosTime OPTIONAL,
           srealm          [9] Realm,
           sname           [10] PrincipalName,
           caddr           [11] HostAddresses OPTIONAL
   }

   LastReq         ::=     SEQUENCE OF SEQUENCE {
           lr-type         [0] Int32,
           lr-value        [1] KerberosTime
   }

   pvno and msg-type
      These fields are described above in Section 5.4.1.  msg-type is
      either KRB_AS_REP or KRB_TGS_REP.

   padata
      This field is described in detail in Section 5.4.1.  One possible
      use for it is to encode an alternate "salt" string to be used with
      a string-to-key algorithm.  This ability is useful for easing
      transitions if a realm name needs to change (e.g., when a company
      is acquired); in such a case all existing password-derived entries
      in the KDC database would be flagged as needing a special salt
      string until the next password change.

   crealm, cname, srealm, and sname
      These fields are the same as those described for the ticket in
      section 5.3.

   ticket
      The newly-issued ticket, from Section 5.3.

   enc-part
      This field is a place holder for the ciphertext and related
      information that forms the encrypted part of a message.  The
      description of the encrypted part of the message follows each
      appearance of this field.

      The key usage value for encrypting this field is 3 in an AS-REP
      message, using the client's long-term key or another key selected
      via pre-authentication mechanisms.  In a TGS-REP message, the key
      usage value is 8 if the TGS session key is used, or 9 if a TGS
      authenticator subkey is used.

      Compatibility note: Some implementations unconditionally send an
      encrypted EncTGSRepPart (application tag number 26) in this field
      regardless of whether the reply is a AS-REP or a TGS-REP.  In the
      interest of compatibility, implementors MAY relax the check on the
      tag number of the decrypted ENC-PART.

   key
      This field is the same as described for the ticket in Section 5.3.

   last-req
      This field is returned by the KDC and specifies the time(s) of the
      last request by a principal.  Depending on what information is
      available, this might be the last time that a request for a TGT
      was made, or the last time that a request based on a TGT was
      successful.  It also might cover all servers for a realm, or just
      the particular server.  Some implementations MAY display this
      information to the user to aid in discovering unauthorized use of
      one's identity.  It is similar in spirit to the last login time
      displayed when logging in to timesharing systems.

   lr-type
      This field indicates how the following lr-value field is to be
      interpreted.  Negative values indicate that the information
      pertains only to the responding server.  Non-negative values
      pertain to all servers for the realm.

      If the lr-type field is zero (0), then no information is conveyed
      by the lr-value subfield.  If the absolute value of the lr-type
      field is one (1), then the lr-value subfield is the time of last
      initial request for a TGT.  If it is two (2), then the lr-value
      subfield is the time of last initial request.  If it is three (3),
      then the lr-value subfield is the time of issue for the newest TGT
      used.  If it is four (4), then the lr-value subfield is the time
      of the last renewal.  If it is five (5), then the lr-value
      subfield is the time of last request (of any type).  If it is (6),
      then the lr-value subfield is the time when the password will
      expire.  If it is (7), then the lr-value subfield is the time when
      the account will expire.

   lr-value
      This field contains the time of the last request.  The time MUST
      be interpreted according to the contents of the accompanying lr-
      type subfield.

   nonce
      This field is described above in Section 5.4.1.

   key-expiration
      The key-expiration field is part of the response from the KDC and
      specifies the time that the client's secret key is due to expire.
      The expiration might be the result of password aging or an account
      expiration.  If present, it SHOULD be set to the earlier of the
      user's key expiration and account expiration.  The use of this
      field is deprecated, and the last-req field SHOULD be used to
      convey this information instead.  This field will usually be left
      out of the TGS reply since the response to the TGS request is
      encrypted in a session key and no client information has to be
      retrieved from the KDC database.  It is up to the application
      client (usually the login program) to take appropriate action
      (such as notifying the user) if the expiration time is imminent.

   flags, authtime, starttime, endtime, renew-till and caddr
      These fields are duplicates of those found in the encrypted
      portion of the attached ticket (see Section 5.3), provided so the
      client MAY verify that they match the intended request and in
      order to assist in proper ticket caching.  If the message is of
      type KRB_TGS_REP, the caddr field will only be filled in if the
      request was for a proxy or forwarded ticket, or if the user is
      substituting a subset of the addresses from the TGT.  If the
      client-requested addresses are not present or not used, then the
      addresses contained in the ticket will be the same as those
      included in the TGT.

5.5.  Client/Server (CS) Message Specifications

   This section specifies the format of the messages used for the
   authentication of the client to the application server.

5.5.1.  KRB_AP_REQ Definition

   The KRB_AP_REQ message contains the Kerberos protocol version number,
   the message type KRB_AP_REQ, an options field to indicate any options
   in use, and the ticket and authenticator themselves.  The KRB_AP_REQ
   message is often referred to as the "authentication header".

   AP-REQ          ::= [APPLICATION 14] SEQUENCE {
           pvno            [0] INTEGER (5),
           msg-type        [1] INTEGER (14),
           ap-options      [2] APOptions,
           ticket          [3] Ticket,
           authenticator   [4] EncryptedData -- Authenticator
   }

   APOptions       ::= KerberosFlags
           -- reserved(0),
           -- use-session-key(1),
           -- mutual-required(2)

   pvno and msg-type
      These fields are described above in Section 5.4.1. msg-type is
      KRB_AP_REQ.

   ap-options
      This field appears in the application request (KRB_AP_REQ) and
      affects the way the request is processed.  It is a bit-field,
      where the selected options are indicated by the bit being set (1),
      and the unselected options and reserved fields by being reset (0).
      The encoding of the bits is specified in Section 5.2.  The
      meanings of the options are as follows:

   Bit(s)  Name             Description

   0       reserved         Reserved for future expansion of this field.

   1       use-session-key  The USE-SESSION-KEY option indicates that
                            the ticket the client is presenting to a
                            server is encrypted in the session key from
                            the server's TGT.  When this option is not
                            specified, the ticket is encrypted in the
                            server's secret key.

   2       mutual-required  The MUTUAL-REQUIRED option tells the server
                            that the client requires mutual
                            authentication, and that it must respond
                            with a KRB_AP_REP message.

   3-31    reserved         Reserved for future use.

   ticket
      This field is a ticket authenticating the client to the server.

   authenticator
      This contains the encrypted authenticator, which includes the
      client's choice of a subkey.

   The encrypted authenticator is included in the AP-REQ; it certifies
   to a server that the sender has recent knowledge of the encryption
   key in the accompanying ticket, to help the server detect replays.
   It also assists in the selection of a "true session key" to use with
   the particular session.  The DER encoding of the following is
   encrypted in the ticket's session key, with a key usage value of 11
   in normal application exchanges, or 7 when used as the PA-TGS-REQ
   PA-DATA field of a TGS-REQ exchange (see Section 5.4.1):

   -- Unencrypted authenticator
   Authenticator   ::= [APPLICATION 2] SEQUENCE  {
           authenticator-vno       [0] INTEGER (5),
           crealm                  [1] Realm,
           cname                   [2] PrincipalName,
           cksum                   [3] Checksum OPTIONAL,
           cusec                   [4] Microseconds,
           ctime                   [5] KerberosTime,
           subkey                  [6] EncryptionKey OPTIONAL,
           seq-number              [7] UInt32 OPTIONAL,
           authorization-data      [8] AuthorizationData OPTIONAL
   }

   authenticator-vno
      This field specifies the version number for the format of the
      authenticator.  This document specifies version 5.

   crealm and cname
      These fields are the same as those described for the ticket in
      section 5.3.

   cksum
      This field contains a checksum of the application data that
      accompanies the KRB_AP_REQ, computed using a key usage value of 10
      in normal application exchanges, or 6 when used in the TGS-REQ
      PA-TGS-REQ AP-DATA field.

   cusec
      This field contains the microsecond part of the client's
      timestamp.  Its value (before encryption) ranges from 0 to 999999.
      It often appears along with ctime.  The two fields are used
      together to specify a reasonably accurate timestamp.

   ctime
      This field contains the current time on the client's host.

   subkey
      This field contains the client's choice for an encryption key to
      be used to protect this specific application session.  Unless an
      application specifies otherwise, if this field is left out, the
      session key from the ticket will be used.

   seq-number
      This optional field includes the initial sequence number to be
      used by the KRB_PRIV or KRB_SAFE messages when sequence numbers
      are used to detect replays.  (It may also be used by application
      specific messages.)  When included in the authenticator, this
      field specifies the initial sequence number for messages from the
      client to the server.  When included in the AP-REP message, the
      initial sequence number is that for messages from the server to
      the client.  When used in KRB_PRIV or KRB_SAFE messages, it is
      incremented by one after each message is sent.  Sequence numbers
      fall in the range 0 through 2^32 - 1 and wrap to zero following
      the value 2^32 - 1.

      For sequence numbers to support the detection of replays
      adequately, they SHOULD be non-repeating, even across connection
      boundaries.  The initial sequence number SHOULD be random and
      uniformly distributed across the full space of possible sequence
      numbers, so that it cannot be guessed by an attacker and so that
      it and the successive sequence numbers do not repeat other
      sequences.  In the event that more than 2^32 messages are to be
      generated in a series of KRB_PRIV or KRB_SAFE messages, rekeying
      SHOULD be performed before sequence numbers are reused with the
      same encryption key.

      Implmentation note: Historically, some implementations transmit
      signed twos-complement numbers for sequence numbers.  In the
      interests of compatibility, implementations MAY accept the
      equivalent negative number where a positive number greater than
      2^31 - 1 is expected.

      Implementation note: As noted before, some implementations omit
      the optional sequence number when its value would be zero.
      Implementations MAY accept an omitted sequence number when
      expecting a value of zero, and SHOULD NOT transmit an
      Authenticator with a initial sequence number of zero.

   authorization-data
      This field is the same as described for the ticket in Section 5.3.
      It is optional and will only appear when additional restrictions
      are to be placed on the use of a ticket, beyond those carried in
      the ticket itself.

5.5.2.  KRB_AP_REP Definition

   The KRB_AP_REP message contains the Kerberos protocol version number,
   the message type, and an encrypted time-stamp.  The message is sent
   in response to an application request (KRB_AP_REQ) for which the
   mutual authentication option has been selected in the ap-options
   field.

   AP-REP          ::= [APPLICATION 15] SEQUENCE {
           pvno            [0] INTEGER (5),
           msg-type        [1] INTEGER (15),
           enc-part        [2] EncryptedData -- EncAPRepPart
   }

   EncAPRepPart    ::= [APPLICATION 27] SEQUENCE {
           ctime           [0] KerberosTime,
           cusec           [1] Microseconds,
           subkey          [2] EncryptionKey OPTIONAL,
           seq-number      [3] UInt32 OPTIONAL
   }

   The encoded EncAPRepPart is encrypted in the shared session key of
   the ticket.  The optional subkey field can be used in an
   application-arranged negotiation to choose a per association session
   key.

   pvno and msg-type
      These fields are described above in Section 5.4.1.  msg-type is
      KRB_AP_REP.

   enc-part
      This field is described above in Section 5.4.2.  It is computed
      with a key usage value of 12.

   ctime
      This field contains the current time on the client's host.

   cusec
      This field contains the microsecond part of the client's
      timestamp.

   subkey
      This field contains an encryption key that is to be used to
      protect this specific application session.  See Section 3.2.6 for
      specifics on how this field is used to negotiate a key.  Unless an
      application specifies otherwise, if this field is left out, the
      sub-session key from the authenticator or if the latter is also
      left out, the session key from the ticket will be used.

   seq-number
      This field is described above in Section 5.3.2.

5.5.3.  Error Message Reply

   If an error occurs while processing the application request, the
   KRB_ERROR message will be sent in response.  See Section 5.9.1 for
   the format of the error message.  The cname and crealm fields MAY be
   left out if the server cannot determine their appropriate values from
   the corresponding KRB_AP_REQ message.  If the authenticator was
   decipherable, the ctime and cusec fields will contain the values from
   it.

5.6.  KRB_SAFE Message Specification

   This section specifies the format of a message that can be used by
   either side (client or server) of an application to send a tamper-
   proof message to its peer.  It presumes that a session key has
   previously been exchanged (for example, by using the
   KRB_AP_REQ/KRB_AP_REP messages).

5.6.1.  KRB_SAFE definition

   The KRB_SAFE message contains user data along with a collision-proof
   checksum keyed with the last encryption key negotiated via subkeys,
   or with the session key if no negotiation has occurred.  The message
   fields are as follows:

   KRB-SAFE        ::= [APPLICATION 20] SEQUENCE {
           pvno            [0] INTEGER (5),
           msg-type        [1] INTEGER (20),
           safe-body       [2] KRB-SAFE-BODY,
           cksum           [3] Checksum
   }

   KRB-SAFE-BODY   ::= SEQUENCE {
           user-data       [0] OCTET STRING,
           timestamp       [1] KerberosTime OPTIONAL,
           usec            [2] Microseconds OPTIONAL,
           seq-number      [3] UInt32 OPTIONAL,
           s-address       [4] HostAddress,
           r-address       [5] HostAddress OPTIONAL
   }

   pvno and msg-type
      These fields are described above in Section 5.4.1.  msg-type is
      KRB_SAFE.

   safe-body
      This field is a placeholder for the body of the KRB-SAFE message.

   cksum
      This field contains the checksum of the application data, computed
      with a key usage value of 15.

      The checksum is computed over the encoding of the KRB-SAFE
      sequence.  First, the cksum is set to a type zero, zero-length
      value, and the checksum is computed over the encoding of the KRB-
      SAFE sequence.  Then the checksum is set to the result of that
      computation.  Finally, the KRB-SAFE sequence is encoded again.
      This method, although different than the one specified in RFC
      1510, corresponds to existing practice.

   user-data
      This field is part of the KRB_SAFE and KRB_PRIV messages, and
      contains the application-specific data that is being passed from
      the sender to the recipient.

   timestamp
      This field is part of the KRB_SAFE and KRB_PRIV messages.  Its
      contents are the current time as known by the sender of the
      message.  By checking the timestamp, the recipient of the message
      is able to make sure that it was recently generated, and is not a
      replay.

   usec
      This field is part of the KRB_SAFE and KRB_PRIV headers.  It
      contains the microsecond part of the timestamp.

   seq-number
      This field is described above in Section 5.3.2.

   s-address
      Sender's address.

      This field specifies the address in use by the sender of the
      message.

   r-address
      This field specifies the address in use by the recipient of the
      message.  It MAY be omitted for some uses (such as broadcast
      protocols), but the recipient MAY arbitrarily reject such
      messages.  This field, along with s-address, can be used to help
      detect messages that have been incorrectly or maliciously
      delivered to the wrong recipient.

5.7.  KRB_PRIV Message Specification

   This section specifies the format of a message that can be used by
   either side (client or server) of an application to send a message to
   its peer securely and privately.  It presumes that a session key has
   previously been exchanged (for example, by using the
   KRB_AP_REQ/KRB_AP_REP messages).

5.7.1.  KRB_PRIV Definition

   The KRB_PRIV message contains user data encrypted in the Session Key.
   The message fields are as follows:

   KRB-PRIV        ::= [APPLICATION 21] SEQUENCE {
           pvno            [0] INTEGER (5),
           msg-type        [1] INTEGER (21),
                           -- NOTE: there is no [2] tag
           enc-part        [3] EncryptedData -- EncKrbPrivPart
   }

   EncKrbPrivPart  ::= [APPLICATION 28] SEQUENCE {
           user-data       [0] OCTET STRING,
           timestamp       [1] KerberosTime OPTIONAL,
           usec            [2] Microseconds OPTIONAL,
           seq-number      [3] UInt32 OPTIONAL,
           s-address       [4] HostAddress -- sender's addr --,
           r-address       [5] HostAddress OPTIONAL -- recip's addr
   }

   pvno and msg-type
      These fields are described above in Section 5.4.1.  msg-type is
      KRB_PRIV.

   enc-part
      This field holds an encoding of the EncKrbPrivPart sequence
      encrypted under the session key, with a key usage value of 13.
      This encrypted encoding is used for the enc-part field of the
      KRB-PRIV message.

   user-data, timestamp, usec, s-address, and r-address
      These fields are described above in Section 5.6.1.

   seq-number
      This field is described above in Section 5.3.2.

5.8.  KRB_CRED Message Specification

   This section specifies the format of a message that can be used to
   send Kerberos credentials from one principal to another.  It is
   presented here to encourage a common mechanism to be used by
   applications when forwarding tickets or providing proxies to
   subordinate servers.  It presumes that a session key has already been
   exchanged, perhaps by using the KRB_AP_REQ/KRB_AP_REP messages.

5.8.1.  KRB_CRED Definition

   The KRB_CRED message contains a sequence of tickets to be sent and
   information needed to use the tickets, including the session key from
   each.  The information needed to use the tickets is encrypted under
   an encryption key previously exchanged or transferred alongside the
   KRB_CRED message.  The message fields are as follows:

   KRB-CRED        ::= [APPLICATION 22] SEQUENCE {
           pvno            [0] INTEGER (5),
           msg-type        [1] INTEGER (22),
           tickets         [2] SEQUENCE OF Ticket,
           enc-part        [3] EncryptedData -- EncKrbCredPart
   }

   EncKrbCredPart  ::= [APPLICATION 29] SEQUENCE {
           ticket-info     [0] SEQUENCE OF KrbCredInfo,
           nonce           [1] UInt32 OPTIONAL,
           timestamp       [2] KerberosTime OPTIONAL,
           usec            [3] Microseconds OPTIONAL,
           s-address       [4] HostAddress OPTIONAL,
           r-address       [5] HostAddress OPTIONAL
   }

   KrbCredInfo     ::= SEQUENCE {
           key             [0] EncryptionKey,
           prealm          [1] Realm OPTIONAL,
           pname           [2] PrincipalName OPTIONAL,
           flags           [3] TicketFlags OPTIONAL,
           authtime        [4] KerberosTime OPTIONAL,
           starttime       [5] KerberosTime OPTIONAL,
           endtime         [6] KerberosTime OPTIONAL,
           renew-till      [7] KerberosTime OPTIONAL,
           srealm          [8] Realm OPTIONAL,
           sname           [9] PrincipalName OPTIONAL,
           caddr           [10] HostAddresses OPTIONAL
   }

   pvno and msg-type
      These fields are described above in Section 5.4.1.  msg-type is
      KRB_CRED.

   tickets
      These are the tickets obtained from the KDC specifically for use
      by the intended recipient.  Successive tickets are paired with the
      corresponding KrbCredInfo sequence from the enc-part of the KRB-
      CRED message.

   enc-part
      This field holds an encoding of the EncKrbCredPart sequence
      encrypted under the session key shared by the sender and the
      intended recipient, with a key usage value of 14.  This encrypted
      encoding is used for the enc-part field of the KRB-CRED message.

      Implementation note: Implementations of certain applications, most
      notably certain implementations of the Kerberos GSS-API mechanism,
      do not separately encrypt the contents of the EncKrbCredPart of
      the KRB-CRED message when sending it.  In the case of those GSS-
      API mechanisms, this is not a security vulnerability, as the
      entire KRB-CRED message is itself embedded in an encrypted
      message.

   nonce
      If practical, an application MAY require the inclusion of a nonce
      generated by the recipient of the message.  If the same value is
      included as the nonce in the message, it provides evidence that
      the message is fresh and has not been replayed by an attacker.  A
      nonce MUST NEVER be reused.

   timestamp and usec
      These fields specify the time that the KRB-CRED message was
      generated.  The time is used to provide assurance that the message
      is fresh.

   s-address and r-address
      These fields are described above in Section 5.6.1.  They are used
      optionally to provide additional assurance of the integrity of the
      KRB-CRED message.

   key
      This field exists in the corresponding ticket passed by the KRB-
      CRED message and is used to pass the session key from the sender
      to the intended recipient.  The field's encoding is described in
      Section 5.2.9.

   The following fields are optional.  If present, they can be
   associated with the credentials in the remote ticket file.  If left
   out, then it is assumed that the recipient of the credentials already
   knows their values.

   prealm and pname
      The name and realm of the delegated principal identity.

   flags, authtime, starttime, endtime, renew-till, srealm, sname,
   and caddr
      These fields contain the values of the corresponding fields from
      the ticket found in the ticket field.  Descriptions of the fields
      are identical to the descriptions in the KDC-REP message.

5.9.  Error Message Specification

   This section specifies the format for the KRB_ERROR message.  The
   fields included in the message are intended to return as much
   information as possible about an error.  It is not expected that all
   the information required by the fields will be available for all
   types of errors.  If the appropriate information is not available
   when the message is composed, the corresponding field will be left
   out of the message.

   Note that because the KRB_ERROR message is not integrity protected,
   it is quite possible for an intruder to synthesize or modify it.  In
   particular, this means that the client SHOULD NOT use any fields in
   this message for security-critical purposes, such as setting a system
   clock or generating a fresh authenticator.  The message can be
   useful, however, for advising a user on the reason for some failure.

5.9.1.  KRB_ERROR Definition

   The KRB_ERROR message consists of the following fields:

   KRB-ERROR       ::= [APPLICATION 30] SEQUENCE {
           pvno            [0] INTEGER (5),
           msg-type        [1] INTEGER (30),
           ctime           [2] KerberosTime OPTIONAL,
           cusec           [3] Microseconds OPTIONAL,
           stime           [4] KerberosTime,
           susec           [5] Microseconds,
           error-code      [6] Int32,
           crealm          [7] Realm OPTIONAL,
           cname           [8] PrincipalName OPTIONAL,
           realm           [9] Realm -- service realm --,
           sname           [10] PrincipalName -- service name --,
           e-text          [11] KerberosString OPTIONAL,

           e-data          [12] OCTET STRING OPTIONAL
   }

   pvno and msg-type
      These fields are described above in Section 5.4.1.  msg-type is
      KRB_ERROR.

   ctime and cusec
      These fields are described above in Section 5.5.2.  If the values
      for these fields are known to the entity generating the error (as
      they would be if the KRB-ERROR is generated in reply to, e.g., a
      failed authentication service request), they should be populated
      in the KRB-ERROR.  If the values are not available, these fields
      can be omitted.

   stime
      This field contains the current time on the server.  It is of type
      KerberosTime.

   susec
      This field contains the microsecond part of the server's
      timestamp.  Its value ranges from 0 to 999999.  It appears along
      with stime.  The two fields are used in conjunction to specify a
      reasonably accurate timestamp.

   error-code
      This field contains the error code returned by Kerberos or the
      server when a request fails.  To interpret the value of this field
      see the list of error codes in Section 7.5.9.  Implementations are
      encouraged to provide for national language support in the display
      of error messages.

   crealm, and cname
      These fields are described above in Section 5.3.  When the entity
      generating the error knows these values, they should be populated
      in the KRB-ERROR.  If the values are not known, the crealm and
      cname fields SHOULD be omitted.

   realm and sname
      These fields are described above in Section 5.3.

   e-text
      This field contains additional text to help explain the error code
      associated with the failed request (for example, it might include
      a principal name which was unknown).

   e-data
      This field contains additional data about the error for use by the
      application to help it recover from or handle the error.  If the
      errorcode is KDC_ERR_PREAUTH_REQUIRED, then the e-data field will
      contain an encoding of a sequence of padata fields, each
      corresponding to an acceptable pre-authentication method and
      optionally containing data for the method:

      METHOD-DATA     ::= SEQUENCE OF PA-DATA

   For error codes defined in this document other than
   KDC_ERR_PREAUTH_REQUIRED, the format and contents of the e-data field
   are implementation-defined.  Similarly, for future error codes, the
   format and contents of the e-data field are implementation-defined
   unless specified otherwise.  Whether defined by the implementation or
   in a future document, the e-data field MAY take the form of TYPED-
   DATA:

   TYPED-DATA      ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE {
           data-type       [0] Int32,
           data-value      [1] OCTET STRING OPTIONAL
   }

5.10.  Application Tag Numbers

   The following table lists the application class tag numbers used by
   various data types defined in this section.

   Tag Number(s)  Type Name      Comments

   0                             unused

   1              Ticket         PDU

   2              Authenticator  non-PDU

   3              EncTicketPart  non-PDU

   4-9                           unused

   10             AS-REQ         PDU

   11             AS-REP         PDU

   12             TGS-REQ        PDU

   13             TGS-REP        PDU

   14             AP-REQ         PDU

   15             AP-REP         PDU

   16             RESERVED16     TGT-REQ (for user-to-user)

   17             RESERVED17     TGT-REP (for user-to-user)

   18-19                         unused

   20             KRB-SAFE       PDU

   21             KRB-PRIV       PDU

   22             KRB-CRED       PDU

   23-24                         unused

   25             EncASRepPart   non-PDU

   26             EncTGSRepPart  non-PDU

   27             EncApRepPart   non-PDU

   28             EncKrbPrivPart non-PDU

   29             EncKrbCredPart non-PDU

   30             KRB-ERROR      PDU

   The ASN.1 types marked above as "PDU" (Protocol Data Unit) are the
   only ASN.1 types intended as top-level types of the Kerberos
   protocol, and are the only types that may be used as elements in
   another protocol that makes use of Kerberos.

6.  Naming Constraints

6.1.  Realm Names

   Although realm names are encoded as GeneralStrings and technically a
   realm can select any name it chooses, interoperability across realm
   boundaries requires agreement on how realm names are to be assigned,
   and what information they imply.

   To enforce these conventions, each realm MUST conform to the
   conventions itself, and it MUST require that any realms with which
   inter-realm keys are shared also conform to the conventions and
   require the same from its neighbors.

   Kerberos realm names are case sensitive.  Realm names that differ
   only in the case of the characters are not equivalent.  There are
   presently three styles of realm names: domain, X500, and other.
   Examples of each style follow:

        domain:   ATHENA.MIT.EDU
          X500:   C=US/O=OSF
         other:   NAMETYPE:rest/of.name=without-restrictions

   Domain style realm names MUST look like domain names: they consist of
   components separated by periods (.) and they contain neither colons
   (:) nor slashes (/).  Though domain names themselves are case
   insensitive, in order for realms to match, the case must match as
   well.  When establishing a new realm name based on an internet domain
   name it is recommended by convention that the characters be converted
   to uppercase.

   X.500 names contain an equals sign (=) and cannot contain a colon (:)
   before the equals sign.  The realm names for X.500 names will be
   string representations of the names with components separated by
   slashes.  Leading and trailing slashes will not be included.  Note
   that the slash separator is consistent with Kerberos implementations
   based on RFC 1510, but it is different from the separator recommended
   in RFC 2253.

   Names that fall into the other category MUST begin with a prefix that
   contains no equals sign (=) or period (.), and the prefix MUST be
   followed by a colon (:) and the rest of the name.  All prefixes
   expect those beginning with used.  Presently none are assigned.

   The reserved category includes strings that do not fall into the
   first three categories.  All names in this category are reserved.  It
   is unlikely that names will be assigned to this category unless there
   is a very strong argument for not using the 'other' category.

   These rules guarantee that there will be no conflicts between the
   various name styles.  The following additional constraints apply to
   the assignment of realm names in the domain and X.500 categories:
   either the name of a realm for the domain or X.500 formats must be
   used by the organization owning (to whom it was assigned) an Internet
   domain name or X.500 name, or, in the case that no such names are
   registered, authority to use a realm name MAY be derived from the
   authority of the parent realm.  For example, if there is no domain
   name for E40.MIT.EDU, then the administrator of the MIT.EDU realm can
   authorize the creation of a realm with that name.

   This is acceptable because the organization to which the parent is
   assigned is presumably the organization authorized to assign names to

   its children in the X.500 and domain name systems as well.  If the
   parent assigns a realm name without also registering it in the domain
   name or X.500 hierarchy, it is the parent's responsibility to make
   sure that in the future there will not exist a name identical to the
   realm name of the child unless it is assigned to the same entity as
   the realm name.

6.2.  Principal Names

   As was the case for realm names, conventions are needed to ensure
   that all agree on what information is implied by a principal name.
   The name-type field that is part of the principal name indicates the
   kind of information implied by the name.  The name-type SHOULD be
   treated only as a hint to interpreting the meaning of a name.  It is
   not significant when checking for equivalence.  Principal names that
   differ only in the name-type identify the same principal.  The name
   type does not partition the name space.  Ignoring the name type, no
   two names can be the same (i.e., at least one of the components, or
   the realm, MUST be different).  The following name types are defined:

   Name Type       Value  Meaning

   NT-UNKNOWN        0    Name type not known
   NT-PRINCIPAL      1    Just the name of the principal as in DCE,
                            or for users
   NT-SRV-INST       2    Service and other unique instance (krbtgt)
   NT-SRV-HST        3    Service with host name as instance
                            (telnet, rcommands)
   NT-SRV-XHST       4    Service with host as remaining components
   NT-UID            5    Unique ID
   NT-X500-PRINCIPAL 6    Encoded X.509 Distinguished name [RFC2253]
   NT-SMTP-NAME      7    Name in form of SMTP email name
                            (e.g., user@example.com)
   NT-ENTERPRISE    10    Enterprise name - may be mapped to principal
                            name

   When a name implies no information other than its uniqueness at a
   particular time, the name type PRINCIPAL SHOULD be used.  The
   principal name type SHOULD be used for users, and it might also be
   used for a unique server.  If the name is a unique machine-generated
   ID that is guaranteed never to be reassigned, then the name type of
   UID SHOULD be used.  (Note that it is generally a bad idea to
   reassign names of any type since stale entries might remain in access
   control lists.)

   If the first component of a name identifies a service and the
   remaining components identify an instance of the service in a
   server-specified manner, then the name type of SRV-INST SHOULD be

   used.  An example of this name type is the Kerberos ticket-granting
   service whose name has a first component of krbtgt and a second
   component identifying the realm for which the ticket is valid.

   If the first component of a name identifies a service and there is a
   single component following the service name identifying the instance
   as the host on which the server is running, then the name type
   SRV-HST SHOULD be used.  This type is typically used for Internet
   services such as telnet and the Berkeley R commands.  If the separate
   components of the host name appear as successive components following
   the name of the service, then the name type SRV-XHST SHOULD be used.
   This type might be used to identify servers on hosts with X.500
   names, where the slash (/) might otherwise be ambiguous.

   A name type of NT-X500-PRINCIPAL SHOULD be used when a name from an
   X.509 certificate is translated into a Kerberos name.  The encoding
   of the X.509 name as a Kerberos principal shall conform to the
   encoding rules specified in RFC 2253.

   A name type of SMTP allows a name to be of a form that resembles an
   SMTP email name.  This name, including an "@" and a domain name, is
   used as the one component of the principal name.

   A name type of UNKNOWN SHOULD be used when the form of the name is
   not known.  When comparing names, a name of type UNKNOWN will match
   principals authenticated with names of any type.  A principal
   authenticated with a name of type UNKNOWN, however, will only match
   other names of type UNKNOWN.

   Names of any type with an initial component of 'krbtgt' are reserved
   for the Kerberos ticket-granting service.  See Section 7.3 for the
   form of such names.

6.2.1.  Name of Server Principals

   The principal identifier for a server on a host will generally be
   composed of two parts: (1) the realm of the KDC with which the server
   is registered, and (2) a two-component name of type NT-SRV-HST, if
   the host name is an Internet domain name, or a multi-component name
   of type NT-SRV-XHST, if the name of the host is of a form (such as
   X.500) that allows slash (/) separators.  The first component of the
   two- or multi-component name will identify the service, and the
   latter components will identify the host.  Where the name of the host
   is not case sensitive (for example, with Internet domain names) the
   name of the host MUST be lowercase.  If specified by the application
   protocol for services such as telnet and the Berkeley R commands that
   run with system privileges, the first component MAY be the string
   'host' instead of a service-specific identifier.

7.  Constants and Other Defined Values

7.1.  Host Address Types

   All negative values for the host address type are reserved for local
   use.  All non-negative values are reserved for officially assigned
   type fields and interpretations.

   Internet (IPv4) Addresses

      Internet (IPv4) addresses are 32-bit (4-octet) quantities, encoded
      in MSB order (most significant byte first).  The IPv4 loopback
      address SHOULD NOT appear in a Kerberos PDU.  The type of IPv4
      addresses is two (2).

   Internet (IPv6) Addresses

      IPv6 addresses [RFC3513] are 128-bit (16-octet) quantities,
      encoded in MSB order (most significant byte first).  The type of
      IPv6 addresses is twenty-four (24).  The following addresses MUST
      NOT appear in any Kerberos PDU:

         *  the Unspecified Address
         *  the Loopback Address
         *  Link-Local addresses

      This restriction applies to the inclusion in the address fields of
      Kerberos PDUs, but not to the address fields of packets that might
      carry such PDUs.  The restriction is necessary because the use of
      an address with non-global scope could allow the acceptance of a
      message sent from a node that may have the same address, but which
      is not the host intended by the entity that added the restriction.
      If the link-local address type needs to be used for communication,
      then the address restriction in tickets must not be used (i.e.,
      addressless tickets must be used).

      IPv4-mapped IPv6 addresses MUST be represented as addresses of
      type 2.

   DECnet Phase IV Addresses

      DECnet Phase IV addresses are 16-bit addresses, encoded in LSB
      order.  The type of DECnet Phase IV addresses is twelve (12).

   Netbios Addresses

      Netbios addresses are 16-octet addresses typically composed of 1
      to 15 alphanumeric characters and padded with the US-ASCII SPC
      character (code 32).  The 16th octet MUST be the US-ASCII NUL
      character (code 0).  The type of Netbios addresses is twenty (20).

   Directional Addresses

      Including the sender address in KRB_SAFE and KRB_PRIV messages is
      undesirable in many environments because the addresses may be
      changed in transport by network address translators.  However, if
      these addresses are removed, the messages may be subject to a
      reflection attack in which a message is reflected back to its
      originator.  The directional address type provides a way to avoid
      transport addresses and reflection attacks.  Directional addresses
      are encoded as four-byte unsigned integers in network byte order.
      If the message is originated by the party sending the original
      KRB_AP_REQ message, then an address of 0 SHOULD be used.  If the
      message is originated by the party to whom that KRB_AP_REQ was
      sent, then the address 1 SHOULD be used.  Applications involving
      multiple parties can specify the use of other addresses.

      Directional addresses MUST only be used for the sender address
      field in the KRB_SAFE or KRB_PRIV messages.  They MUST NOT be used
      as a ticket address or in a KRB_AP_REQ message.  This address type
      SHOULD only be used in situations where the sending party knows
      that the receiving party supports the address type.  This
      generally means that directional addresses may only be used when
      the application protocol requires their support.  Directional
      addresses are type (3).

7.2.  KDC Messaging: IP Transports

   Kerberos defines two IP transport mechanisms for communication
   between clients and servers: UDP/IP and TCP/IP.

7.2.1.  UDP/IP transport

   Kerberos servers (KDCs) supporting IP transports MUST accept UDP
   requests and SHOULD listen for them on port 88 (decimal) unless
   specifically configured to listen on an alternative UDP port.
   Alternate ports MAY be used when running multiple KDCs for multiple
   realms on the same host.

   Kerberos clients supporting IP transports SHOULD support the sending
   of UDP requests.  Clients SHOULD use KDC discovery [7.2.3] to
   identify the IP address and port to which they will send their
   request.

   When contacting a KDC for a KRB_KDC_REQ request using UDP/IP
   transport, the client shall send a UDP datagram containing only an
   encoding of the request to the KDC.  The KDC will respond with a
   reply datagram containing only an encoding of the reply message
   (either a KRB_ERROR or a KRB_KDC_REP) to the sending port at the
   sender's IP address.  The response to a request made through UDP/IP
   transport MUST also use UDP/IP transport.  If the response cannot be
   handled using UDP (for example, because it is too large), the KDC
   MUST return KRB_ERR_RESPONSE_TOO_BIG, forcing the client to retry the
   request using the TCP transport.

7.2.2.  TCP/IP Transport

   Kerberos servers (KDCs) supporting IP transports MUST accept TCP
   requests and SHOULD listen for them on port 88 (decimal) unless
   specifically configured to listen on an alternate TCP port.
   Alternate ports MAY be used when running multiple KDCs for multiple
   realms on the same host.

   Clients MUST support the sending of TCP requests, but MAY choose to
   try a request initially using the UDP transport.  Clients SHOULD use
   KDC discovery [7.2.3] to identify the IP address and port to which
   they will send their request.

   Implementation note: Some extensions to the Kerberos protocol will
   not succeed if any client or KDC not supporting the TCP transport is
   involved.  Implementations of RFC 1510 were not required to support
   TCP/IP transports.

   When the KRB_KDC_REQ message is sent to the KDC over a TCP stream,
   the response (KRB_KDC_REP or KRB_ERROR message) MUST be returned to
   the client on the same TCP stream that was established for the
   request.  The KDC MAY close the TCP stream after sending a response,
   but MAY leave the stream open for a reasonable period of time if it
   expects a follow-up.  Care must be taken in managing TCP/IP
   connections on the KDC to prevent denial of service attacks based on
   the number of open TCP/IP connections.

   The client MUST be prepared to have the stream closed by the KDC at
   any time after the receipt of a response.  A stream closure SHOULD
   NOT be treated as a fatal error.  Instead, if multiple exchanges are
   required (e.g., certain forms of pre-authentication), the client may
   need to establish a new connection when it is ready to send

   subsequent messages.  A client MAY close the stream after receiving a
   response, and SHOULD close the stream if it does not expect to send
   follow-up messages.

   A client MAY send multiple requests before receiving responses,
   though it must be prepared to handle the connection being closed
   after the first response.

   Each request (KRB_KDC_REQ) and response (KRB_KDC_REP or KRB_ERROR)
   sent over the TCP stream is preceded by the length of the request as
   4 octets in network byte order.  The high bit of the length is
   reserved for future expansion and MUST currently be set to zero.  If
   a KDC that does not understand how to interpret a set high bit of the
   length encoding receives a request with the high order bit of the
   length set, it MUST return a KRB-ERROR message with the error
   KRB_ERR_FIELD_TOOLONG and MUST close the TCP stream.

   If multiple requests are sent over a single TCP connection and the
   KDC sends multiple responses, the KDC is not required to send the
   responses in the order of the corresponding requests.  This may
   permit some implementations to send each response as soon as it is
   ready, even if earlier requests are still being processed (for
   example, waiting for a response from an external device or database).

7.2.3.  KDC Discovery on IP Networks

   Kerberos client implementations MUST provide a means for the client
   to determine the location of the Kerberos Key Distribution Centers
   (KDCs).  Traditionally, Kerberos implementations have stored such
   configuration information in a file on each client machine.
   Experience has shown that this method of storing configuration
   information presents problems with out-of-date information and
   scaling, especially when using cross-realm authentication.  This
   section describes a method for using the Domain Name System [RFC1035]
   for storing KDC location information.

7.2.3.1.  DNS vs. Kerberos: Case Sensitivity of Realm Names

   In Kerberos, realm names are case sensitive.  Although it is strongly
   encouraged that all realm names be all uppercase, this recommendation
   has not been adopted by all sites.  Some sites use all lowercase
   names and other use mixed case.  DNS, on the other hand, is case
   insensitive for queries.  Because the realm names "MYREALM",
   "myrealm", and "MyRealm" are all different, but resolve the same in
   the domain name system, it is necessary that only one of the possible
   combinations of upper- and lowercase characters be used in realm
   names.

7.2.3.2.  Specifying KDC Location Information with DNS SRV records

   KDC location information is to be stored using the DNS SRV RR
   [RFC2782].  The format of this RR is as follows:

      _Service._Proto.Realm TTL Class SRV Priority Weight Port Target

   The Service name for Kerberos is always "kerberos".

   The Proto can be either "udp" or "tcp".  If these SRV records are to
   be used, both "udp" and "tcp" records MUST be specified for all KDC
   deployments.

   The Realm is the Kerberos realm that this record corresponds to.  The
   realm MUST be a domain-style realm name.

   TTL, Class, SRV, Priority, Weight, and Target have the standard
   meaning as defined in RFC 2782.

   As per RFC 2782, the Port number used for "_udp" and "_tcp" SRV
   records SHOULD be the value assigned to "kerberos" by the Internet
   Assigned Number Authority: 88 (decimal), unless the KDC is configured
   to listen on an alternate TCP port.

   Implementation note: Many existing client implementations do not
   support KDC Discovery and are configured to send requests to the IANA
   assigned port (88 decimal), so it is strongly recommended that KDCs
   be configured to listen on that port.

7.2.3.3.  KDC Discovery for Domain Style Realm Names on IP Networks

   These are DNS records for a Kerberos realm EXAMPLE.COM.  It has two
   Kerberos servers, kdc1.example.com and kdc2.example.com.  Queries
   should be directed to kdc1.example.com first as per the specified
   priority.  Weights are not used in these sample records.

     _kerberos._udp.EXAMPLE.COM.     IN   SRV   0 0 88 kdc1.example.com.
     _kerberos._udp.EXAMPLE.COM.     IN   SRV   1 0 88 kdc2.example.com.
     _kerberos._tcp.EXAMPLE.COM.     IN   SRV   0 0 88 kdc1.example.com.
     _kerberos._tcp.EXAMPLE.COM.     IN   SRV   1 0 88 kdc2.example.com.

7.3.  Name of the TGS

   The principal identifier of the ticket-granting service shall be
   composed of three parts: the realm of the KDC issuing the TGS ticket,
   and a two-part name of type NT-SRV-INST, with the first part "krbtgt"
   and the second part the name of the realm that will accept the TGT.
   For example, a TGT issued by the ATHENA.MIT.EDU realm to be used to

   get tickets from the ATHENA.MIT.EDU KDC has a principal identifier of
   "ATHENA.MIT.EDU" (realm), ("krbtgt", "ATHENA.MIT.EDU") (name).  A TGT
   issued by the ATHENA.MIT.EDU realm to be used to get tickets from the
   MIT.EDU realm has a principal identifier of "ATHENA.MIT.EDU" (realm),
   ("krbtgt", "MIT.EDU") (name).

7.4.  OID Arc for KerberosV5

   This OID MAY be used to identify Kerberos protocol messages
   encapsulated in other protocols.  It also designates the OID arc for
   KerberosV5-related OIDs assigned by future IETF action.
   Implementation note: RFC 1510 had an incorrect value (5) for "dod" in
   its OID.

   id-krb5         OBJECT IDENTIFIER ::= {
           iso(1) identified-organization(3) dod(6) internet(1)
           security(5) kerberosV5(2)
   }

   Assignment of OIDs beneath the id-krb5 arc must be obtained by
   contacting the registrar for the id-krb5 arc, or its designee.  At
   the time of the issuance of this RFC, such registrations can be
   obtained by contacting krb5-oid-registrar@mit.edu.

7.5.  Protocol Constants and Associated Values

   The following tables list constants used in the protocol and define
   their meanings.  In the "specification" section, ranges are specified
   that limit the values of constants for which values are defined here.
   This allows implementations to make assumptions about the maximum
   values that will be received for these constants.  Implementations
   receiving values outside the range specified in the "specification"
   section MAY reject the request, but they MUST recover cleanly.

7.5.1.  Key Usage Numbers

   The encryption and checksum specifications in [RFC3961] require as
   input a "key usage number", to alter the encryption key used in any
   specific message in order to make certain types of cryptographic
   attack more difficult.  These are the key usage values assigned in
   this document:

           1.  AS-REQ PA-ENC-TIMESTAMP padata timestamp, encrypted with
               the client key (Section 5.2.7.2)

           2.  AS-REP Ticket and TGS-REP Ticket (includes TGS session
               key or application session key), encrypted with the
               service key (Section 5.3)
           3.  AS-REP encrypted part (includes TGS session key or
               application session key), encrypted with the client key
               (Section 5.4.2)
           4.  TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with
               the TGS session key (Section 5.4.1)
           5.  TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with
               the TGS authenticator subkey (Section 5.4.1)
           6.  TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator cksum,
               keyed with the TGS session key (Section 5.5.1)
           7.  TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator (includes
               TGS authenticator subkey), encrypted with the TGS session
               key (Section 5.5.1)
           8.  TGS-REP encrypted part (includes application session
               key), encrypted with the TGS session key (Section 5.4.2)
           9.  TGS-REP encrypted part (includes application session
               key), encrypted with the TGS authenticator subkey
               (Section 5.4.2)
          10.  AP-REQ Authenticator cksum, keyed with the application
               session key (Section 5.5.1)
          11.  AP-REQ Authenticator (includes application authenticator
               subkey), encrypted with the application session key
               (Section 5.5.1)
          12.  AP-REP encrypted part (includes application session
               subkey), encrypted with the application session key
               (Section 5.5.2)
          13.  KRB-PRIV encrypted part, encrypted with a key chosen by
               the application (Section 5.7.1)
          14.  KRB-CRED encrypted part, encrypted with a key chosen by
               the application (Section 5.8.1)
          15.  KRB-SAFE cksum, keyed with a key chosen by the
               application (Section 5.6.1)
       16-18.  Reserved for future use in Kerberos and related
               protocols.
          19.  AD-KDC-ISSUED checksum (ad-checksum in 5.2.6.4)
       20-21.  Reserved for future use in Kerberos and related
               protocols.
       22-25.  Reserved for use in the Kerberos Version 5 GSS-API
               mechanisms [RFC4121].
      26-511.  Reserved for future use in Kerberos and related
               protocols.
    512-1023.  Reserved for uses internal to a Kerberos implementation.
        1024.  Encryption for application use in protocols that do not
               specify key usage values

        1025.  Checksums for application use in protocols that do not
               specify key usage values
   1026-2047.  Reserved for application use.

7.5.2.  PreAuthentication Data Types

   Padata and Data Type    Padata-type   Comment
                            Value

   PA-TGS-REQ                  1
   PA-ENC-TIMESTAMP            2
   PA-PW-SALT                  3
   [reserved]                  4
   PA-ENC-UNIX-TIME            5        (deprecated)
   PA-SANDIA-SECUREID          6
   PA-SESAME                   7
   PA-OSF-DCE                  8
   PA-CYBERSAFE-SECUREID       9
   PA-AFS3-SALT                10
   PA-ETYPE-INFO               11
   PA-SAM-CHALLENGE            12       (sam/otp)
   PA-SAM-RESPONSE             13       (sam/otp)
   PA-PK-AS-REQ_OLD            14       (pkinit)
   PA-PK-AS-REP_OLD            15       (pkinit)
   PA-PK-AS-REQ                16       (pkinit)
   PA-PK-AS-REP                17       (pkinit)
   PA-ETYPE-INFO2              19       (replaces pa-etype-info)
   PA-USE-SPECIFIED-KVNO       20
   PA-SAM-REDIRECT             21       (sam/otp)
   PA-GET-FROM-TYPED-DATA      22       (embedded in typed data)
   TD-PADATA                   22       (embeds padata)
   PA-SAM-ETYPE-INFO           23       (sam/otp)
   PA-ALT-PRINC                24       (crawdad@fnal.gov)
   PA-SAM-CHALLENGE2           30       (kenh@pobox.com)
   PA-SAM-RESPONSE2            31       (kenh@pobox.com)
   PA-EXTRA-TGT                41       Reserved extra TGT
   TD-PKINIT-CMS-CERTIFICATES  101      CertificateSet from CMS
   TD-KRB-PRINCIPAL            102      PrincipalName
   TD-KRB-REALM                103      Realm
   TD-TRUSTED-CERTIFIERS       104      from PKINIT
   TD-CERTIFICATE-INDEX        105      from PKINIT
   TD-APP-DEFINED-ERROR        106      application specific
   TD-REQ-NONCE                107      INTEGER
   TD-REQ-SEQ                  108      INTEGER
   PA-PAC-REQUEST              128      (jbrezak@exchange.microsoft.com)

7.5.3.  Address Types

   Address Type                   Value

   IPv4                             2
   Directional                      3
   ChaosNet                         5
   XNS                              6
   ISO                              7
   DECNET Phase IV                 12
   AppleTalk DDP                   16
   NetBios                         20
   IPv6                            24

7.5.4.  Authorization Data Types

   Authorization Data Type          Ad-type Value

   AD-IF-RELEVANT                     1
   AD-INTENDED-FOR-SERVER             2
   AD-INTENDED-FOR-APPLICATION-CLASS  3
   AD-KDC-ISSUED                      4
   AD-AND-OR                          5
   AD-MANDATORY-TICKET-EXTENSIONS     6
   AD-IN-TICKET-EXTENSIONS            7
   AD-MANDATORY-FOR-KDC               8
   Reserved values                 9-63
   OSF-DCE                           64
   SESAME                            65
   AD-OSF-DCE-PKI-CERTID             66 (hemsath@us.ibm.com)
   AD-WIN2K-PAC                     128 (jbrezak@exchange.microsoft.com)
   AD-ETYPE-NEGOTIATION             129  (lzhu@windows.microsoft.com)

7.5.5.  Transited Encoding Types

   Transited Encoding Type         Tr-type Value

   DOMAIN-X500-COMPRESS            1
   Reserved values                 All others

7.5.6.  Protocol Version Number

   Label               Value   Meaning or MIT Code

   pvno                  5     Current Kerberos protocol version number

7.5.7.  Kerberos Message Types

   Message Type   Value  Meaning

   KRB_AS_REQ      10    Request for initial authentication
   KRB_AS_REP      11    Response to KRB_AS_REQ request
   KRB_TGS_REQ     12    Request for authentication based on TGT
   KRB_TGS_REP     13    Response to KRB_TGS_REQ request
   KRB_AP_REQ      14    Application request to server
   KRB_AP_REP      15    Response to KRB_AP_REQ_MUTUAL
   KRB_RESERVED16  16    Reserved for user-to-user krb_tgt_request
   KRB_RESERVED17  17    Reserved for user-to-user krb_tgt_reply
   KRB_SAFE        20    Safe (checksummed) application message
   KRB_PRIV        21    Private (encrypted) application message
   KRB_CRED        22    Private (encrypted) message to forward
                           credentials
   KRB_ERROR       30    Error response

7.5.8.  Name Types

   Name Type           Value  Meaning

   KRB_NT_UNKNOWN        0    Name type not known
   KRB_NT_PRINCIPAL      1    Just the name of the principal as in DCE,
                                or for users
   KRB_NT_SRV_INST       2    Service and other unique instance (krbtgt)
   KRB_NT_SRV_HST        3    Service with host name as instance
                                (telnet, rcommands)
   KRB_NT_SRV_XHST       4    Service with host as remaining components
   KRB_NT_UID            5    Unique ID
   KRB_NT_X500_PRINCIPAL 6    Encoded X.509 Distinguished name [RFC2253]
   KRB_NT_SMTP_NAME      7    Name in form of SMTP email name
                                (e.g., user@example.com)
   KRB_NT_ENTERPRISE    10    Enterprise name; may be mapped to
                                principal name

7.5.9.  Error Codes

   Error Code                         Value  Meaning

   KDC_ERR_NONE                           0  No error
   KDC_ERR_NAME_EXP                       1  Client's entry in database
                                               has expired
   KDC_ERR_SERVICE_EXP                    2  Server's entry in database
                                               has expired
   KDC_ERR_BAD_PVNO                       3  Requested protocol version
                                               number not supported

   KDC_ERR_C_OLD_MAST_KVNO                4  Client's key encrypted in
                                               old master key
   KDC_ERR_S_OLD_MAST_KVNO                5  Server's key encrypted in
                                               old master key
   KDC_ERR_C_PRINCIPAL_UNKNOWN            6  Client not found in
                                               Kerberos database
   KDC_ERR_S_PRINCIPAL_UNKNOWN            7  Server not found in
                                               Kerberos database
   KDC_ERR_PRINCIPAL_NOT_UNIQUE           8  Multiple principal entries
                                               in database
   KDC_ERR_NULL_KEY                       9  The client or server has a
                                               null key
   KDC_ERR_CANNOT_POSTDATE               10  Ticket not eligible for
                                               postdating
   KDC_ERR_NEVER_VALID                   11  Requested starttime is
                                               later than end time
   KDC_ERR_POLICY                        12  KDC policy rejects request
   KDC_ERR_BADOPTION                     13  KDC cannot accommodate
                                               requested option
   KDC_ERR_ETYPE_NOSUPP                  14  KDC has no support for
                                               encryption type
   KDC_ERR_SUMTYPE_NOSUPP                15  KDC has no support for
                                               checksum type
   KDC_ERR_PADATA_TYPE_NOSUPP            16  KDC has no support for
                                               padata type
   KDC_ERR_TRTYPE_NOSUPP                 17  KDC has no support for
                                               transited type
   KDC_ERR_CLIENT_REVOKED                18  Clients credentials have
                                               been revoked
   KDC_ERR_SERVICE_REVOKED               19  Credentials for server have
                                               been revoked
   KDC_ERR_TGT_REVOKED                   20  TGT has been revoked
   KDC_ERR_CLIENT_NOTYET                 21  Client not yet valid; try
                                               again later
   KDC_ERR_SERVICE_NOTYET                22  Server not yet valid; try
                                               again later
   KDC_ERR_KEY_EXPIRED                   23  Password has expired;
                                               change password to reset
   KDC_ERR_PREAUTH_FAILED                24  Pre-authentication
                                               information was invalid
   KDC_ERR_PREAUTH_REQUIRED              25  Additional pre-
                                               authentication required
   KDC_ERR_SERVER_NOMATCH                26  Requested server and ticket
                                               don't match
   KDC_ERR_MUST_USE_USER2USER            27  Server principal valid for
                                               user2user only
   KDC_ERR_PATH_NOT_ACCEPTED             28  KDC Policy rejects
                                               transited path

   KDC_ERR_SVC_UNAVAILABLE               29  A service is not available
   KRB_AP_ERR_BAD_INTEGRITY              31  Integrity check on
                                               decrypted field failed
   KRB_AP_ERR_TKT_EXPIRED                32  Ticket expired
   KRB_AP_ERR_TKT_NYV                    33  Ticket not yet valid
   KRB_AP_ERR_REPEAT                     34  Request is a replay
   KRB_AP_ERR_NOT_US                     35  The ticket isn't for us
   KRB_AP_ERR_BADMATCH                   36  Ticket and authenticator
                                               don't match
   KRB_AP_ERR_SKEW                       37  Clock skew too great
   KRB_AP_ERR_BADADDR                    38  Incorrect net address
   KRB_AP_ERR_BADVERSION                 39  Protocol version mismatch
   KRB_AP_ERR_MSG_TYPE                   40  Invalid msg type
   KRB_AP_ERR_MODIFIED                   41  Message stream modified
   KRB_AP_ERR_BADORDER                   42  Message out of order
   KRB_AP_ERR_BADKEYVER                  44  Specified version of key is
                                               not available
   KRB_AP_ERR_NOKEY                      45  Service key not available
   KRB_AP_ERR_MUT_FAIL                   46  Mutual authentication
                                               failed
   KRB_AP_ERR_BADDIRECTION               47  Incorrect message direction
   KRB_AP_ERR_METHOD                     48  Alternative authentication
                                               method required
   KRB_AP_ERR_BADSEQ                     49  Incorrect sequence number
                                               in message
   KRB_AP_ERR_INAPP_CKSUM                50  Inappropriate type of
                                               checksum in message
   KRB_AP_PATH_NOT_ACCEPTED              51  Policy rejects transited
                                               path
   KRB_ERR_RESPONSE_TOO_BIG              52  Response too big for UDP;
                                               retry with TCP
   KRB_ERR_GENERIC                       60  Generic error (description
                                               in e-text)
   KRB_ERR_FIELD_TOOLONG                 61  Field is too long for this
                                               implementation
   KDC_ERROR_CLIENT_NOT_TRUSTED          62  Reserved for PKINIT
   KDC_ERROR_KDC_NOT_TRUSTED             63  Reserved for PKINIT
   KDC_ERROR_INVALID_SIG                 64  Reserved for PKINIT
   KDC_ERR_KEY_TOO_WEAK                  65  Reserved for PKINIT
   KDC_ERR_CERTIFICATE_MISMATCH          66  Reserved for PKINIT
   KRB_AP_ERR_NO_TGT                     67  No TGT available to
                                               validate USER-TO-USER
   KDC_ERR_WRONG_REALM                   68  Reserved for future use
   KRB_AP_ERR_USER_TO_USER_REQUIRED      69  Ticket must be for
                                               USER-TO-USER
   KDC_ERR_CANT_VERIFY_CERTIFICATE       70  Reserved for PKINIT
   KDC_ERR_INVALID_CERTIFICATE           71  Reserved for PKINIT
   KDC_ERR_REVOKED_CERTIFICATE           72  Reserved for PKINIT

   KDC_ERR_REVOCATION_STATUS_UNKNOWN     73  Reserved for PKINIT
   KDC_ERR_REVOCATION_STATUS_UNAVAILABLE 74  Reserved for PKINIT
   KDC_ERR_CLIENT_NAME_MISMATCH          75  Reserved for PKINIT
   KDC_ERR_KDC_NAME_MISMATCH             76  Reserved for PKINIT

8.  Interoperability Requirements

   Version 5 of the Kerberos protocol supports a myriad of options.
   Among these are multiple encryption and checksum types; alternative
   encoding schemes for the transited field; optional mechanisms for
   pre-authentication; the handling of tickets with no addresses;
   options for mutual authentication; user-to-user authentication;
   support for proxies; the format of realm names; the handling of
   authorization data; and forwarding, postdating, and renewing tickets.

   In order to ensure the interoperability of realms, it is necessary to
   define a minimal configuration that must be supported by all
   implementations.  This minimal configuration is subject to change as
   technology does.  For example, if at some later date it is discovered
   that one of the required encryption or checksum algorithms is not
   secure, it will be replaced.

8.1.  Specification 2

   This section defines the second specification of these options.
   Implementations which are configured in this way can be said to
   support Kerberos Version 5 Specification 2 (5.2).  Specification 1
   (deprecated) may be found in RFC 1510.

   Transport

      TCP/IP and UDP/IP transport MUST be supported by clients and KDCs
      claiming conformance to specification 2.

   Encryption and Checksum Methods

      The following encryption and checksum mechanisms MUST be
      supported:

      Encryption: AES256-CTS-HMAC-SHA1-96 [RFC3962]
      Checksums: HMAC-SHA1-96-AES256 [RFC3962]

      Implementations SHOULD support other mechanisms as well, but the
      additional mechanisms may only be used when communicating with
      principals known to also support them.  The following mechanisms
      from [RFC3961] and [RFC3962] SHOULD be supported:

      Encryption: AES128-CTS-HMAC-SHA1-96, DES-CBC-MD5, DES3-CBC-SHA1-KD
      Checksums: DES-MD5, HMAC-SHA1-DES3-KD, HMAC-SHA1-96-AES128

      Implementations MAY support other mechanisms as well, but the
      additional mechanisms may only be used when communicating with
      principals known to support them also.

      Implementation note: Earlier implementations of Kerberos generate
      messages using the CRC-32 and RSA-MD5 checksum methods.  For
      interoperability with these earlier releases, implementors MAY
      consider supporting these checksum methods but should carefully
      analyze the security implications to limit the situations within
      which these methods are accepted.

   Realm Names

      All implementations MUST understand hierarchical realms in both
      the Internet Domain and the X.500 style.  When a TGT for an
      unknown realm is requested, the KDC MUST be able to determine the
      names of the intermediate realms between the KDCs realm and the
      requested realm.

   Transited Field Encoding

      DOMAIN-X500-COMPRESS (described in Section 3.3.3.2) MUST be
      supported.  Alternative encodings MAY be supported, but they may
      only be used when that encoding is supported by ALL intermediate
      realms.

   Pre-authentication Methods

      The TGS-REQ method MUST be supported.  It is not used on the
      initial request.  The PA-ENC-TIMESTAMP method MUST be supported by
      clients, but whether it is enabled by default MAY be determined on
      a realm-by-realm basis.  If the method is not used in the initial
      request and the error KDC_ERR_PREAUTH_REQUIRED is returned
      specifying PA-ENC-TIMESTAMP as an acceptable method, the client
      SHOULD retry the initial request using the PA-ENC-TIMESTAMP pre-
      authentication method.  Servers need not support the PA-ENC-
      TIMESTAMP method, but if it is not supported the server SHOULD
      ignore the presence of PA-ENC-TIMESTAMP pre-authentication in a
      request.

      The ETYPE-INFO2 method MUST be supported; this method is used to
      communicate the set of supported encryption types, and
      corresponding salt and string to key parameters.  The ETYPE-INFO
      method SHOULD be supported for interoperability with older
      implementation.

   Mutual Authentication

      Mutual authentication (via the KRB_AP_REP message) MUST be
      supported.

   Ticket Addresses and Flags

      All KDCs MUST pass through tickets that carry no addresses (i.e.,
      if a TGT contains no addresses, the KDC will return derivative
      tickets).  Implementations SHOULD default to requesting
      addressless tickets, as this significantly increases
      interoperability with network address translation.  In some cases,
      realms or application servers MAY require that tickets have an
      address.

      Implementations SHOULD accept directional address type for the
      KRB_SAFE and KRB_PRIV message and SHOULD include directional
      addresses in these messages when other address types are not
      available.

      Proxies and forwarded tickets MUST be supported.  Individual
      realms and application servers can set their own policy on when
      such tickets will be accepted.

      All implementations MUST recognize renewable and postdated
      tickets, but they need not actually implement them.  If these
      options are not supported, the starttime and endtime in the ticket
      SHALL specify a ticket's entire useful life.  When a postdated
      ticket is decoded by a server, all implementations SHALL make the
      presence of the postdated flag visible to the calling server.

   User-to-User Authentication

      Support for user-to-user authentication (via the ENC-TKT-IN-SKEY
      KDC option) MUST be provided by implementations, but individual
      realms MAY decide as a matter of policy to reject such requests on
      a per-principal or realm-wide basis.

   Authorization Data

      Implementations MUST pass all authorization data subfields from
      TGTs to any derivative tickets unless they are directed to
      suppress a subfield as part of the definition of that registered
      subfield type.  (It is never incorrect to pass on a subfield, and
      no registered subfield types presently specify suppression at the
      KDC.)

      Implementations MUST make the contents of any authorization data
      subfields available to the server when a ticket is used.
      Implementations are not required to allow clients to specify the
      contents of the authorization data fields.

   Constant Ranges

      All protocol constants are constrained to 32-bit (signed) values
      unless further constrained by the protocol definition.  This limit
      is provided to allow implementations to make assumptions about the
      maximum values that will be received for these constants.
      Implementations receiving values outside this range MAY reject the
      request, but they MUST recover cleanly.

8.2.  Recommended KDC Values

   Following is a list of recommended values for a KDC configuration.

      Minimum lifetime              5 minutes
      Maximum renewable lifetime    1 week
      Maximum ticket lifetime       1 day
      Acceptable clock skew         5 minutes
      Empty addresses               Allowed
      Proxiable, etc.               Allowed

9.  IANA Considerations

   Section 7 of this document specifies protocol constants and other
   defined values required for the interoperability of multiple
   implementations.  Until a subsequent RFC specifies otherwise, or the
   Kerberos working group is shut down, allocations of additional
   protocol constants and other defined values required for extensions
   to the Kerberos protocol will be administered by the Kerberos working
   group.  Following the recommendations outlined in [RFC2434], guidance
   is provided to the IANA as follows:

   "reserved" realm name types in Section 6.1 and "other" realm types
   except those beginning with "X-" or "x-" will not be registered
   without IETF standards action, at which point guidelines for further
   assignment will be specified.  Realm name types beginning with "X-"
   or "x-" are for private use.

   For host address types described in Section 7.1, negative values are
   for private use.  Assignment of additional positive numbers is
   subject to review by the Kerberos working group or other expert
   review.

   Additional key usage numbers, as defined in Section 7.5.1, will be
   assigned subject to review by the Kerberos working group or other
   expert review.

   Additional preauthentication data type values, as defined in section
   7.5.2, will be assigned subject to review by the Kerberos working
   group or other expert review.

   Additional authorization data types as defined in Section 7.5.4, will
   be assigned subject to review by the Kerberos working group or other
   expert review.  Although it is anticipated that there may be
   significant demand for private use types, provision is intentionally
   not made for a private use portion of the namespace because conflicts
   between privately assigned values could have detrimental security
   implications.

   Additional transited encoding types, as defined in Section 7.5.5,
   present special concerns for interoperability with existing
   implementations.  As such, such assignments will only be made by
   standards action, except that the Kerberos working group or another
   other working group with competent jurisdiction may make preliminary
   assignments for documents that are moving through the standards
   process.

   Additional Kerberos message types, as described in Section 7.5.7,
   will be assigned subject to review by the Kerberos working group or
   other expert review.

   Additional name types, as described in Section 7.5.8, will be
   assigned subject to review by the Kerberos working group or other
   expert review.

   Additional error codes described in Section 7.5.9 will be assigned
   subject to review by the Kerberos working group or other expert
   review.

10.  Security Considerations

   As an authentication service, Kerberos provides a means of verifying
   the identity of principals on a network.  By itself, Kerberos does
   not provide authorization.  Applications should not accept the
   issuance of a service ticket by the Kerberos server as granting
   authority to use the service, since such applications may become
   vulnerable to the bypass of this authorization check in an
   environment where they inter-operate with other KDCs or where other
   options for application authentication are provided.

   Denial of service attacks are not solved with Kerberos.  There are
   places in the protocols where an intruder can prevent an application
   from participating in the proper authentication steps.  Because
   authentication is a required step for the use of many services,
   successful denial of service attacks on a Kerberos server might
   result in the denial of other network services that rely on Kerberos
   for authentication.  Kerberos is vulnerable to many kinds of denial
   of service attacks: those on the network, which would prevent clients
   from contacting the KDC; those on the domain name system, which could
   prevent a client from finding the IP address of the Kerberos server;
   and those by overloading the Kerberos KDC itself with repeated
   requests.

   Interoperability conflicts caused by incompatible character-set usage
   (see 5.2.1) can result in denial of service for clients that utilize
   character-sets in Kerberos strings other than those stored in the KDC
   database.

   Authentication servers maintain a database of principals (i.e., users
   and servers) and their secret keys.  The security of the
   authentication server machines is critical.  The breach of security
   of an authentication server will compromise the security of all
   servers that rely upon the compromised KDC, and will compromise the
   authentication of any principals registered in the realm of the
   compromised KDC.

   Principals must keep their secret keys secret.  If an intruder
   somehow steals a principal's key, it will be able to masquerade as
   that principal or impersonate any server to the legitimate principal.

   Password-guessing attacks are not solved by Kerberos.  If a user
   chooses a poor password, it is possible for an attacker to
   successfully mount an off-line dictionary attack by repeatedly
   attempting to decrypt, with successive entries from a dictionary,
   messages obtained that are encrypted under a key derived from the
   user's password.

   Unless pre-authentication options are required by the policy of a
   realm, the KDC will not know whether a request for authentication
   succeeds.  An attacker can request a reply with credentials for any
   principal.  These credentials will likely not be of much use to the
   attacker unless it knows the client's secret key, but the
   availability of the response encrypted in the client's secret key
   provides the attacker with ciphertext that may be used to mount brute
   force or dictionary attacks to decrypt the credentials, by guessing
   the user's password.  For this reason it is strongly encouraged that
   Kerberos realms require the use of pre-authentication.  Even with

   pre-authentication, attackers may try brute force or dictionary
   attacks against credentials that are observed by eavesdropping on the
   network.

   Because a client can request a ticket for any server principal and
   can attempt a brute force or dictionary attack against the server
   principal's key using that ticket, it is strongly encouraged that
   keys be randomly generated (rather than generated from passwords) for
   any principals that are usable as the target principal for a
   KRB_TGS_REQ or KRB_AS_REQ messages.  [RFC4086]

   Although the DES-CBC-MD5 encryption method and DES-MD5 checksum
   methods are listed as SHOULD be implemented for backward
   compatibility, the single DES encryption algorithm on which these are
   based is weak, and stronger algorithms should be used whenever
   possible.

   Each host on the network must have a clock that is loosely
   synchronized to the time of the other hosts; this synchronization is
   used to reduce the bookkeeping needs of application servers when they
   do replay detection.  The degree of "looseness" can be configured on
   a per-server basis, but it is typically on the order of 5 minutes.
   If the clocks are synchronized over the network, the clock
   synchronization protocol MUST itself be secured from network
   attackers.

   Principal identifiers must not recycled on a short-term basis.  A
   typical mode of access control will use access control lists (ACLs)
   to grant permissions to particular principals.  If a stale ACL entry
   remains for a deleted principal and the principal identifier is
   reused, the new principal will inherit rights specified in the stale
   ACL entry.  By not reusing principal identifiers, the danger of
   inadvertent access is removed.

   Proper decryption of an KRB_AS_REP message from the KDC is not
   sufficient for the host to verify the identity of the user; the user
   and an attacker could cooperate to generate a KRB_AS_REP format
   message that decrypts properly but is not from the proper KDC.  To
   authenticate a user logging on to a local system, the credentials
   obtained in the AS exchange may first be used in a TGS exchange to
   obtain credentials for a local server.  Those credentials must then
   be verified by a local server through successful completion of the
   Client/Server exchange.

   Many RFC 1510-compliant implementations ignore unknown authorization
   data elements.  Depending on these implementations to honor
   authorization data restrictions may create a security weakness.

   Kerberos credentials contain clear-text information identifying the
   principals to which they apply.  If privacy of this information is
   needed, this exchange should itself be encapsulated in a protocol
   providing for confidentiality on the exchange of these credentials.

   Applications must take care to protect communications subsequent to
   authentication, either by using the KRB_PRIV or KRB_SAFE messages as
   appropriate, or by applying their own confidentiality or integrity
   mechanisms on such communications.  Completion of the KRB_AP_REQ and
   KRB_AP_REP exchange without subsequent use of confidentiality and
   integrity mechanisms provides only for authentication of the parties
   to the communication and not confidentiality and integrity of the
   subsequent communication.  Applications applying confidentiality and
   integrity protection mechanisms other than KRB_PRIV and KRB_SAFE must
   make sure that the authentication step is appropriately linked with
   the protected communication channel that is established by the
   application.

   Unless the application server provides its own suitable means to
   protect against replay (for example, a challenge-response sequence
   initiated by the server after authentication, or use of a server-
   generated encryption subkey), the server must utilize a replay cache
   to remember any authenticator presented within the allowable clock
   skew.  All services sharing a key need to use the same replay cache.
   If separate replay caches are used, then an authenticator used with
   one such service could later be replayed to a different service with
   the same service principal.

   If a server loses track of authenticators presented within the
   allowable clock skew, it must reject all requests until the clock
   skew interval has passed, providing assurance that any lost or
   replayed authenticators will fall outside the allowable clock skew
   and can no longer be successfully replayed.

   Implementations of Kerberos should not use untrusted directory
   servers to determine the realm of a host.  To allow this would allow
   the compromise of the directory server to enable an attacker to
   direct the client to accept authentication with the wrong principal
   (i.e., one with a similar name, but in a realm with which the
   legitimate host was not registered).

   Implementations of Kerberos must not use DNS to map one name to
   another (canonicalize) in order to determine the host part of the
   principal name with which one is to communicate.  To allow this
   canonicalization would allow a compromise of the DNS to result in a
   client obtaining credentials and correctly authenticating to the

   wrong principal.  Though the client will know who it is communicating
   with, it will not be the principal with which it intended to
   communicate.

   If the Kerberos server returns a TGT for a realm 'closer' than the
   desired realm, the client may use local policy configuration to
   verify that the authentication path used is an acceptable one.
   Alternatively, a client may choose its own authentication path rather
   than rely on the Kerberos server to select one.  In either case, any
   policy or configuration information used to choose or validate
   authentication paths, whether by the Kerberos server or client, must
   be obtained from a trusted source.

   The Kerberos protocol in its basic form does not provide perfect
   forward secrecy for communications.  If traffic has been recorded by
   an eavesdropper, then messages encrypted using the KRB_PRIV message,
   or messages encrypted using application-specific encryption under
   keys exchanged using Kerberos can be decrypted if the user's,
   application server's, or KDC's key is subsequently discovered.  This
   is because the session key used to encrypt such messages, when
   transmitted over the network, is encrypted in the key of the
   application server.  It is also encrypted under the session key from
   the user's TGT when it is returned to the user in the KRB_TGS_REP
   message.  The session key from the TGT is sent to the user in the
   KRB_AS_REP message encrypted in the user's secret key and embedded in
   the TGT, which was encrypted in the key of the KDC.  Applications
   requiring perfect forward secrecy must exchange keys through
   mechanisms that provide such assurance, but may use Kerberos for
   authentication of the encrypted channel established through such
   other means.

11.  Acknowledgements

   This document is a revision to RFC 1510 which was co-authored with
   John Kohl.  The specification of the Kerberos protocol described in
   this document is the result of many years of effort.  Over this
   period, many individuals have contributed to the definition of the
   protocol and to the writing of the specification.  Unfortunately, it
   is not possible to list all contributors as authors of this document,
   though there are many not listed who are authors in spirit, including
   those who contributed text for parts of some sections, who
   contributed to the design of parts of the protocol, and who
   contributed significantly to the discussion of the protocol in the
   IETF common authentication technology (CAT) and Kerberos working
   groups.

   Among those contributing to the development and specification of
   Kerberos were Jeffrey Altman, John Brezak, Marc Colan, Johan
   Danielsson, Don Davis, Doug Engert, Dan Geer, Paul Hill, John Kohl,
   Marc Horowitz, Matt Hur, Jeffrey Hutzelman, Paul Leach, John Linn,
   Ari Medvinsky, Sasha Medvinsky, Steve Miller, Jon Rochlis, Jerome
   Saltzer, Jeffrey Schiller, Jennifer Steiner, Ralph Swick, Mike Swift,
   Jonathan Trostle, Theodore Ts'o, Brian Tung, Jacques Vidrine, Assar
   Westerlund, and Nicolas Williams.  Many other members of MIT Project
   Athena, the MIT networking group, and the Kerberos and CAT working
   groups of the IETF contributed but are not listed.

A.  ASN.1 module

KerberosV5Spec2 {
        iso(1) identified-organization(3) dod(6) internet(1)
        security(5) kerberosV5(2) modules(4) krb5spec2(2)
} DEFINITIONS EXPLICIT TAGS ::= BEGIN

-- OID arc for KerberosV5
--
-- This OID may be used to identify Kerberos protocol messages
-- encapsulated in other protocols.
--
-- This OID also designates the OID arc for KerberosV5-related OIDs.
--
-- NOTE: RFC 1510 had an incorrect value (5) for "dod" in its OID.
id-krb5         OBJECT IDENTIFIER ::= {
        iso(1) identified-organization(3) dod(6) internet(1)
        security(5) kerberosV5(2)
}

Int32           ::= INTEGER (-2147483648..2147483647)
                    -- signed values representable in 32 bits

UInt32          ::= INTEGER (0..4294967295)
                    -- unsigned 32 bit values

Microseconds    ::= INTEGER (0..999999)
                    -- microseconds

KerberosString  ::= GeneralString (IA5String)

Realm           ::= KerberosString

PrincipalName   ::= SEQUENCE {
        name-type       [0] Int32,
        name-string     [1] SEQUENCE OF KerberosString
}

KerberosTime    ::= GeneralizedTime -- with no fractional seconds

HostAddress     ::= SEQUENCE  {
        addr-type       [0] Int32,
        address         [1] OCTET STRING
}

-- NOTE: HostAddresses is always used as an OPTIONAL field and
-- should not be empty.
HostAddresses   -- NOTE: subtly different from rfc1510,

                -- but has a value mapping and encodes the same
        ::= SEQUENCE OF HostAddress

-- NOTE: AuthorizationData is always used as an OPTIONAL field and
-- should not be empty.
AuthorizationData       ::= SEQUENCE OF SEQUENCE {
        ad-type         [0] Int32,
        ad-data         [1] OCTET STRING
}

PA-DATA         ::= SEQUENCE {
        -- NOTE: first tag is [1], not [0]
        padata-type     [1] Int32,
        padata-value    [2] OCTET STRING -- might be encoded AP-REQ
}

KerberosFlags   ::= BIT STRING (SIZE (32..MAX))
                    -- minimum number of bits shall be sent,
                    -- but no fewer than 32

EncryptedData   ::= SEQUENCE {
        etype   [0] Int32 -- EncryptionType --,
        kvno    [1] UInt32 OPTIONAL,
        cipher  [2] OCTET STRING -- ciphertext
}

EncryptionKey   ::= SEQUENCE {
        keytype         [0] Int32 -- actually encryption type --,
        keyvalue        [1] OCTET STRING
}

Checksum        ::= SEQUENCE {
        cksumtype       [0] Int32,
        checksum        [1] OCTET STRING
}

Ticket          ::= [APPLICATION 1] SEQUENCE {
        tkt-vno         [0] INTEGER (5),
        realm           [1] Realm,
        sname           [2] PrincipalName,
        enc-part        [3] EncryptedData -- EncTicketPart
}

-- Encrypted part of ticket
EncTicketPart   ::= [APPLICATION 3] SEQUENCE {
        flags                   [0] TicketFlags,
        key                     [1] EncryptionKey,
        crealm                  [2] Realm,

        cname                   [3] PrincipalName,
        transited               [4] TransitedEncoding,
        authtime                [5] KerberosTime,
        starttime               [6] KerberosTime OPTIONAL,
        endtime                 [7] KerberosTime,
        renew-till              [8] KerberosTime OPTIONAL,
        caddr                   [9] HostAddresses OPTIONAL,
        authorization-data      [10] AuthorizationData OPTIONAL
}

-- encoded Transited field
TransitedEncoding       ::= SEQUENCE {
        tr-type         [0] Int32 -- must be registered --,
        contents        [1] OCTET STRING
}

TicketFlags     ::= KerberosFlags
        -- reserved(0),
        -- forwardable(1),
        -- forwarded(2),
        -- proxiable(3),
        -- proxy(4),
        -- may-postdate(5),
        -- postdated(6),
        -- invalid(7),
        -- renewable(8),
        -- initial(9),
        -- pre-authent(10),
        -- hw-authent(11),
-- the following are new since 1510
        -- transited-policy-checked(12),
        -- ok-as-delegate(13)

AS-REQ          ::= [APPLICATION 10] KDC-REQ

TGS-REQ         ::= [APPLICATION 12] KDC-REQ

KDC-REQ         ::= SEQUENCE {
        -- NOTE: first tag is [1], not [0]
        pvno            [1] INTEGER (5) ,
        msg-type        [2] INTEGER (10 -- AS -- | 12 -- TGS --),
        padata          [3] SEQUENCE OF PA-DATA OPTIONAL
                            -- NOTE: not empty --,
        req-body        [4] KDC-REQ-BODY
}

KDC-REQ-BODY    ::= SEQUENCE {
        kdc-options             [0] KDCOptions,

        cname                   [1] PrincipalName OPTIONAL
                                    -- Used only in AS-REQ --,
        realm                   [2] Realm
                                    -- Server's realm
                                    -- Also client's in AS-REQ --,
        sname                   [3] PrincipalName OPTIONAL,
        from                    [4] KerberosTime OPTIONAL,
        till                    [5] KerberosTime,
        rtime                   [6] KerberosTime OPTIONAL,
        nonce                   [7] UInt32,
        etype                   [8] SEQUENCE OF Int32 -- EncryptionType
                                    -- in preference order --,
        addresses               [9] HostAddresses OPTIONAL,
        enc-authorization-data  [10] EncryptedData OPTIONAL
                                    -- AuthorizationData --,
        additional-tickets      [11] SEQUENCE OF Ticket OPTIONAL
                                        -- NOTE: not empty
}

KDCOptions      ::= KerberosFlags
        -- reserved(0),
        -- forwardable(1),
        -- forwarded(2),
        -- proxiable(3),
        -- proxy(4),
        -- allow-postdate(5),
        -- postdated(6),
        -- unused7(7),
        -- renewable(8),
        -- unused9(9),
        -- unused10(10),
        -- opt-hardware-auth(11),
        -- unused12(12),
        -- unused13(13),
-- 15 is reserved for canonicalize
        -- unused15(15),
-- 26 was unused in 1510
        -- disable-transited-check(26),
--
        -- renewable-ok(27),
        -- enc-tkt-in-skey(28),
        -- renew(30),
        -- validate(31)

AS-REP          ::= [APPLICATION 11] KDC-REP

TGS-REP         ::= [APPLICATION 13] KDC-REP

KDC-REP         ::= SEQUENCE {
        pvno            [0] INTEGER (5),
        msg-type        [1] INTEGER (11 -- AS -- | 13 -- TGS --),
        padata          [2] SEQUENCE OF PA-DATA OPTIONAL
                                -- NOTE: not empty --,
        crealm          [3] Realm,
        cname           [4] PrincipalName,
        ticket          [5] Ticket,
        enc-part        [6] EncryptedData
                                -- EncASRepPart or EncTGSRepPart,
                                -- as appropriate
}

EncASRepPart    ::= [APPLICATION 25] EncKDCRepPart

EncTGSRepPart   ::= [APPLICATION 26] EncKDCRepPart

EncKDCRepPart   ::= SEQUENCE {
        key             [0] EncryptionKey,
        last-req        [1] LastReq,
        nonce           [2] UInt32,
        key-expiration  [3] KerberosTime OPTIONAL,
        flags           [4] TicketFlags,
        authtime        [5] KerberosTime,
        starttime       [6] KerberosTime OPTIONAL,
        endtime         [7] KerberosTime,
        renew-till      [8] KerberosTime OPTIONAL,
        srealm          [9] Realm,
        sname           [10] PrincipalName,
        caddr           [11] HostAddresses OPTIONAL
}

LastReq         ::=     SEQUENCE OF SEQUENCE {
        lr-type         [0] Int32,
        lr-value        [1] KerberosTime
}

AP-REQ          ::= [APPLICATION 14] SEQUENCE {
        pvno            [0] INTEGER (5),
        msg-type        [1] INTEGER (14),
        ap-options      [2] APOptions,
        ticket          [3] Ticket,
        authenticator   [4] EncryptedData -- Authenticator
}

APOptions       ::= KerberosFlags
        -- reserved(0),
        -- use-session-key(1),

        -- mutual-required(2)

-- Unencrypted authenticator
Authenticator   ::= [APPLICATION 2] SEQUENCE  {
        authenticator-vno       [0] INTEGER (5),
        crealm                  [1] Realm,
        cname                   [2] PrincipalName,
        cksum                   [3] Checksum OPTIONAL,
        cusec                   [4] Microseconds,
        ctime                   [5] KerberosTime,
        subkey                  [6] EncryptionKey OPTIONAL,
        seq-number              [7] UInt32 OPTIONAL,
        authorization-data      [8] AuthorizationData OPTIONAL
}

AP-REP          ::= [APPLICATION 15] SEQUENCE {
        pvno            [0] INTEGER (5),
        msg-type        [1] INTEGER (15),
        enc-part        [2] EncryptedData -- EncAPRepPart
}

EncAPRepPart    ::= [APPLICATION 27] SEQUENCE {
        ctime           [0] KerberosTime,
        cusec           [1] Microseconds,
        subkey          [2] EncryptionKey OPTIONAL,
        seq-number      [3] UInt32 OPTIONAL
}

KRB-SAFE        ::= [APPLICATION 20] SEQUENCE {
        pvno            [0] INTEGER (5),
        msg-type        [1] INTEGER (20),
        safe-body       [2] KRB-SAFE-BODY,
        cksum           [3] Checksum
}

KRB-SAFE-BODY   ::= SEQUENCE {
        user-data       [0] OCTET STRING,
        timestamp       [1] KerberosTime OPTIONAL,
        usec            [2] Microseconds OPTIONAL,
        seq-number      [3] UInt32 OPTIONAL,
        s-address       [4] HostAddress,
        r-address       [5] HostAddress OPTIONAL
}

KRB-PRIV        ::= [APPLICATION 21] SEQUENCE {
        pvno            [0] INTEGER (5),
        msg-type        [1] INTEGER (21),
                        -- NOTE: there is no [2] tag

        enc-part        [3] EncryptedData -- EncKrbPrivPart
}

EncKrbPrivPart  ::= [APPLICATION 28] SEQUENCE {
        user-data       [0] OCTET STRING,
        timestamp       [1] KerberosTime OPTIONAL,
        usec            [2] Microseconds OPTIONAL,
        seq-number      [3] UInt32 OPTIONAL,
        s-address       [4] HostAddress -- sender's addr --,
        r-address       [5] HostAddress OPTIONAL -- recip's addr
}

KRB-CRED        ::= [APPLICATION 22] SEQUENCE {
        pvno            [0] INTEGER (5),
        msg-type        [1] INTEGER (22),
        tickets         [2] SEQUENCE OF Ticket,
        enc-part        [3] EncryptedData -- EncKrbCredPart
}

EncKrbCredPart  ::= [APPLICATION 29] SEQUENCE {
        ticket-info     [0] SEQUENCE OF KrbCredInfo,
        nonce           [1] UInt32 OPTIONAL,
        timestamp       [2] KerberosTime OPTIONAL,
        usec            [3] Microseconds OPTIONAL,
        s-address       [4] HostAddress OPTIONAL,
        r-address       [5] HostAddress OPTIONAL
}

KrbCredInfo     ::= SEQUENCE {
        key             [0] EncryptionKey,
        prealm          [1] Realm OPTIONAL,
        pname           [2] PrincipalName OPTIONAL,
        flags           [3] TicketFlags OPTIONAL,
        authtime        [4] KerberosTime OPTIONAL,
        starttime       [5] KerberosTime OPTIONAL,
        endtime         [6] KerberosTime OPTIONAL,
        renew-till      [7] KerberosTime OPTIONAL,
        srealm          [8] Realm OPTIONAL,
        sname           [9] PrincipalName OPTIONAL,
        caddr           [10] HostAddresses OPTIONAL
}

KRB-ERROR       ::= [APPLICATION 30] SEQUENCE {
        pvno            [0] INTEGER (5),
        msg-type        [1] INTEGER (30),
        ctime           [2] KerberosTime OPTIONAL,
        cusec           [3] Microseconds OPTIONAL,
        stime           [4] KerberosTime,

        susec           [5] Microseconds,
        error-code      [6] Int32,
        crealm          [7] Realm OPTIONAL,
        cname           [8] PrincipalName OPTIONAL,
        realm           [9] Realm -- service realm --,
        sname           [10] PrincipalName -- service name --,
        e-text          [11] KerberosString OPTIONAL,
        e-data          [12] OCTET STRING OPTIONAL
}

METHOD-DATA     ::= SEQUENCE OF PA-DATA

TYPED-DATA      ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE {
        data-type       [0] Int32,
        data-value      [1] OCTET STRING OPTIONAL
}

-- preauth stuff follows

PA-ENC-TIMESTAMP        ::= EncryptedData -- PA-ENC-TS-ENC

PA-ENC-TS-ENC           ::= SEQUENCE {
        patimestamp     [0] KerberosTime -- client's time --,
        pausec          [1] Microseconds OPTIONAL
}

ETYPE-INFO-ENTRY        ::= SEQUENCE {
        etype           [0] Int32,
        salt            [1] OCTET STRING OPTIONAL
}

ETYPE-INFO              ::= SEQUENCE OF ETYPE-INFO-ENTRY

ETYPE-INFO2-ENTRY       ::= SEQUENCE {
        etype           [0] Int32,
        salt            [1] KerberosString OPTIONAL,
        s2kparams       [2] OCTET STRING OPTIONAL
}

ETYPE-INFO2             ::= SEQUENCE SIZE (1..MAX) OF ETYPE-INFO2-ENTRY

AD-IF-RELEVANT          ::= AuthorizationData

AD-KDCIssued            ::= SEQUENCE {
        ad-checksum     [0] Checksum,
        i-realm         [1] Realm OPTIONAL,
        i-sname         [2] PrincipalName OPTIONAL,
        elements        [3] AuthorizationData

}

AD-AND-OR               ::= SEQUENCE {
        condition-count [0] Int32,
        elements        [1] AuthorizationData
}

AD-MANDATORY-FOR-KDC    ::= AuthorizationData

END

B.  Changes since RFC 1510

   This document replaces RFC 1510 and clarifies specification of items
   that were not completely specified.  Where changes to recommended
   implementation choices were made, or where new options were added,
   those changes are described within the document and listed in this
   section.  More significantly, "Specification 2" in Section 8 changes
   the required encryption and checksum methods to bring them in line
   with the best current practices and to deprecate methods that are no
   longer considered sufficiently strong.

   Discussion was added to Section 1 regarding the ability to rely on
   the KDC to check the transited field, and on the inclusion of a flag
   in a ticket indicating that this check has occurred.  This is a new
   capability not present in RFC 1510.  Pre-existing implementations may
   ignore or not set this flag without negative security implications.

   The definition of the secret key says that in the case of a user the
   key may be derived from a password.  In RFC 1510, it said that the
   key was derived from the password.  This change was made to
   accommodate situations where the user key might be stored on a
   smart-card, or otherwise obtained independently of a password.

   The introduction mentions the use of public key cryptography for
   initial authentication in Kerberos by reference.  RFC 1510 did not
   include such a reference.

   Section 1.3 was added to explain that while Kerberos provides
   authentication of a named principal, it is still the responsibility
   of the application to ensure that the authenticated name is the
   entity with which the application wishes to communicate.

   Discussion of extensibility has been added to the introduction.

   Discussion of how extensibility affects ticket flags and KDC options
   was added to the introduction of Section 2.  No changes were made to
   existing options and flags specified in RFC 1510, though some of the

   sections in the specification were renumbered, and text was revised
   to make the description and intent of existing options clearer,
   especially with respect to the ENC-TKT-IN-SKEY option (now section
   2.9.2) which is used for user-to-user authentication.  The new option
   and ticket flag transited policy checking (Section 2.7) was added.

   A warning regarding generation of session keys for application use
   was added to Section 3, urging the inclusion of key entropy from the
   KDC generated session key in the ticket.  An example regarding use of
   the sub-session key was added to Section 3.2.6.  Descriptions of the
   pa-etype-info, pa-etype-info2, and pa-pw-salt pre-authentication data
   items were added.  The recommendation for use of pre-authentication
   was changed from "MAY" to "SHOULD" and a note was added regarding
   known plaintext attacks.

   In RFC 1510, Section 4 described the database in the KDC.  This
   discussion was not necessary for interoperability and unnecessarily
   constrained implementation.  The old Section 4 was removed.

   The current Section 4 was formerly Section 6 on encryption and
   checksum specifications.  The major part of this section was brought
   up to date to support new encryption methods, and moved to a separate
   document.  Those few remaining aspects of the encryption and checksum
   specification specific to Kerberos are now specified in Section 4.

   Significant changes were made to the layout of Section 5 to clarify
   the correct behavior for optional fields.  Many of these changes were
   made necessary because of improper ASN.1 description in the original
   Kerberos specification which left the correct behavior
   underspecified.  Additionally, the wording in this section was
   tightened wherever possible to ensure that implementations conforming
   to this specification will be extensible with the addition of new
   fields in future specifications.

   Text was added describing time_t=0 issues in the ASN.1.  Text was
   also added, clarifying issues with implementations treating omitted
   optional integers as zero.  Text was added clarifying behavior for
   optional SEQUENCE or SEQUENCE OF that may be empty.  Discussion was
   added regarding sequence numbers and behavior of some
   implementations, including "zero" behavior and negative numbers.  A
   compatibility note was added regarding the unconditional sending of
   EncTGSRepPart regardless of the enclosing reply type.  Minor changes
   were made to the description of the HostAddresses type.  Integer
   types were constrained.  KerberosString was defined as a
   (significantly) constrained GeneralString.  KerberosFlags was defined
   to reflect existing implementation behavior that departs from the

   definition in RFC 1510.  The transited-policy-checked(12) and the
   ok-as-delegate(13) ticket flags were added.  The disable-transited-
   check(26) KDC option was added.

   Descriptions of commonly implemented PA-DATA were added to Section 5.
   The description of KRB-SAFE has been updated to note the existing
   implementation behavior of double-encoding.

   There were two definitions of METHOD-DATA in RFC 1510.  The second
   one, intended for use with KRB_AP_ERR_METHOD was removed leaving the
   SEQUENCE OF PA-DATA definition.

   Section 7, naming constraints, from RFC 1510 was moved to Section 6.

   Words were added describing the convention that domain-based realm
   names for newly-created realms should be specified as uppercase.
   This recommendation does not make lowercase realm names illegal.
   Words were added highlighting that the slash-separated components in
   the X.500 style of realm names is consistent with existing RFC 1510
   based implementations, but that it conflicts with the general
   recommendation of X.500 name representation specified in RFC 2253.

   Section 8, network transport, constants and defined values, from RFC
   1510 was moved to Section 7.  Since RFC 1510, the definition of the
   TCP transport for Kerberos messages was added, and the encryption and
   checksum number assignments have been moved into a separate document.

   "Specification 2" in Section 8 of the current document changes the
   required encryption and checksum methods to bring them in line with
   the best current practices and to deprecate methods that are no
   longer considered sufficiently strong.

   Two new sections, on IANA considerations and security considerations
   were added.

   The pseudo-code has been removed from the appendix.  The pseudo-code
   was sometimes misinterpreted to limit implementation choices and in
   RFC 1510, it was not always consistent with the words in the
   specification.  Effort was made to clear up any ambiguities in the
   specification, rather than to rely on the pseudo-code.

   An appendix was added containing the complete ASN.1 module drawn from
   the discussion in Section 5 of the current document.

END NOTES

   (*TM) Project Athena, Athena, and Kerberos are trademarks of the
   Massachusetts Institute of Technology (MIT).

Normative References

   [RFC3961]          Raeburn, K., "Encryption and Checksum
                      Specifications for Kerberos 5", RFC 3961, February
                      2005.

   [RFC3962]          Raeburn, K., "Advanced Encryption Standard (AES)
                      Encryption for Kerberos 5", RFC 3962, February
                      2005.

   [ISO-646/ECMA-6]   International Organization for Standardization,
                      "7-bit Coded Character Set for Information
                      Interchange", ISO/IEC 646:1991.

   [ISO-2022/ECMA-35] International Organization for Standardization,
                      "Character code structure and extension
                      techniques", ISO/IEC 2022:1994.

   [RFC1035]          Mockapetris, P., "Domain names - implementation
                      and specification", STD 13, RFC 1035, November
                      1987.

   [RFC2119]          Bradner, S., "Key words for use in RFCs to
                      Indicate Requirement Levels", BCP 14, RFC 2119,
                      March 1997.

   [RFC2434]          Narten, T. and H. Alvestrand, "Guidelines for
                      Writing an IANA Considerations Section in RFCs",
                      BCP 26, RFC 2434, October 1998.

   [RFC2782]          Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS
                      RR for specifying the location of services (DNS
                      SRV)", RFC 2782, February 2000.

   [RFC2253]          Wahl, M., Kille, S., and T. Howes, "Lightweight
                      Directory Access Protocol (v3): UTF-8 String
                      Representation of Distinguished Names", RFC 2253,
                      December 1997.

   [RFC3513]          Hinden, R. and S. Deering, "Internet Protocol
                      Version 6 (IPv6) Addressing Architecture", RFC
                      3513, April 2003.

   [X680]             Abstract Syntax Notation One (ASN.1):
                      Specification of Basic Notation, ITU-T
                      Recommendation X.680 (1997) | ISO/IEC
                      International Standard 8824-1:1998.

   [X690]             ASN.1 encoding rules: Specification of Basic
                      Encoding Rules (BER), Canonical Encoding Rules
                      (CER) and Distinguished Encoding Rules (DER),
                      ITU-T Recommendation X.690 (1997)| ISO/IEC
                      International Standard 8825-1:1998.

Informative References

   [ISO-8859]         International Organization for Standardization,
                      "8-bit Single-byte Coded Graphic Character Sets --
                      Latin Alphabet", ISO/IEC 8859.

   [RFC1964]          Linn, J., "The Kerberos Version 5 GSS-API
                      Mechanism", RFC 1964, June 1996.

   [DGT96]            Don Davis, Daniel Geer, and Theodore Ts'o,
                      "Kerberos With Clocks Adrift: History, Protocols,
                      and Implementation", USENIX Computing Systems 9:1,
                      January 1996.

   [DS81]             Dorothy E. Denning and Giovanni Maria Sacco,
                      "Time-stamps in Key Distribution Protocols,"
                      Communications of the ACM, Vol. 24 (8), p. 533-
                      536, August 1981.

   [KNT94]            John T. Kohl, B. Clifford Neuman, and Theodore Y.
                      Ts'o, "The Evolution of the Kerberos
                      Authentication System". In Distributed Open
                      Systems, pages 78-94. IEEE Computer Society Press,
                      1994.

   [MNSS87]           S. P. Miller, B. C. Neuman, J. I. Schiller, and J.
                      H. Saltzer, Section E.2.1: Kerberos Authentication
                      and Authorization System, M.I.T. Project Athena,
                      Cambridge, Massachusetts, December 21, 1987.

   [NS78]             Roger M. Needham and Michael D. Schroeder, "Using
                      Encryption for Authentication in Large Networks of
                      Computers," Communications of the ACM, Vol. 21
                      (12), pp. 993-999, December 1978.

   [Neu93]            B. Clifford Neuman, "Proxy-Based Authorization and
                      Accounting for Distributed Systems," in
                      Proceedings of the 13th International Conference
                      on Distributed Computing Systems, Pittsburgh, PA,
                      May 1993.

   [NT94]             B. Clifford Neuman and Theodore Y. Ts'o, "An
                      Authentication Service for Computer Networks,"
                      IEEE Communications Magazine, Vol. 32 (9), p. 33-
                      38, September 1994.

   [Pat92]            J. Pato, Using Pre-Authentication to Avoid
                      Password Guessing Attacks, Open Software
                      Foundation DCE Request for Comments 26 (December
                      1992.

   [RFC1510]          Kohl, J. and C. Neuman, "The Kerberos Network
                      Authentication Service (V5)", RFC 1510, September
                      1993.

   [RFC4086]          Eastlake, D., 3rd, Schiller, J., and S. Crocker,
                      "Randomness Requirements for Security", BCP 106,
                      RFC 4086, June 2005.

   [SNS88]            J. G. Steiner, B. C. Neuman, and J. I. Schiller,
                      "Kerberos: An Authentication Service for Open
                      Network Systems," p. 191-202, Usenix Conference
                      Proceedings, Dallas, Texas, February 1988.

   [RFC4121]          Zhu, L., Jaganathan, K., and S. Hartman, "The
                      Kerberos Version 5 Generic Security Service
                      Application Program Interface (GSS-API) Mechanism:
                      Version 2", RFC 4121, July 2005.

Authors' Addresses

   Clifford Neuman
   Information Sciences Institute
   University of Southern California
   4676 Admiralty Way
   Marina del Rey, CA 90292, USA

   EMail: bcn@isi.edu

   Tom Yu
   Massachusetts Institute of Technology
   77 Massachusetts Avenue
   Cambridge, MA 02139, USA

   EMail: tlyu@mit.edu

   Sam Hartman
   Massachusetts Institute of Technology
   77 Massachusetts Avenue
   Cambridge, MA 02139, USA

   EMail: hartmans-ietf@mit.edu

   Kenneth Raeburn
   Massachusetts Institute of Technology
   77 Massachusetts Avenue
   Cambridge, MA 02139, USA

   EMail: raeburn@mit.edu

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