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RFC 1352 - SNMP Security Protocols


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Network Working Group                                         J. Galvin
Request for Comments: 1352            Trusted Information Systems, Inc.
                                                          K. McCloghrie
                                               Hughes LAN Systems, Inc.
                                                               J. Davin
                                    MIT Laboratory for Computer Science
                                                              July 1992

                        SNMP Security Protocols

Status of this Memo

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

Table of Contents

   1.    Abstract . . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.1   Threats  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.2   Goals and Constraints  . . . . . . . . . . . . . . . . . . .   5
   2.3   Security Services  . . . . . . . . . . . . . . . . . . . . .   6
   2.4   Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . .   6
   2.4.1   Message Digest Algorithm . . . . . . . . . . . . . . . . .   7
   2.4.2   Symmetric Encryption Algorithm . . . . . . . . . . . . . .   8
   3.    SNMP Party   . . . . . . . . . . . . . . . . . . . . . . . .   9
   4.    Digest Authentication Protocol . . . . . . . . . . . . . . .  11
   4.1   Generating a Message   . . . . . . . . . . . . . . . . . . .  14
   4.2   Receiving a Message  . . . . . . . . . . . . . . . . . . . .  15
   5.    Symmetric Privacy Protocol . . . . . . . . . . . . . . . . .  16
   5.1   Generating a Message   . . . . . . . . . . . . . . . . . . .  17
   5.2   Receiving a Message  . . . . . . . . . . . . . . . . . . . .  18
   6.    Clock and Secret Distribution  . . . . . . . . . . . . . . .  19
   6.1   Initial Configuration    . . . . . . . . . . . . . . . . . .  20
   6.2   Clock Distribution   . . . . . . . . . . . . . . . . . . . .  22
   6.3   Clock Synchronization  . . . . . . . . . . . . . . . . . . .  24
   6.4   Secret Distribution  . . . . . . . . . . . . . . . . . . . .  26
   6.5   Crash Recovery   . . . . . . . . . . . . . . . . . . . . . .  28
   7.    Security Considerations  . . . . . . . . . . . . . . . . . .  30
   7.1   Recommended Practices  . . . . . . . . . . . . . . . . . . .  30
   7.2   Conformance    . . . . . . . . . . . . . . . . . . . . . . .  33
   7.3   Protocol Correctness . . . . . . . . . . . . . . . . . . . .  34
   7.3.1   Clock Monotonicity Mechanism . . . . . . . . . . . . . . .  35
   7.3.2   Data Integrity Mechanism . . . . . . . . . . . . . . . . .  36

   7.3.3   Data Origin Authentication Mechanism . . . . . . . . . . .  36
   7.3.4   Restricted Administration Mechanism  . . . . . . . . . . .  36
   7.3.5   Ordered Delivery Mechanism   . . . . . . . . . . . . . . .  37
   7.3.6   Message Timeliness Mechanism . . . . . . . . . . . . . . .  38
   7.3.7   Selective Clock Acceleration Mechanism . . . . . . . . . .  38
   7.3.8   Confidentiality Mechanism  . . . . . . . . . . . . . . . .  39
   8.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  39
   9.    References . . . . . . . . . . . . . . . . . . . . . . . . .  40
   10.   Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  41

1.  Abstract

   The Simple Network Management Protocol (SNMP) specification [1]
   allows for the protection of network management operations by a
   variety of security protocols.  The SNMP administrative model
   described in [2] provides a framework for securing SNMP network
   management. In the context of that framework, this memo defines
   protocols to support the following three security services:

     o data integrity,

     o data origin authentication, and

     o data confidentiality.

   Please send comments to the SNMP Security Developers mailing list
   (snmp-sec-dev@tis.com).

2.  Introduction

   In the model described in [2], each SNMP party is, by definition,
   associated with a single authentication protocol.  The authentication
   protocol provides a mechanism by which SNMP management communications
   transmitted by the party may be reliably identified as having
   originated from that party. The authentication protocol defined in
   this memo also reliably determines that the message received is the
   message that was sent.

   Similarly, each SNMP party is, by definition, associated with a
   single privacy protocol. The privacy protocol provides a mechanism by
   which SNMP management communications transmitted to said party are
   protected from disclosure. The privacy protocol in this memo
   specifies that only authenticated messages may be protected from
   disclosure.

   These protocols are secure alternatives to the so-called "trivial"
   protocol defined in [1].

      USE OF THE TRIVIAL PROTOCOL ALONE DOES NOT CONSTITUTE SECURE
      NETWORK MANAGEMENT. THEREFORE, A NETWORK MANAGEMENT SYSTEM THAT
      IMPLEMENTS ONLY THE TRIVIAL PROTOCOL IS NOT CONFORMANT TO THIS
      SPECIFICATION.

   The Digest Authentication Protocol is described in Section 4.  It
   provides a data integrity service by transmitting a message digest --
   computed by the originator and verified by the recipient -- with each
   SNMP message. The data origin authentication service is provided by
   prefixing the message with a secret value known only to the
   originator and recipient, prior to computing the digest. Thus, data
   integrity is supported explicitly while data origin authentication is
   supported implicitly in the verification of the digest.

   The Symmetric Privacy Protocol is described in Section 5. It protects
   messages from disclosure by encrypting their contents according to a
   secret cryptographic key known only to the originator and recipient.
   The additional functionality afforded by this protocol is assumed to
   justify its additional computational cost.

   The Digest Authentication Protocol depends on the existence of
   loosely synchronized clocks between the originator and recipient of a
   message. The protocol specification makes no assumptions about the
   strategy by which such clocks are synchronized. Section 6.3 presents
   one strategy that is particularly suited to the demands of SNMP
   network management.

   Both protocols described here require the sharing of secret
   information between the originator of a message and its recipient.
   The protocol specifications assume the existence of the necessary
   secrets. The selection of such secrets and their secure distribution
   to appropriate parties may be accomplished by a variety of
   strategies. Section 6.4 presents one such strategy that is
   particularly suited to the demands of SNMP network management.

2.1   Threats

   Several of the classical threats to network protocols are applicable
   to the network management problem and therefore would be applicable
   to any SNMP security protocol. Other threats are not applicable to
   the network management problem. This section discusses principal
   threats, secondary threats, and threats which are of lesser
   importance.

   The principal threats against which any SNMP security protocol should
   provide protection are:

   Modification of Information.
      The SNMP protocol provides the means for management stations to
      interrogate and to manipulate the value of objects in a managed
      agent.  The modification threat is the danger that some party may
      alter in-transit messages generated by an authorized party in such
      a way as to effect unauthorized management operations, including
      falsifying the value of an object.

   Masquerade.
      The SNMP administrative model includes an access control model.
      Access control necessarily depends on knowledge of the origin of a
      message.  The masquerade threat is the danger that management
      operations not authorized for some party may be attempted by that
      party by assuming the identity of another party that has the
      appropriate authorizations.

   Two secondary threats are also identified. The security protocols
   defined in this memo do provide protection against:

   Message Stream Modification.
      The SNMP protocol is based upon connectionless transport services.
      The message stream modification threat is the danger that messages
      may be arbitrarily re-ordered, delayed or replayed to effect
      unauthorized management operations.  This threat may arise either
      by the work of a malicious attacker or by the natural operation of
      a subnetwork service.

   Disclosure.
      The disclosure threat is the danger of eavesdropping on the
      exchanges between managed agents and a management station.
      Protecting against this threat is mandatory when the SNMP is used
      to administer private parameters on which its security is based.
      Protecting against the disclosure threat may also be required as a
      matter of local policy.

   There are at least two threats that a SNMP security protocol need not
   protect against. The security protocols defined in this memo do not
   provide protection against:

   Denial of Service.
      A SNMP security protocol need not attempt to address the broad
      range of attacks by which service to authorized parties is denied.
      Indeed, such denial-of-service attacks are in many cases
      indistinguishable from the type of network failures with which any
      viable network management protocol must cope as a matter of
      course.

   Traffic Analysis.
      In addition, a SNMP security protocol need not attempt to address
      traffic analysis attacks.  Indeed, many traffic patterns are
      predictable -- agents may be managed on a regular basis by a
      relatively small number of management stations -- and therefore
      there is no significant advantage afforded by protecting against
      traffic analysis.

2.2   Goals and Constraints

   Based on the foregoing account of threats in the SNMP network
   management environment, the goals of a SNMP security protocol are
   enumerated below.

    1. The protocol should provide for verification that each
       received SNMP message has not been modified during
       its transmission through the network in such a way that
       an unauthorized management operation might result.

    2. The protocol should provide for verification of the
       identity of the originator of each received SNMP
       message.

    3. The protocol should provide that the apparent time of
       generation for each received SNMP message is recent.

    4. The protocol should provide that the apparent time of
       generation for each received SNMP message is
       subsequent to that for all previously delivered messages
       of similar origin.

    5. The protocol should provide, when necessary, that the
       contents of each received SNMP message are protected
       from disclosure.

   In addition to the principal goal of supporting secure network
   management, the design of any SNMP security protocol is also
   influenced by the following constraints:

    1. When the requirements of effective management in times
       of network stress are inconsistent with those of security,
       the former are preferred.

    2. Neither the security protocol nor its underlying security
       mechanisms should depend upon the ready availability
       of other network services (e.g., Network Time Protocol
       (NTP) or secret/key management protocols).

    3. A security mechanism should entail no changes to the
       basic SNMP network management philosophy.

2.3   Security Services

   The security services necessary to support the goals of a SNMP
   security protocol are as follows.

   Data Integrity   is the provision of the property that data
       and data sequences have not been altered or destroyed
       in an unauthorized manner.

   Data Origin Authentication    is the provision of the
       property that the claimed origin of received data is
       corroborated.

   Data Confidentiality   is the provision of the property that
       information is not made available or disclosed to
       unauthorized individuals, entities, or processes.

      The protocols specified in this memo require both data
      integrity and data origin authentication to be used at all
      times. For these protocols, it is not possible to realize data
      integrity without data origin authentication, nor is it possible
      to realize data origin authentication without data integrity.

      Further, there is no provision for data confidentiality without
      both data integrity and data origin authentication.

2.4   Mechanisms

      The security protocols defined in this memo employ several
      types of mechanisms in order to realize the goals and security
      services described above:

     o In support of data integrity, a message digest algorithm
       is required. A digest is calculated over an appropriate
       portion of a SNMP message and included as part of the
       message sent to the recipient.

     o In support of data origin authentication and data
       integrity, the portion of a SNMP message that is
       digested is first prefixed with a secret value shared by
       the originator of that message and its intended recipient.

     o To protect against the threat of message reordering, a
       timestamp value is included in each message generated.
       A recipient evaluates the timestamp to determine if the

       message is recent and it uses the timestamp to determine
       if the message is ordered relative to other messages it
       has received. In conjunction with other readily available
       information (e.g., the request-id), the timestamp also
       indicates whether or not the message is a replay of a
       previous message. This protection against the threat of
       message reordering implies no protection against
       unauthorized deletion or suppression of messages.

     o In support of data confidentiality, a symmetric
       encryption algorithm is required. An appropriate
       portion of the message is encrypted prior to being
       transmitted to its recipient.

   The security protocols in this memo are defined independently of the
   particular choice of a message digest and encryption algorithm --
   owing principally to the lack of a suitable metric by which to
   evaluate the security of particular algorithm choices. However, in
   the interests of completeness and in order to guarantee
   interoperability, Sections 2.4.1 and 2.4.2 specify particular
   choices, which are considered acceptably secure as of this writing.
   In the future, this memo may be updated by the publication of a memo
   specifying substitute or alternate choices of algorithms, i.e., a
   replacement for or addition to the sections below.

2.4.1   Message Digest Algorithm

   In support of data integrity, the use of the MD5 [3] message digest
   algorithm is chosen. A 128-bit digest is calculated over the
   designated portion of a SNMP message and included as part of the
   message sent to the recipient.

   An appendix of [3] contains a C Programming Language implementation
   of the algorithm. This code was written with portability being the
   principal objective. Implementors may wish to optimize the
   implementation with respect to the characteristics of their hardware
   and software platforms.

   The use of this algorithm in conjunction with the Digest
   Authentication Protocol (see Section 4) is identified by the ASN.1
   object identifier value md5AuthProtocol, defined in [4].

   For any SNMP party for which the authentication protocol is
   md5AuthProtocol, the size of its private authentication key is 16
   octets.

   Within an authenticated management communication generated by such a
   party, the size of the authDigest component of that communication

   (see Section 4) is 16 octets.

2.4.2   Symmetric Encryption Algorithm

   In support of data confidentiality, the use of the Data Encryption
   Standard (DES) in the Cipher Block Chaining mode of operation is
   chosen. The designated portion of a SNMP message is encrypted and
   included as part of the message sent to the recipient.

   Two organizations have published specifications defining the DES: the
   National Institute of Standards and Technology (NIST) [5] and the
   American National Standards Institute [6].  There is a companion
   Modes of Operation specification for each definition (see [7] and
   [8], respectively).

   The NIST has published three additional documents that implementors
   may find useful.

     o There is a document with guidelines for implementing
       and using the DES, including functional specifications
       for the DES and its modes of operation [9].

     o There is a specification of a validation test suite for the
       DES [10]. The suite is designed to test all aspects of the
       DES and is useful for pinpointing specific problems.

     o There is a specification of a maintenance test for the
       DES [11]. The test utilizes a minimal amount of data
       and processing to test all components of the DES. It
       provides a simple yes-or-no indication of correct
       operation and is useful to run as part of an initialization
       step, e.g., when a computer reboots.

   The use of this algorithm in conjunction with the Symmetric Privacy
   Protocol (see Section 5) is identified by the ASN.1 object identifier
   value desPrivProtocol, defined in [4].

   For any SNMP party for which the privacy protocol is desPrivProtocol,
   the size of the private privacy key is 16 octets, of which the first
   8 octets are a DES key and the second 8 octets are a DES
   Initialization Vector. The 64-bit DES key in the first 8 octets of
   the private key is a 56 bit quantity used directly by the algorithm
   plus 8 parity bits -- arranged so that one parity bit is the least
   significant bit of each octet. The setting of the parity bits is
   ignored.

   The length of the octet sequence to be encrypted by the DES must be

   an integral multiple of 8. When encrypting, the data should be padded
   at the end as necessary; the actual pad value is insignificant.

   If the length of the octet sequence to be decrypted is not an
   integral multiple of 8 octets, the processing of the octet sequence
   should be halted and an appropriate exception noted. Upon decrypting,
   the padding should be ignored.

3.  SNMP Party

   Recall from [2] that a SNMP party is a conceptual, virtual execution
   context whose operation is restricted (for security or other
   purposes) to an administratively defined subset of all possible
   operations of a particular SNMP protocol entity. A SNMP protocol
   entity is an actual process which performs network management
   operations by generating and/or responding to SNMP protocol messages
   in the manner specified in [1]. Architecturally, every SNMP protocol
   entity maintains a local database that represents all SNMP parties
   known to it.

   A SNMP party may be represented by an ASN.1 value with the following
   syntax.

      SnmpParty ::= SEQUENCE {
        partyIdentity
           OBJECT IDENTIFIER,
        partyTDomain
           OBJECT IDENTIFIER,
        partyTAddr
           OCTET STRING,
        partyProxyFor
           OBJECT IDENTIFIER,
        partyMaxMessageSize
           INTEGER,
        partyAuthProtocol
           OBJECT IDENTIFIER,
        partyAuthClock
           INTEGER,
        partyAuthLastMsg
           INTEGER,
        partyAuthNonce
           INTEGER,
        partyAuthPrivate
           OCTET STRING,
        partyAuthPublic
           OCTET STRING,
        partyAuthLifetime

           INTEGER,
        partyPrivProtocol
           OBJECT IDENTIFIER,
        partyPrivPrivate
           OCTET STRING,
        partyPrivPublic
           OCTET STRING
      }

   For each SnmpParty value that represents a SNMP party, the generic
   significance of each of its components is defined in [2]. For each
   SNMP party that supports the generation of messages using the Digest
   Authentication Protocol, additional, special significance is
   attributed to certain components of that party's representation:

     o Its partyAuthProtocol component is called the
       authentication protocol and identifies a combination of
       the Digest Authentication Protocol with a particular
       digest algorithm (such as that defined in Section 2.4.1).
       This combined mechanism is used to authenticate the
       origin and integrity of all messages generated by the
       party.

     o Its partyAuthClock component is called the
       authentication clock and represents a notion of the
       current time that is specific to the party.

     o Its partyAuthLastMsg component is called the
       last-timestamp and represents a notion of time
       associated with the most recent, authentic protocol
       message generated by the party.

     o Its partyAuthNonce component is called the nonce
       and represents a monotonically increasing integer
       associated with the most recent, authentic protocol
       message generated by the party. The nonce associated
       with a particular message distinguishes it among all
       others transmitted in the same unit time interval.

     o Its partyAuthPrivate component is called the private
       authentication key and represents any secret value
       needed to support the Digest Authentication Protocol
       and associated digest algorithm.

     o Its partyAuthPublic component is called the public
       authentication key and represents any public value that
       may be needed to support the authentication protocol.

       This component is not significant except as suggested in
       Section 6.4.

     o Its partyAuthLifetime component is called the
       lifetime and represents an administrative upper bound
       on acceptable delivery delay for protocol messages
       generated by the party.

   For each SNMP party that supports the receipt of messages via the
   Symmetric Privacy Protocol, additional, special significance is
   attributed to certain components of that party's representation:

     o Its partyPrivProtocol component is called the privacy
       protocol and identifies a combination of the Symmetric
       Privacy Protocol with a particular encryption algorithm
       (such as that defined in Section 2.4.2). This combined
       mechanism is used to protect from disclosure all protocol
       messages received by the party.

     o Its partyPrivPrivate component is called the private
       privacy key and represents any secret value needed to
       support the Symmetric Privacy Protocol and associated
       encryption algorithm.

     o Its partyPrivPublic component is called the public
       privacy key and represents any public value that may be
       needed to support the privacy protocol. This component
       is not significant except as suggested in Section 6.4.

4.  Digest Authentication Protocol

   This section describes the Digest Authentication Protocol. It
   provides both for verifying the integrity of a received message
   (i.e., the message received is the message sent) and for verifying
   the origin of a message (i.e., the reliable identification of the
   originator). The integrity of the message is protected by computing a
   digest over an appropriate portion of a message. The digest is
   computed by the originator of the message, transmitted with the
   message, and verified by the recipient of the message.

   A secret value known only to the originator and recipient of the
   message is prefixed to the message prior to the digest computation.
   Thus, the origin of the message is known implicitly with the
   verification of the digest.

   Recall from [2] that a SNMP management communication is represented
   by an ASN.1 value with the following syntax.

      SnmpMgmtCom ::= [1] IMPLICIT SEQUENCE {
        dstParty
           OBJECT IDENTIFIER,
        srcParty
           OBJECT IDENTIFIER,
        pdu   PDUs
      }

   For each SnmpMgmtCom value that represents a SNMP management
   communication, the following statements are true:

     o Its dstParty component is called the destination and
       identifies the SNMP party to which the communication
       is directed.

     o Its srcParty component is called the source and
       identifies the SNMP party from which the
       communication is originated.

     o Its pdu component has the form and significance
       attributed to it in [1].

   Recall from [2] that a SNMP authenticated management communication is
   represented by an ASN.1 value with the following syntax.

      SnmpAuthMsg ::= [1] IMPLICIT SEQUENCE {
        authInfo
           ANY, - defined by authentication protocol
        authData
           SnmpMgmtCom
      }

   For each SnmpAuthMsg value that represents a SNMP authenticated
   management communication, the following statements are true:

     o Its authInfo component is called the authentication
       information and represents information required in
       support of the authentication protocol used by the
       SNMP party originating the message. The detailed
       significance of the authentication information is specific
       to the authentication protocol in use; it has no effect on
       the application semantics of the communication other
       than its use by the authentication protocol in
       determining whether the communication is authentic or
       not.

     o Its authData component is called the authentication
       data and represents a SNMP management
       communication.

   In support of the Digest Authentication Protocol, an authInfo
   component is of type AuthInformation:

      AuthInformation ::= [1] IMPLICIT SEQUENCE {
        authTimestamp
           INTEGER (0..2147483647),
        authNonce
           INTEGER (0..2147483647),
        authDigest
           OCTET STRING
      }

   For each AuthInformation value that represents authentication
   information, the following statements are true:

     o Its authTimestamp component is called the
       authentication timestamp and represents the time of the
       generation of the message according to the
       partyAuthClock of the SNMP party that originated
       it. Note that the granularity of the authentication
       timestamp is 1 second.

     o Its authNonce component is called the authentication
       nonce and represents a non-negative integer value
       evaluated according to the authTimestamp value. In
       order not to limit transmission frequency of management
       communications to the granularity of the authentication
       timestamp, the authentication nonce is provided to
       differentiate between multiple messages sent with the
       same value of authTimestamp. The authentication
       nonce is a monotonically increasing sequence number,
       that is reset for each new authentication timestamp
       value.

     o Its authDigest component is called the authentication
       digest and represents the digest computed over an
       appropriate portion of the message, where the message is
       temporarily prefixed with a secret value for the purposes
       of computing the digest.

4.1   Generating a Message

   This section describes the behavior of a SNMP protocol entity when it
   acts as a SNMP party for which the authentication protocol is
   administratively specified as the Digest Authentication Protocol.
   Insofar as the behavior of a SNMP protocol entity when transmitting
   protocol messages is defined generically in [2], only those aspects
   of that behavior that are specific to the Digest Authentication
   Protocol are described below. In particular, this section describes
   the encapsulation of a SNMP management communication into a SNMP
   authenticated management communication.

   According to [2], a SnmpAuthMsg value is constructed during Step 3 of
   generic processing. In particular, it states the authInfo component
   is constructed according to the authentication protocol identified
   for the SNMP party originating the message. When the relevant
   authentication protocol is the Digest Authentication Protocol, the
   procedure performed by a SNMP protocol entity whenever a management
   communication is to be transmitted by a SNMP party is as follows.

    1. The local database is consulted to determine the
       authentication clock, last-timestamp, nonce, and private
       authentication key (extracted, for example, according to
       the conventions defined in Section 2.4.1) of the SNMP
       party originating the message.

    2. The authTimestamp component is set to the retrieved
       authentication clock value.

    3. If the last-timestamp is equal to the authentication
       clock, the nonce is incremented. Otherwise the nonce is
       set to zero. The authNonce component is set to the
       nonce value. In the local database, the originating
       SNMP party's nonce and last-timestamp are set to the
       nonce value and the authentication clock, respectively.

    4. The authentication digest is temporarily set to the
       private authentication key. The SnmpAuthMsg value
       is serialized according to the conventions of [12] and [1].
       A digest is computed over the octet sequence
       representing that serialized value using, for example, the
       algorithm specified in Section 2.4.1. The authDigest
       component is set to the computed digest value.

   As set forth in [2], the SnmpAuthMsg value is then encapsulated
   according to the appropriate privacy protocol into a SnmpPrivMsg
   value. This latter value is then serialized and transmitted to the
   receiving SNMP party.

4.2   Receiving a Message

   This section describes the behavior of a SNMP protocol entity upon
   receipt of a protocol message from a SNMP party for which the
   authentication protocol is administratively specified as the Digest
   Authentication Protocol. Insofar as the behavior of a SNMP protocol
   entity when receiving protocol messages is defined generically in
   [2], only those aspects of that behavior that are specific to the
   Digest Authentication Protocol are described below.

   According to [2], a SnmpAuthMsg value is evaluated during Step 9 of
   generic processing. In particular, it states the SnmpAuthMsg value is
   evaluated according to the authentication protocol identified for the
   SNMP party that originated the message. When the relevant
   authentication protocol is the Digest Authentication Protocol, the
   procedure performed by a SNMP protocol entity whenever a management
   communication is received by a SNMP party is as follows.

    1. If the ASN.1 type of the authInfo component is not
       AuthInformation, the message is evaluated as
       unauthentic. Otherwise, the authTimestamp,
       authNonce, and authDigest components are
       extracted from the SnmpAuthMsg value.

    2. The local database is consulted to determine the
       authentication clock, last-timestamp, nonce, private
       authentication key (extracted, for example, according to
       the conventions defined in Section 2.4.1), and lifetime of
       the SNMP party that originated the message.

    3. If the authTimestamp component plus the lifetime is
       less than the authentication clock, the message is
       evaluated as unauthentic.

    4. If the authTimestamp component is less than the
       last-timestamp recorded for the originating party in the
       local database, the message is evaluated as unauthentic.

    5. If the authTimestamp component is equal to the
       last-timestamp and if the authNonce component is less
       than or equal to the nonce, the message is evaluated as
       unauthentic.

    6. The authDigest component is extracted and
       temporarily recorded.

    7. A new SnmpAuthMsg value is constructed such that
       its authDigest component is set to the private

       authentication key and its other components are set to
       the value of the corresponding components in the
       received SnmpAuthMsg value. This new
       SnmpAuthMsg value is serialized according to the
       conventions of [12] and [1]. A digest is computed over
       the octet sequence representing that serialized value
       using, for example, the algorithm specified in
       Section 2.4.1.

    8. If the computed digest value is not equal to the
       previously recorded digest value, the message is
       evaluated as unauthentic.

    9. The message is evaluated as authentic.

   10. The last-timestamp and nonce values locally recorded
       for the originating SNMP party are set to the
       authTimestamp value and the authNonce value,
       respectively.

   11. The authentication clock value locally recorded for the
       originating SNMP party is advanced to the
       authTimestamp value if this latter exceeds the
       recorded value.

   If the SnmpAuthMsg value is evaluated as unauthentic, an
   authentication failure is noted and the received message is discarded
   without further processing. Otherwise, processing of the received
   message continues as specified in [2].

5.  Symmetric Privacy Protocol

   This section describes the Symmetric Privacy Protocol. It provides
   for protection from disclosure of a received message.  An appropriate
   portion of the message is encrypted according to a secret key known
   only to the originator and recipient of the message.

   This protocol assumes the underlying mechanism is a symmetric
   encryption algorithm. In addition, the message to be encrypted must
   be protected according to the conventions of the Digest
   Authentication Protocol.

   Recall from [2] that a SNMP private management communication is
   represented by an ASN.1 value with the following syntax.

      SnmpPrivMsg ::= [1] IMPLICIT SEQUENCE {
        privDst
           OBJECT IDENTIFIER,
        privData
           [1] IMPLICIT OCTET STRING
      }

   For each SnmpPrivMsg value that represents a SNMP private management
   communication, the following statements are true:

     o Its privDst component is called the privacy destination
       and identifies the SNMP party to which the
       communication is directed.

     o Its privData component is called the privacy data and
       represents the (possibly encrypted) serialization
       (according to the conventions of [12] and [1]) of a SNMP
       authenticated management communication.

5.1   Generating a Message

   This section describes the behavior of a SNMP protocol entity when it
   communicates with a SNMP party for which the privacy protocol is
   administratively specified as the Symmetric Privacy Protocol. Insofar
   as the behavior of a SNMP protocol entity when transmitting a
   protocol message is defined generically in [2], only those aspects of
   that behavior that are specific to the Symmetric Privacy Protocol are
   described below. In particular, this section describes the
   encapsulation of a SNMP authenticated management communication into a
   SNMP private management communication.

   According to [2], a SnmpPrivMsg value is constructed during Step 5 of
   generic processing. In particular, it states the privData component
   is constructed according to the privacy protocol identified for the
   SNMP party receiving the message.  When the relevant privacy protocol
   is the Symmetric Privacy Protocol, the procedure performed by a SNMP
   protocol entity whenever a management communication is to be
   transmitted by a SNMP party is as follows.

    1. If the SnmpAuthMsg value is not authenticated
       according to the conventions of the Digest
       Authentication Protocol, the generation of the private
       management communication fails according to a local
       procedure, without further processing.

    2. The local database is consulted to determine the private
       privacy key of the SNMP party receiving the message

       (represented, for example, according to the conventions
       defined in Section 2.4.2).

    3. The SnmpAuthMsg value is serialized according to the
       conventions of [12] and [1].

    4. The octet sequence representing the serialized
       SnmpAuthMsg value is encrypted using, for example,
       the algorithm specified in Section 2.4.2 and the
       extracted private privacy key.

    5. The privData component is set to the encrypted value.

      As set forth in [2], the SnmpPrivMsg value is then serialized
      and transmitted to the receiving SNMP party.

5.2   Receiving a Message

   This section describes the behavior of a SNMP protocol entity when it
   acts as a SNMP party for which the privacy protocol is
   administratively specified as the Symmetric Privacy Protocol. Insofar
   as the behavior of a SNMP protocol entity when receiving a protocol
   message is defined generically in [2], only those aspects of that
   behavior that are specific to the Symmetric Privacy Protocol are
   described below.

   According to [2], the privData component of a received SnmpPrivMsg
   value is evaluated during Step 4 of generic processing. In
   particular, it states the privData component is evaluated according
   to the privacy protocol identified for the SNMP party receiving the
   message. When the relevant privacy protocol is the Symmetric Privacy
   Protocol, the procedure performed by a SNMP protocol entity whenever
   a management communication is received by a SNMP party is as follows.

    1. The local database is consulted to determine the private
       privacy key of the SNMP party receiving the message
       (represented, for example, according to the conventions
       defined in Section 2.4.2).

    2. The contents octets of the privData component are
       decrypted using, for example, the algorithm specified in
       Section 2.4.2 and the extracted private privacy key.

      Processing of the received message continues as specified in [2].

6.  Clock and Secret Distribution

   The protocols described in Sections 4 and 5 assume the existence of
   loosely synchronized clocks and shared secret values. Three
   requirements constrain the strategy by which clock values and secrets
   are distributed.

     o If the value of an authentication clock is decreased, the
       last-timestamp and private authentication key must be
       changed concurrently.

       When the value of an authentication clock is decreased,
       messages that have been sent with a timestamp value
       between the value of the authentication clock and its
       new value may be replayed. Changing the private
       authentication key obviates this threat. However,
       changing the authentication clock and the private
       authentication key is not sufficient to ensure proper
       operation. If the last-timestamp is not reduced similarly
       to the authentication clock, no message will be
       considered authentic until the value of the authentication
       clock exceeds the value of the last-timestamp.

     o The private authentication key and private privacy key
       must be known only to the parties requiring knowledge
       of them.

       Protecting the secrets from disclosure is critical to the
       security of the protocols. In particular, if the secrets are
       distributed via a network, the secrets must be protected
       with a protocol that supports confidentiality, e.g., the
       Symmetric Privacy Protocol. Further, knowledge of the
       secrets must be as restricted as possible within an
       implementation. In particular, although the secrets may
       be known to one or more persons during the initial
       configuration of a device, the secrets should be changed
       immediately after configuration such that their actual
       value is known only to the software. A management
       station has the additional responsibility of recovering the
       state of all parties whenever it boots, and it may address
       this responsibility by recording the secrets on a
       long-term storage device. Access to information on this
       device must be as restricted as is practically possible.

     o There must exist at least one SNMP protocol entity that
       assumes the role of a responsible management station.

       This management station is responsible for ensuring that

       all authentication clocks are synchronized and for
       changing the secret values when necessary. Although
       more than one management station may share this
       responsibility, their coordination is essential to the
       secure management of the network. The mechanism by
       which multiple management stations ensure that no
       more than one of them attempts to synchronize the
       clocks or update the secrets at any one time is a local
       implementation issue.

       A responsible management station may either support
       clock synchronization and secret distribution as separate
       functions, or combine them into a single functional unit.

   The first section below specifies the procedures by which a SNMP
   protocol entity is initially configured. The next two sections
   describe one strategy for distributing clock values and one for
   determining a synchronized clock value among SNMP parties supporting
   the Digest Authentication Protocol. For SNMP parties supporting the
   Symmetric Privacy Protocol, the next section describes a strategy for
   distributing secret values. The last section specifies the procedures
   by which a SNMP protocol entity recovers from a "crash."

6.1   Initial Configuration

   This section describes the initial configuration of a SNMP protocol
   entity that supports the Digest Authentication Protocol or both the
   Digest Authentication Protocol and the Symmetric Privacy Protocol.

   When a network device is first installed, its initial, secure
   configuration must be done manually, i.e., a person must physically
   visit the device and enter the initial secret values for at least its
   first secure SNMP party. This requirement suggests that the person
   will have knowledge of the initial secret values.

   In general, the security of a system is enhanced as the number of
   entities that know a secret is reduced. Requiring a person to
   physically visit a device every time a SNMP party is configured not
   only exposes the secrets unnecessarily but is administratively
   prohibitive. In particular, when MD5 is used, the initial
   authentication secret is 128 bits long and when DES is used an
   additional 128 bits are needed -- 64 bits each for the key and
   initialization vector. Clearly, these values will need to be recorded
   on a medium in order to be transported between a responsible
   management station and a managed agent. The recommended procedure is
   to configure a small set of initial SNMP parties for each SNMP
   protocol entity, one pair of which may be used initially to configure
   all other SNMP parties.

   In fact, there is a minimal, useful set of SNMP parties that could be
   configured between each responsible management station and managed
   agent. This minimal set includes one of each of the following for
   both the responsible management station and the managed agent:

     o a SNMP party for which the authentication protocol and
       privacy protocol are the values noAuth and noPriv,
       respectively,

     o a SNMP party for which the authentication protocol
       identifies the mechanism defined in Section 2.4.1 and its
       privacy protocol is the value noPriv, and

     o a SNMP party for which the authentication protocol and
       privacy protocol identify the mechanisms defined in
       Section 2.4.1 and Section 2.4.2, respectively.

   The last of these SNMP parties in both the responsible management
   station and the managed agent could be used to configure all other
   SNMP parties. It is the only suitable party for this purpose because
   it is the only party that supports data confidentiality, which is
   necessary in order to protect the distributed secrets from disclosure
   to unauthorized entities.

   Configuring one pair of SNMP parties to be used to configure all
   other parties has the advantage of exposing only one pair of secrets
   -- the secrets used to configure the minimal, useful set identified
   above. To limit this exposure, the responsible management station
   should change these values as its first operation upon completion of
   the initial configuration. In this way, secrets are known only to the
   peers requiring knowledge of them in order to communicate.

   The Management Information Base (MIB) document [4] supporting these
   security protocols specifies 6 initial party identities and initial
   values, which, by convention, are assigned to the parties and their
   associated parameters.

   All 6 parties should be configured in each new managed agent and its
   responsible management station. The responsible management station
   should be configured first, since the management station can be used
   to generate the initial secrets and provide them to a person, on a
   suitable medium, for distribution to the managed agent. The following
   sequence of steps describes the initial configuration of a managed
   agent and its responsible management station.

    1. Determine the initial values for each of the attributes of
       the SNMP party to be configured. Some of these values
       may be computed by the responsible management

       station, some may be specified in the MIB document,
       and some may be administratively determined.

    2. Configure the parties in the responsible management
       station, according to the set of initial values. If the
       management station is computing some initial values to
       be entered into the agent, an appropriate medium must
       be present to record the values.

    3. Configure the parties in the managed agent, according to
       the set of initial values.

    4. The responsible management station must synchronize
       the authentication clock values for each party it shares
       with each managed agent. Section 6.3 specifies one
       strategy by which this could be accomplished.

    5. The responsible management station should change the
       secret values manually configured to ensure the actual
       values are known only to the peers requiring knowledge
       of them in order to communicate. To do this, the
       management station generates new secrets for each party
       to be reconfigured and distributes those secrets with a
       strategy that uses a protocol that protects them from
       disclosure, e.g., Symmetric Privacy Protocol (see
       Section 6.4). Upon receiving positive acknowledgement
       that the new values have been distributed, the
       management station should update its local database
       with the new values.

   If the managed agent does not support a protocol that protects
   messages from disclosure, then automatic maintenance and
   configuration of parties is not possible, i.e., the last step above
   is not possible. The secrets can only be changed by a physical visit
   to the device.

   If there are other SNMP protocol entities requiring knowledge of the
   secrets, the responsible management station must distribute the
   information upon completion of the initial configuration. The
   mechanism used must protect the secrets from disclosure to
   unauthorized entities. The Symmetric Privacy Protocol, for example,
   is an acceptable mechanism.

6.2   Clock Distribution

   A responsible management station must ensure that the authentication
   clock value for each SNMP party for which it is responsible

     o is loosely synchronized among all the local databases in
       which it appears,

     o is reset, as indicated below, upon reaching its maximal
       value, and

     o is non-decreasing, except as indicated below.

   The skew among the clock values must be accounted for in the lifetime
   value, in addition to the expected communication delivery delay.

   A skewed authentication clock may be detected by a number of
   strategies, including knowledge of the accuracy of the system clock,
   unauthenticated queries of the party database, and recognition of
   authentication failures originated by the party.

   Whenever clock skew is detected, and whenever the SNMP entities at
   both the responsible management station and the relevant managed
   agent support an appropriate privacy protocol (e.g., the Symmetric
   Privacy Protocol), a straightforward strategy for the correction of
   clock skew is simultaneous alteration of authentication clock and
   private key for the relevant SNMP party. If the request to alter the
   key and clock for a particular party originates from that same party,
   then, prior to transmitting that request, the local notion of the
   authentication clock is artificially advanced to assure acceptance of
   the request as authentic.

   More generally, however, since an authentication clock value need not
   be protected from disclosure, it is not necessary that a managed
   agent support a privacy protocol in order for a responsible
   management station to correct skewed clock values. The procedure for
   correcting clock skew in the general case is presented in Section
   6.3.

   In addition to correcting skewed notions of authentication clocks,
   every SNMP entity must react correctly as an authentication clock
   approaches its maximal value. If the authentication clock for a
   particular SNMP party ever reaches the maximal time value, the clock
   must halt at that value.  (The value of interest may be the maximum
   less lifetime.  When authenticating a message, its authentication
   timestamp is added to lifetime and compared to the authentication
   clock.  A SNMP protocol entity must guarantee that the sum is never
   greater than the maximal time value.) In this state, the only
   authenticated request a management station should generate for this
   party is one that alters the value of at least its authentication
   clock and private authentication key. In order to reset these values,
   the responsible management station may set the authentication
   timestamp in the message to the maximal time value. In this case, the

   nonce value may be used to distinguish multiple messages.

   The value of the authentication clock for a particular SNMP party
   must never be altered such that its new value is less than its old
   value, unless its last-timestamp and private authentication key are
   also altered at the same time.

6.3   Clock Synchronization

   Unless the secrets are changed at the same time, the correct way to
   synchronize clocks is to advance the slower clock to be equal to the
   faster clock. Suppose that party agentParty is realized by the SNMP
   entity in a managed agent; suppose that party mgrParty is realized by
   the SNMP entity in the corresponding responsible management station.
   For any pair of parties, there are four possible conditions of the
   authentication clocks that could require correction:

    1. The management station's notion of the value of the
       authentication clock for agentParty exceeds the agent's
       notion.

    2. The management station's notion of the value of the
       authentication clock for mgrParty exceeds the agent's
       notion.

    3. The agent's notion of the value of the authentication
       clock for agentParty exceeds the management station's
       notion.

    4. The agent's notion of the value of the authentication
       clock for mgrParty exceeds the management station's
       notion.

   The selective clock acceleration mechanism intrinsic to the protocol
   corrects conditions 2 and 3 as part of the normal processing of an
   authentic message. Therefore, the clock adjustment procedure below
   does not provide for any adjustments in those cases. Rather, the
   following sequence of steps specifies how the clocks may be
   synchronized when condition 1, condition 4, or both of those
   conditions are manifest.

    1. The responsible management station saves its existing
       notions of the authentication clocks for the two parties
       agentParty and mgrParty.

    2. The responsible management station retrieves the
       authentication clock values for both agentParty and
       mgrParty from the agent. This retrieval must be an

       unauthenticated request, since the management station
       does not know if the clocks are synchronized. If the
       request fails, the clocks cannot be synchronized, and the
       clock adjustment procedure is aborted without further
       processing.

    3. If the management station's notion of the authentication
       clock for agentParty exceeds the notion just retrieved
       from the agent by more than the amount of the
       communications delay between the two protocol entities,
       then condition 1 is manifest. The recommended estimate
       of communication delay in this context is one half of the
       lifetime value recorded for agentParty.

    4. If the notion of the authentication clock for mgrParty
       just retrieved from the agent exceeds the management
       station's notion, then condition 4 is manifest, and the
       responsible management station advances its notion of
       the authentication clock for mgrParty to match the
       agent's notion.

    5. If condition 1 is manifest, then the responsible
       management station sends an authenticated
       management operation to the agent that advances the
       agent's notion of the authentication clock for
       agentParty to be equal to the management station's
       notion. If this management operation fails, then the
       management station restores its previously saved notions
       of the clock values, and the clock adjustment procedure
       is aborted without further processing.

    6. The responsible management station retrieves the
       authentication clock values for both agentParty and
       mgrParty from the agent. This retrieval must be an
       authenticated request, in order that the management
       station may verify that the clock values are properly
       synchronized. If this authenticated query fails, then the
       management station restores its previously saved notions
       of the clock values, and the clock adjustment procedure
       is aborted without further processing. Otherwise, clock
       synchronization has been successfully realized.

   It is important to note step 4 above must be completed before
   attempting step 5. Otherwise, the agent may evaluate the request in
   step 5 as unauthentic. Similarly, step 5 above must be completed
   before attempting step 6. Otherwise, the management station may
   evaluate the query response in step 6 as unauthentic.

   Administrative advancement of a clock as described above does not
   introduce any new vulnerabilities, since the value of the clock is
   intended to increase with the passage of time. A potential
   operational problem is the rejection of management operations that
   are authenticated using a previous value of the relevant party clock.
   This possibility may be avoided if a management station suppresses
   generation of management traffic between relevant parties while this
   clock adjustment procedure is in progress.

6.4   Secret Distribution

   This section describes one strategy by which a SNMP protocol entity
   that supports both the Digest Authentication Protocol and the
   Symmetric Privacy Protocol can change the secrets for a particular
   SNMP party.

   The frequency with which the secrets of a SNMP party should be
   changed is a local administrative issue. However, the more frequently
   a secret is used, the more frequently it should be changed. At a
   minimum, the secrets must be changed whenever the associated
   authentication clock approaches its maximal value (see Section 7).
   Note that, owing to both administrative and automatic advances of the
   authentication clock described in this memo, the authentication clock
   for a SNMP party may well approach its maximal value sooner than
   might otherwise be expected.

   The following sequence of steps specifies how a responsible
   management station alters a secret value (i.e., the private
   authentication key or the private privacy key) for a particular SNMP
   party.

    1. The responsible management station generates a new
       secret value.

    2. The responsible management station encapsulates a
       SNMP Set request in a SNMP private management
       communication with at least the following properties.

        o Its source supports the Digest Authentication
          Protocol and the Symmetric Privacy Protocol.

        o Its destination supports the Symmetric Privacy
          Protocol and the Digest Authentication Protocol.

    3. The SNMP private management communication is
       transmitted to its destination.

    4. Upon receiving the request, the recipient processes the

       message according to [1] and [2].

    5. The recipient encapsulates a SNMP Set response in a
       SNMP private management communication with at least
       the following properties.

        o Its source supports the Digest Authentication
          Protocol and the Symmetric Privacy Protocol.

        o Its destination supports the Symmetric Privacy
          Protocol and the Digest Authentication Protocol.

    6. The SNMP private management communication is
       transmitted to its destination.

    7. Upon receiving the response, the responsible
       management station updates its local database with the
       new value.

   If the responsible management station does not receive a response to
   its request, there are two possible causes.

     o The request may not have been delivered to the
       destination.

     o The response may not have been delivered to the
       originator of the request.

   In order to distinguish the two possible error conditions, a
   responsible management station could check the destination to see if
   the change has occurred. Unfortunately, since the secret values are
   unreadable, this is not directly possible.

   The recommended strategy for verifying key changes is to set the
   public value corresponding to the secret being changed to a
   recognizable, novel value: that is, alter the public authentication
   key value for the relevant party when changing its private
   authentication key, or alter its public privacy key value when
   changing its private privacy key. In this way, the responsible
   management station may retrieve the public value when a response is
   not received, and verify whether or not the change has taken place.
   (This strategy is available since the public values are not used by
   the protocols defined in this memo. If this strategy is employed,
   then the public values are significant in this context. Of course,
   protocols using the public values may make use of this strategy
   directly.)

   One other scenario worthy of mention is using a SNMP party to change

   its own secrets. In this case, the destination will change its local
   database prior to generating a response. Thus, the response will be
   constructed according to the new value.  However, the responsible
   management station will not update its local database until after the
   response is received. This suggests the responsible management
   station may receive a response which will be evaluated as
   unauthentic, unless the correct secret is used. The responsible
   management station may either account for this scenario as a special
   case, or use an alteration of the relevant public values (as
   described above) to verify the key change.

   Note, during the period of time after the request has been sent and
   before the response is received, the management station must keep
   track of both the old and new secret values. Since the delay may be
   the result of a network failure, the management station must be
   prepared to retain both values for an extended period of time,
   including across reboots.

6.5   Crash Recovery

   This section describes the requirements for SNMP protocol entities in
   connection with recovery from system crashes or other service
   interruptions.

   For each SNMP party in the local database for a particular SNMP
   protocol entity, its identity, authentication clock, private
   authentication key, and private privacy key must enjoy non-volatile,
   incorruptible representations. If possible, lifetime should also
   enjoy a non-volatile, incorruptible representation.  If said protocol
   entity supports other security protocols or algorithms in addition to
   the two defined in this memo, then the authentication protocol and
   the privacy protocol for each party also require non-volatile,
   incorruptible representation.

   The authentication clock of a SNMP party is a critical component of
   the overall security of the protocols. The inclusion of a reliable
   representation of a clock in a SNMP protocol entity enhances overall
   security. A reliable clock representation continues to increase
   according to the passage of time, even when the local SNMP protocol
   entity -- due to power loss or other system failure -- may not be
   operating.  An example of a reliable clock representation is that
   provided by battery-powered clock-calendar devices incorporated into
   some contemporary systems. It is assumed that management stations
   always support reliable clock representations, where clock adjustment
   by a human operator during crash recovery may contribute to that
   reliability.

   If a managed agent crashes and does not reboot in time for its

   responsible management station to prevent its authentication clock
   from reaching its maximal value, upon reboot the clock must be halted
   at its maximal value. The procedures specified in Section 6.3 would
   then apply.

   If a managed network element supports a reliable clock
   representation, recovering from a crash requires few special actions.
   Upon recovery, those attributes of each SNMP party that do not enjoy
   non-volatile or reliable representation are initialized as follows.

     o If the private authentication key is not the OCTET
       STRING of zero length, the authentication protocol is
       set to identify use of the Digest Authentication Protocol
       in conjunction with the algorithm specified in
       Section 2.4.1.

     o The last-timestamp is initialized to the value of the
       authentication clock.

     o The nonce is initialized to zero.

     o If the lifetime is not retained, it should be initialized to
       zero.

     o If the private privacy key is not the OCTET STRING
       of zero length, the privacy protocol is set to identify use
       of the Symmetric Privacy Protocol in conjunction with
       the algorithm specified in Section 2.4.2.

   Upon detecting that a managed agent has rebooted, a responsible
   management station must reset all other party attributes, including
   the lifetime if it was not retained. In order to reset the lifetime,
   the responsible management station should set the authentication
   timestamp in the message to the sum of the authentication clock and
   desired lifetime. This is an artificial advancement of the
   authentication timestamp in order to guarantee the message will be
   authentic when received by the recipient.

   If, alternatively, a managed network element does not support a
   reliable clock representation, then those attributes of each SNMP
   party that do not enjoy non-volatile representation are initialized
   as follows.

     o If the private authentication key is not the OCTET
       STRING of zero length, the authentication protocol is
       set to identify use of the Digest Authentication Protocol
       in conjunction with the algorithm specified in
       Section 2.4.1.

     o The authentication clock is initialized to the maximal
       time value.

     o The last-timestamp is initialized to the maximal time
       value.

     o The nonce is initialized to zero.

     o If the lifetime is not retained, it should be initialized to
       zero.

     o If the private privacy key is not the OCTET STRING
       of zero length, the privacy protocol is set to identify use
       of the Symmetric Privacy Protocol in conjunction with
       the algorithm specified in Section 2.4.2.

   The only authenticated request a management station should generate
   for a party in this initial state is one that alters the value of at
   least its authentication clock, private authentication key, and
   lifetime (if that was not retained). In order to reset these values,
   the responsible management station must set the authentication
   timestamp in the message to the maximal time value. The nonce value
   may be used to distinguish multiple messages.

7.  Security Considerations

   This section highlights security considerations relevant to the
   protocols and procedures defined in this memo. Practices that
   contribute to secure, effective operation of the mechanisms defined
   here are described first. Constraints on implementation behavior that
   are necessary to the security of the system are presented next.
   Finally, an informal account of the contribution of each mechanism of
   the protocols to the required goals is presented.

7.1   Recommended Practices

   This section describes practices that contribute to the secure,
   effective operation of the mechanisms defined in this memo.

     o A management station should discard SNMP responses
       for which neither the request-id component nor the
       represented management information corresponds to any
       currently outstanding request.

       Although it would be typical for a management station
       to do this as a matter of course, in the context of these
       security protocols it is significant owing to the possibility
       of message duplication (malicious or otherwise).

     o A management station should not interpret an agent's
       lack of response to an authenticated SNMP management
       communication as a conclusive indication of agent or
       network failure.

       It is possible for authentication failure traps to be lost or
       suppressed as a result of authentication clock skew or
       inconsistent notions of shared secrets. In order either to
       facilitate administration of such SNMP parties or to
       provide for continued management in times of network
       stress, a management station implementation may
       provide for arbitrary, artificial advancement of the
       timestamp or selection of shared secrets on locally
       generated messages.

     o The lifetime value for a SNMP party should be chosen
       (by the local administration) to be as small as possible,
       given the accuracy of clock devices available, relevant
       round-trip communications delays, and the frequency
       with which a responsible management station will be
       able to verify all clock values.

       A large lifetime increases the vulnerability to malicious
       delays of SNMP messages. The implementation of a
       management station may, when explicitly authorized,
       provide for dynamic adjustment of the lifetime in order
       to accommodate changing network conditions.

     o When sending state altering messages to a managed
       agent, a management station should delay sending
       successive messages to the managed agent until a
       positive acknowledgement is received for the previous
       message or until the previous message expires.

       When using the noAuth protocol, no message ordering
       is imposed by the SNMP. Messages may be received in
       any order relative to their time of generation and each
       will be processed in the ordered received. In contrast,
       the security protocols guarantee that received messages
       are ordered insofar as each received message must have
       been sent subsequent to the sending of a previously
       received message.

       When an authenticated message is sent to a managed
       agent, it will be valid for a period of time that does not
       exceed lifetime under normal circumstances. During the
       period of time this message is valid, if the management
       station sends another authenticated message to the

       managed agent that is received and processed prior to
       the first message, the first message will be considered
       unauthentic when it is received by the managed agent.

       Indeed, a management station must cope with the loss
       and re-ordering of messages resulting from anomalies in
       the network as a matter of course. A management
       station implementation may choose to prevent the loss
       of messages resulting from re-ordering when using the
       security protocols defined in this memo by delaying
       sending successive messages.

     o The frequency with which the secrets of a SNMP party
       should be changed is indirectly related to the frequency
       of their use.

       Protecting the secrets from disclosure is critical to the
       overall security of the protocols. Frequent use of a secret
       provides a continued source of data that may be useful
       to a cryptanalyst in exploiting known or perceived
       weaknesses in an algorithm. Frequent changes to the
       secret avoid this vulnerability.

       Changing a secret after each use is is generally regarded
       as the most secure practice, but a significant amount of
       overhead may be associated with that approach.

       Note, too, in a local environment the threat of disclosure
       may be insignificant, and as such the changing of secrets
       may be less frequent. However, when public data
       networks are the communication paths, more caution is
       prudent.

     o In order to foster the greatest degree of security, a
       management station implementation must support
       constrained, pairwise sharing of secrets among SNMP
       entities as its default mode of operation.

       Owing to the use of symmetric cryptography in the
       protocols defined here, the secrets associated with a
       particular SNMP party must be known to all other
       SNMP parties with which that party may wish to
       communicate. As the number of locations at which
       secrets are known and used increases, the likelihood of
       their disclosure also increases, as does the potential
       impact of that disclosure. Moreover, if the set of SNMP
       protocol entities with knowledge of a particular secret
       numbers more than two, data origin cannot be reliably

       authenticated because it is impossible to determine with
       any assurance which entity of that set may be the
       originator of a particular SNMP message. Thus, the
       greatest degree of security is afforded by configurations
       in which the secrets for each SNMP party are known to
       at most two protocol entities.

7.2   Conformance

   A SNMP protocol entity implementation that claims conformance to this
   memo must satisfy the following requirements:

    1. It must implement the noAuth and noPriv protocols
       whose object identifiers are defined in [4].

       noAuth  This protocol signifies that messages generated
          by a party using it are not protected as to origin or
          integrity. It is required to ensure that a party's
          authentication clock is always accessible.
       noPriv  This protocol signifies that messages received
          by a party using it are not protected from
          disclosure. It is required to ensure that a party's
          authentication clock is always accessible.

    2. It must implement the Digest Authentication Protocol in
       conjunction with the algorithm defined in Section 2.4.1.

    3. It must include in its local database at least one SNMP
       party with the following parameters set as follows:

        o partyAuthProtocol is set to noAuth and
        o partyPrivProtocol is set to noPriv.

       This party must have a MIB view [2] specified that
       includes at least the authentication clock of all other
       parties. Alternatively, the authentication clocks of the
       other parties may be partitioned among several similarly
       configured parties according to a local implementation
       convention.

    4. For each SNMP party about which it maintains
       information in a local database, an implementation must
       satisfy the following requirements:

      (a) It must not allow a party's parameters to be set to
          a value inconsistent with its expected syntax. In
          particular, Section 2.4 specifies constraints for the
          chosen mechanisms.

      (b) It must, to the maximal extent possible, prohibit
          read-access to the private authentication key and
          private encryption key under all circumstances
          except as required to generate and/or validate
          SNMP messages with respect to that party. This
          prohibition includes prevention of read-access by
          the entity's human operators.
      (c) It must allow the party's authentication clock to be
          publicly accessible. The correct operation of the
          Digest Authentication Protocol requires that it be
          possible to determine this value at all times in
          order to guarantee that skewed authentication
          clocks can be resynchronized.
      (d) It must prohibit alterations to its record of the
          authentication clock for that party independently of
          alterations to its record of the private
          authentication key (unless the clock alteration is an
          advancement).
      (e) It must never allow its record of the authentication
          clock for that party to be incremented beyond the
          maximal time value and so "roll-over" to zero.
      (f) It must never increase its record of the lifetime for
          that party except as may be explicitly authorized
          (via imperative command or securely represented
          configuration information) by the responsible
          network administrator.
      (g) In the event that the non-volatile, incorruptible
          representations of a party's parameters (in
          particular, either the private authentication key or
          private encryption key) are lost or destroyed, it
          must alter its record of these quantities to random
          values so subsequent interaction with that party
          requires manual redistribution of new secrets and
          other parameters.

    5. If it selects new value(s) for a party's secret(s), it must
       avoid bad or obvious choices for said secret(s). Choices
       to be avoided are boundary values (such as all-zeros)
       and predictable values (such as the same value as
       previously or selecting from a predetermined set).

7.3   Protocol Correctness

   The correctness of these SNMP security protocols with respect to the
   stated goals depends on the following assumptions:

    1. The chosen message digest algorithm satisfies its design
       criteria. In particular, it must be computationally
       infeasible to discover two messages that share the same
       digest value.

    2. It is computationally infeasible to determine the secret
       used in calculating a digest on the concatenation of the
       secret and a message when both the digest and the
       message are known.

    3. The chosen symmetric encryption algorithm satisfies its
       design criteria. In particular, it must be computationally
       infeasible to determine the cleartext message from the
       ciphertext message without knowledge of the key used in
       the transformation.

    4. Local notions of a party's authentication clock while it is
       associated with a specific private key value are
       monotonically non-decreasing (i.e., they never run
       backwards) in the absence of administrative
       manipulations.

    5. The secrets for a particular SNMP party are known only
       to authorized SNMP protocol entities.

    6. Local notions of the authentication clock for a particular
       SNMP party are never altered such that the
       authentication clock's new value is less than the current
       value without also altering the private authentication
       key.

   For each mechanism of the protocol, an informal account of its
   contribution to the required goals is presented below.  Pseudocode
   fragments are provided where appropriate to exemplify possible
   implementations; they are intended to be self-explanatory.

7.3.1   Clock Monotonicity Mechanism

   By pairing each sequence of a clock's values with a unique key, the
   protocols partially realize goals 3 and 4, and the conjunction of
   this property with assumption 6 above is sufficient for the claim
   that, with respect to a specific private key value, all local notions
   of a party's authentication clock are, in general, non-decreasing
   with time.

7.3.2   Data Integrity Mechanism

   The protocols require computation of a message digest computed over
   the SNMP message prepended by the secret for the relevant party. By
   virtue of this mechanism and assumptions 1 and 2, the protocols
   realize goal 1.

   Normally, the inclusion of the message digest value with the digested
   message would not be sufficient to guarantee data integrity, since
   the digest value can be modified in addition to the message while it
   is enroute. However, since not all of the digested message is
   included in the transmission to the destination, it is not possible
   to substitute both a message and a digest value while enroute to a
   destination.

   Strictly speaking, the specified strategy for data integrity does not
   detect a SNMP message modification which appends extraneous material
   to the end of such messages. However, owing to the representation of
   SNMP messages as ASN.1 values, such modifications cannot --
   consistent with goal 1 -- result in unauthorized management
   operations.

   The data integrity mechanism specified in this memo protects only
   against unauthorized modification of individual SNMP messages. A more
   general data integrity service that affords protection against the
   threat of message stream modification is not realized by this
   mechanism, although limited protection against reordering, delay, and
   duplication of messages within a message stream are provided by other
   mechanisms of the protocol.

7.3.3   Data Origin Authentication Mechanism

   The data integrity mechanism requires the use of a secret value known
   only to communicating parties. By virtue of this mechanism and
   assumptions 1 and 2, the protocols explicitly prevent unauthorized
   modification of messages. Data origin authentication is implicit if
   the message digest value can be verified. That is, the protocols
   realize goal 2.

7.3.4   Restricted Administration Mechanism

   This memo requires that implementations preclude administrative
   alterations of the authentication clock for a particular party
   independently from its private authentication key (unless that clock
   alteration is an advancement). An example of an efficient
   implementation of this restriction is provided in a pseudocode
   fragment below. This pseudocode fragment meets the requirements of
   assumption 6.

   Pseudocode Fragment. Observe that the requirement is not for
   simultaneous alteration but to preclude independent alteration. This
   latter requirement is fairly easily realized in a way that is
   consistent with the defined semantics of the SNMP Set operation.

   Void partySetKey (party, newKeyValue)
   {
       if (party->clockAltered) {
          party->clockAltered = FALSE;
          party->keyAltered = FALSE;
          party->keyInUse = newKeyValue;
          party->clockInUse = party->clockCache;
       }
       else {
          party->keyAltered = TRUE;
          party->keyCache = newKeyValue;
       }
   }

   Void partySetClock (party, newClockValue)
   {
       if (party->keyAltered) {
          party->keyAltered = FALSE;
          party->clockAltered = FALSE;
          party->clockInUse = newClockValue;
          party->keyInUse = party->keyCache;
       }
       else {
          party->clockAltered = TRUE;
          party->clockCache = newClockValue;
       }
   }

7.3.5   Ordered Delivery Mechanism

   The definition of the Digest Authentication Protocol requires that,
   if the timestamp value on a received message does not exceed the
   timestamp of the most recent validated message locally delivered from
   the originating party, then that message is not delivered. Otherwise,
   the record of the timestamp for the most recent locally delivered
   validated message is updated.

   if (msgIsValidated) {
       if (timestampOfReceivedMsg >
          party->timestampOfLastDeliveredMsg) {

          party->timestampOfLastDeliveredMsg =
             timestampOfReceivedMsg;
       }
       else {
          msgIsValidated = FALSE;
       }
   }

   Although not explicitly represented in the pseudocode above, in the
   Digest Authentication Protocol, the ordered delivery mechanism must
   ensure that, when the authentication timestamp of the received
   message is equal to the last-timestamp, received messages continue to
   be delivered as long as their nonce values are monotonically
   increasing. By virtue of this mechanism, the protocols realize goal
   4.

7.3.6   Message Timeliness Mechanism

   The definition of the SNMP security protocols requires that, if the
   authentication timestamp value on a received message -- augmented by
   an administratively chosen lifetime value -- is less than the local
   notion of the clock for the originating SNMP party, the message is
   not delivered.

   if (timestampOfReceivedMsg +
          party->administrativeLifetime <=
          party->localNotionOfClock) {
          msgIsValidated = FALSE;
   }

   By virtue of this mechanism, the protocols realize goal 3. In cases
   in which the local notions of a particular SNMP party clock are
   moderately well-synchronized, the timeliness mechanism effectively
   limits the age of validly delivered messages. Thus, if an attacker
   diverts all validated messages for replay much later, the delay
   introduced by this attack is limited to a period that is proportional
   to the skew among local notions of the party clock.

7.3.7   Selective Clock Acceleration Mechanism

   The definition of the SNMP security protocols requires that, if the
   timestamp value on a received, validated message exceeds the local
   notion of the clock for the originating party, then that notion is
   adjusted forward to correspond to said timestamp value. This
   mechanism is neither strictly necessary nor sufficient to the

   security of the protocol; rather, it fosters the clock
   synchronization on which valid message delivery depends -- thereby
   enhancing the effectiveness of the protocol in a management context.

   if (msgIsValidated) {
          if (timestampOfReceivedMsg >
                party->localNotionOfClock) {
                party->localNotionOfClock =
                      timestampOfReceivedMsg;
          }
   }

   The effect of this mechanism is to synchronize local notions of the
   party clock more closely in the case where a sender's notion is more
   advanced than a receiver's. In the opposite case, this mechanism has
   no effect on local notions of the party clock and either the received
   message is validly delivered or not according to other mechanisms of
   the protocol.

   Operation of this mechanism does not, in general, improve the
   probability of validated delivery for messages generated by party
   participants whose local notion of the party clock is relatively less
   advanced. In this case, queries from a management station may not be
   validly delivered and the management station needs to react
   appropriately (e.g., by administratively resynchronizing local
   notions of the clock in conjunction with a key change). In contrast,
   the delivery of SNMP trap messages generated by an agent that suffers
   from a less advanced notion of a party clock is more problematic, for
   an agent may lack the capacity to recognize and react to security
   failures that prevent delivery of its messages. Thus, the inherently
   unreliable character of trap messages is likely to be compounded by
   attempts to provide for their validated delivery.

7.3.8   Confidentiality Mechanism

   The protocols require the use of a symmetric encryption algorithm
   when the data confidentiality service is required. By virtue of this
   mechanism and assumption 3, the protocols realize goal 5.

8.  Acknowledgements

   The authors would like to thank the members of the SNMP Security
   Working Group of the IETF for their patience and comments. Special
   thanks go to Jeff Case who provided the first implementation of the
   protocols. Dave Balenson, John Linn, Dan Nessett, and all the members
   of the Privacy and Security Research Group provided many valuable and

   detailed comments.

9.  References

   [1] Case, J., M. Fedor, M. Schoffstall, and J. Davin, The Simple
       Network Management Protocol", RFC 1157, University of Tennessee
       at Knoxville, Performance Systems International, Performance
       Systems International, and the MIT Laboratory for Computer
       Science, May 1990.  (Obsoletes RFC 1098.)

   [2] Davin, J., Galvin, J., and K. McCloghrie, "SNMP Administrative
       Model", RFC 1351, MIT Laboratory for Computer Science, Trusted
       Information Systems, Inc., Hughes LAN Systems, Inc., July 1992.

   [3] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, MIT
       Laboratory for Computer Science, April 1992.

   [4] McCloghrie, K., Davin, J., and J. Galvin, "Definitions of Managed
       Objects for Administration of SNMP Parties", RFC 1353, Hughes LAN
       Systems, Inc., MIT Laboratory for Computer Science, Trusted
       Information Systems, Inc., July 1992.

   [5] FIPS Publication 46-1, "Data Encryption Standard", National
       Institute of Standards and Technology, Federal Information
       Processing Standard (FIPS); Supersedes FIPS Publication 46,
       January 15, 1977; Reaffirmed January 22, 1988.

   [6] ANSI X3.92-1981, "Data Encryption Algorithm", American National
       Standards Institute, December 30, 1980.

   [7] FIPS Publication 81, "DES Modes of Operation", National Institute
       of Standards and Technology, December 2, 1980, Federal
       Information Processing Standard (FIPS).

   [8] ANSI X3.106-1983, "Data Encryption Algorithm - Modes of
       Operation", American National Standards Institute, May 16, 1983.

   [9] FIPS Publication 74, "Guidelines for Implementing and Using the
       NBS Data Encryption Standard", National Institute of Standards
       and Technology, April 1, 1981.  Federal Information Processing
       Standard (FIPS).

  [10] Special Publication 500-20, "Validating the Correctness of
       Hardware Implementations of the NBS Data Encryption Standard",
       National Institute of Standards and Technology.

  [11] Special Publication 500-61, "Maintenance Testing for the Data
       Encryption Standard", National Institute of Standards and

       Technology, August 1980.

  [12] Information Processing -- Open Systems Interconnection --
       Specification of Basic Encoding Rules for Abstract Syntax
       Notation One (ASN.1), International Organization for
       Standardization/International Electrotechnical Institute, 1987,
       International Standard 8825.

10.  Authors' Addresses

       James M. Galvin
       Trusted Information Systems, Inc.
       3060 Washington Road, Route 97
       Glenwood, MD 21738

       Phone:  (301) 854-6889
       EMail:  galvin@tis.com

       Keith McCloghrie
       Hughes LAN Systems, Inc.
       1225 Charleston Road
       Mountain View, CA 94043

       Phone:  (415) 966-7934
       EMail:  kzm@hls.com

       James R. Davin
       MIT Laboratory for Computer Science
       545 Technology Square
       Cambridge, MA 02139

       Phone:  (617) 253-6020
       EMail:  jrd@ptt.lcs.mit.edu

 

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