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RFC 5421 - Basic Password Exchange within the Flexible Authentic

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Network Working Group                                      N. Cam-Winget
Request for Comments: 5421                                       H. Zhou
Category: Informational                                    Cisco Systems
                                                              March 2009

       Basic Password Exchange within the Flexible Authentication
   via Secure Tunneling Extensible Authentication Protocol (EAP-FAST)

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

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   document authors.  All rights reserved.

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   EAP-FAST has been implemented by many vendors and it is used in the
   Internet.  Publication of this specification is intended to promote
   interoperability by documenting current use of existing EAP methods
   within EAP-FAST.

   The EAP method EAP-FAST-GTC reuses the EAP type code assigned to EAP-
   GTC (6).  The reuse of previously assigned EAP Type Codes is
   incompatible with EAP method negotiation as defined in RFC 3748.

   Since EAP-GTC does not support method-specific version negotiation,
   the use of EAP-FAST-GTC is implied when used inside the EAP-FAST
   tunnel during authentication.  This behavior may cause problems in
   implementations where the use of another vendor's EAP-GTC is
   required.  Since such support requires special case execution of a
   method within a tunnel, it also complicates implementations that use
   the same method code both within and outside of the tunnel method.
   If EAP-FAST were to be designed today, these difficulties could be
   avoided by utilization of unique EAP Type codes.  Given these issues,
   assigned method types must not be re-used with different meaning
   inside tunneled methods in the future.


   The Flexible Authentication via Secure Tunneling Extensible
   Authentication Protocol (EAP-FAST) method enables secure
   communication between a peer and a server by using Transport Layer
   Security (TLS) to establish a mutually authenticated tunnel.  Within
   this tunnel, a basic password exchange, based on the Generic Token
   Card method (EAP-GTC), may be executed to authenticate the peer.

Table of Contents

   1. Introduction ....................................................2
      1.1. Specification Requirements .................................3
   2. EAP-FAST GTC Authentication .....................................3
   3. Security Considerations .........................................7
      3.1. Security Claims ............................................7
   4. IANA Considerations .............................................8
   5. Acknowledgments .................................................9
   6. References ......................................................9
      6.1. Normative References .......................................9
      6.2. Informative References .....................................9

1.  Introduction

   EAP-FAST [RFC4851] is an EAP method that can be used to mutually
   authenticate a peer and server.  This document describes the EAP-FAST
   inner EAP method, EAP-FAST-GTC, which is used to authenticate the
   peer through a basic password exchange.  EAP-FAST-GTC was developed
   to support using cleartext passwords to authenticate to legacy user
   databases, to facilitate password change, and to support one time
   password features such as new pin mode.  Message exchanges, including
   user credentials, are cleartext strings transferred within the
   encrypted TLS tunnel and thus are considered secure.  For historical
   reasons, EAP-FAST-GTC uses EAP Type 6, originally allocated to EAP-
   GTC [RFC3748].  Note that EAP-FAST-GTC payloads used in EAP-FAST
   require specific formatting and therefore will not necessarily be

   compatible with EAP-GTC mechanisms used outside of EAP-FAST.  To
   avoid interference between these two methods, EAP-FAST-GTC MUST NOT
   be used outside an EAP-FAST tunnel, and EAP-GTC MUST NOT be used
   inside an EAP-FAST tunnel.  All EAP-FAST-GTC packets sent within the
   TLS tunnel must be encapsulated in EAP Payload TLVs, described in

   It is assumed that a reader of this document is familiar with EAP-
   FAST [RFC4851].

1.1.  Specification Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  EAP-FAST GTC Authentication

   All EAP-FAST-GTC packets inside EAP-FAST other than the empty
   acknowledgment packet MUST follow the "LABEL=Value" format.  All
   Labels are in ASCII text and SHALL NOT contain the space character.
   Currently, three Labels are defined:

   o  "CHALLENGE", the server request packet MUST be in the form of
      "CHALLENGE=Value", where Value is the server challenge, such as
      "please enter your password".

   o  "RESPONSE", the peer response packet MUST be in the form of
      "RESPONSE=Value", where Value is the peer response.

   o  "E", the server failure packet MUST be in the form of "E=Value",
      where Value is the error message generated by the server.

   If the peer or the server receives an EAP-FAST-GTC request or
   response that is not in the format specified above, it SHOULD fail
   the authentication by sending a Result TLV with a failure.

   After the TLS encryption tunnel is established and EAP-FAST
   Authentication phase 2 starts, the EAP server sends an EAP-FAST-GTC
   Request, which contains a server challenge.  The server challenge is
   a displayable message for use by the peer to prompt the user.

   A peer MAY prompt the user for the user credentials, or decide to use
   the user credentials gained through some other means without
   prompting the user.  The peer sends the user credentials back in the
   EAP-FAST-GTC Response using the following format:


   where "user@example.com" is the actual username and "secret" is the
   actual password.  The NULL character "\0" is used to separate the
   username and password.

   The username and password are included in a single message in the
   first response packet as an optimization by eliminating the inner
   method EAP-Identity exchange to save an extra round trip.

   Once the EAP-FAST server receives the user credentials, it SHOULD
   first validate the user identity with the Initiator ID (I-ID)
   [RFC5422] in the PAC-Opaque (Protected Access Credential) and if it
   matches, it will continue to authenticate the user with internal or
   external user databases.

   Additional exchanges MAY occur between the EAP-FAST server and peer
   to facilitate various user authentications.  The EAP-FAST server
   might send additional challenges to prompt the peer for additional
   information, such as a request for the next token or a new pin in the
   one time password case, or a server failure packet to indicate an
   error.  The peer displays the prompt to the user again and sends back
   the needed information in an EAP-FAST-GTC Response.  The exchange
   ends when a Result TLV is received.

   An EAP-FAST-GTC server implementation within EAP-FAST uses the
   following format to indicate an error if an authentication fails:

       "E=eeeeeeeeee R=r M=<msg>"


   The "eeeeeeeeee" is the ASCII representation of a decimal error code
   corresponding to one of those listed below, though peer
   implementations SHOULD deal with codes not on this list gracefully.

   The error code need not be 10 digits long.

   Below is a complete list of predefined error codes:


      Indicates that access is attempted outside the allowed hours.
      Peer implementations SHOULD display the error message to the user
      and ask the user to try at a later time.


      Indicates that the requested account is disabled.  Peer
      implementations SHOULD display the error message to the user,
      which helps the user to resolve the issue with the administrator.


      Indicates that the password has expired and a password change is
      required.  Peer implementations SHOULD prompt the user for a new
      password and send back the new password in the peer response


      Indicates that access has been denied due to lack of dial-in
      permission.  Peer implementations SHOULD display the error message
      to the user, which helps the user to resolve the issue with the


      Indicates that there was authentication failure due to an
      incorrect username or password.  Based on the retry flag described
      below, peer implementations MAY prompt the user again for a new
      set of username and password or simply send back an empty
      acknowledgment packet to acknowledge the failure and go into the
      termination phase of the authentication session.


      Indicates that the password change failed, most likely because the
      new password fails to meet the password complexity policy.  Peer
      implementations SHOULD display the error message and prompt the
      user again for the new password.


      Indicates that the PAC used to establish the EAP-FAST session
      cannot be used to authenticate to this user account.  Based on the
      retry flag described below, peer implementations MAY prompt the
      user again for a new set of username and password or simply send
      back an empty acknowledgment packet to acknowledge the failure and
      go into the termination phase of the authentication session.

   The "r" is a single character ASCII flag set to '1' if a retry is
   allowed, and '0' if not.  When the server sets this flag to '1', it
   disables short timeouts, expecting the peer to prompt the user for

   new credentials and to resubmit the response.  When the server sets
   this flag to '0', the peer SHOULD NOT prompt the user for new
   credentials to try again without restarting the EAP-FAST
   authentication from the beginning.

   The <msg> is human-readable ASCII text.  Current implementations only
   support ASCII text.

   The server failure packet can be broken into Label/Value pairs using
   the space character as the separator.  The only value that may
   contain the space character is the <msg> value, which is always the
   last value pair in the failure packet.  The peer SHOULD ignore any
   unknown label/value pair in the failure packet.

   The error format described above is similar to what is defined in the
   Microsoft Challenge Handshake Authentication Protocol version 2
   (MSCHAPv2) [RFC2759], except for the omission of a server challenge.
   So if the EAP-FAST server is distributing MSCHAPv2 exchanges to the
   backend inner method server, it can simply return what the backend
   inner method server returns less the server challenge.  In the case
   of connecting to a one time password or Lightweight Directory Access
   Protocol (LDAP) [RFC4511] server, the EAP-FAST server can translate
   the error message into this format.  With the addition of the retry
   count, the peer can potentially prompt the user for new credentials
   to try again without restarting the EAP-FAST authentication from the
   beginning.  The peer will respond to the error code with another EAP-
   FAST-GTC Response packet with both the new username and password, or
   in case of other unrecoverable failures, an empty EAP-FAST-GTC packet
   for acknowledgement.  The peer uses empty EAP-FAST-GTC payload as an
   acknowledgment of the unrecoverable failure.

   If the EAP-FAST server finishes authentication for the EAP-FAST-GTC
   inner method, it will proceed to Protected Termination as described
   in [RFC4851].  In the case of an unrecoverable EAP-FAST-GTC
   authentication failure, the EAP server can send an EAP-FAST-GTC error
   code as described above, along with the Result TLV for protected
   termination.  This way, no extra round trips will occur.  The peer
   can acknowledge the EAP-FAST-GTC failure as well as the Result TLV
   within the same EAP-FAST packet.  Once the server receives the
   acknowledgement, the TLS tunnel will be torn down and a clear text
   EAP-Failure will be sent.

   The username and password, as well as server challenges, MAY support
   non-ASCII characters.  In this case, international username,
   password, and messages are based on the use of Unicode characters,
   encoded as UTF-8 [RFC3629] and processed with a certain algorithm to

   ensure a canonical representation.  The username and password input
   SHOULD be processed according to Section 2.4 of [RFC4282], and the
   server challenges SHOULD be processed according to [RFC5198].

   Since EAP-FAST-GTC does not generate session keys, the MSKi (Master
   Session Key) used for crypto-binding for EAP-FAST will be filled with
   all zeros.

3.  Security Considerations

   The EAP-FAST-GTC method sends password information in the clear and
   MUST NOT be used outside of a protected tunnel providing strong
   protection, such as the one provided by EAP-FAST.  Weak encryption,
   such as 40-bit encryption or NULL cipher, MUST NOT be used.  In
   addition, the peer MUST authenticate the server before disclosing its
   credentials.  Since EAP-FAST Server-Unauthenticated Provisioning Mode
   does not authenticate the server, EAP-FAST-GTC MUST NOT be used as
   the inner method in this mode.  EAP-FAST-GTC MAY be used in EAP-FAST
   authentication and Server-Authenticated Provisioning Mode [RFC5422],
   where the server is authenticated.  Since EAP-FAST-GTC requires the
   server to have access to the actual authentication secret, it is
   RECOMMENDED to vary the stored authentication validation data by
   domain so that a compromise of a server at one location does not
   compromise others.

3.1.  Security Claims

   This section provides the needed security claim requirement for EAP

   Auth. mechanism:         Password based.
   Ciphersuite negotiation: No.  However, such negotiation is provided
                            by EAP-FAST for the outer authentication.
   Mutual authentication:   No.  However, EAP-FAST provides server-side
   Integrity protection:    No.  However, any method executed within the
                            EAP-FAST tunnel is protected.
   Replay protection:       See above.
   Confidentiality:         See above.
   Key derivation:          Keys are not generated, see Section 2.
                            However, when used inside EAP-FAST, the
                            outer method will provide keys.  See
                            [RFC4851] for the properties of those keys.
   Key strength:            See above.
   Dictionary attack prot.: No.  However, when used inside the EAP-FAST
                            tunnel, the protection provided by the TLS
                            tunnel prevents an off-line dictionary

   Fast reconnect:          No.  However, EAP-FAST provides a fast
                            reconnect capability that allows the reuse
                            of an earlier session authenticated by EAP-
   Cryptographic binding:   No.  Given that no keys are generated, EAP-
                            FAST-GTC or its use within EAP-FAST cannot
                            provide a cryptographic assurance that no
                            binding attack has occurred.  EAP-FAST-GTC
                            is required only to run within a protected
                            tunnel, but even the use of the same
                            credentials in some other, unprotected
                            context might lead to a vulnerability.  As a
                            result, credentials used in EAP-FAST-GTC
                            SHOULD NOT be used in other unprotected
                            authentication mechanisms.
   Session independence:    No.  However, EAP-FAST provides session
   Fragmentation:           No.  However, EAP-FAST provides support for
   Key Hierarchy:           Not applicable.
   Channel binding:         No, though EAP-FAST can be extended for

4.  IANA Considerations

   EAP-FAST-GTC uses the assigned value of 6 (EAP-GTC) for the EAP Type
   in [RFC3748].

   This document defines a registry for EAP-FAST-GTC error codes when
   running inside EAP-FAST, named "EAP-FAST GTC Error Codes".  It may be
   assigned by Specification Required as defined in [RFC5226].  A
   summary of the error codes defined so far is given below:








   No IANA registry will be created for Labels, as current
   implementations only support the Labels defined in this document and
   new Labels are not expected; if necessary, new Labels can be defined
   in documents updating this document.

5.  Acknowledgments

   The authors would like thank Joe Salowey and Amir Naftali for their
   contributions of the problem space, and Jouni Malinen, Pasi Eronen,
   Jari Arkko, and Chris Newman for reviewing this document.

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, "Extensible Authentication Protocol (EAP)",
              RFC 3748, June 2004.

   [RFC4282]  Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
              Network Access Identifier", RFC 4282, December 2005.

   [RFC4851]  Cam-Winget, N., McGrew, D., Salowey, J., and H. Zhou, "The
              Flexible Authentication via Secure Tunneling Extensible
              Authentication Protocol Method (EAP-FAST)", RFC 4851,
              May 2007.

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, March 2008.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

6.2.  Informative References

   [RFC2759]  Zorn, G., "Microsoft PPP CHAP Extensions, Version 2",
              RFC 2759, January 2000.

   [RFC4511]  Sermersheim, J., "Lightweight Directory Access Protocol
              (LDAP): The Protocol", RFC 4511, June 2006.

   [RFC5422]  Cam-Winget, N., McGrew, D., Salowey, J., and H. Zhou,
              "Dynamic Provisioning Using Flexible Authentication via
              Secure Tunneling Extensible Authentication Protocol (EAP-
              FAST)", RFC 5422, March 2009.

Authors' Addresses

   Nancy Cam-Winget
   Cisco Systems
   3625 Cisco Way
   San Jose, CA  95134

   EMail: ncamwing@cisco.com

   Hao Zhou
   Cisco Systems
   4125 Highlander Parkway
   Richfield, OH  44286

   EMail: hzhou@cisco.com


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