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INTERNET-DRAFT                                           M. VanHeyningen
<draft-ietf-aft-socks-cram-00>                      Aventail Corporation
Expires 20 September 1997                                  20 March 1997


         Challenge-Response Authentication Method for SOCKS V5

Status of this Memo

      This document is an Internet-Draft.  Internet-Drafts are working
      documents of the Internet Engineering Task Force (IETF), its
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Abstract

   This document specifies a general Challenge-Response Authentication
   Method (CRAM) for use with SOCKS Version 5 [RFC 1928].  It is
   intended to support various challenge-response mechanisms, such as
   S/KEY and OTP [RFC 1938] as well as authentication tokens.

Introduction

   The protocol specification for SOCKS Version 5 [RFC 1928] specifies a
   generalized framework for the use of arbitrary authentication
   protocols in the initial SOCKS connection setup.  This document
   suggests a general framework for a Challenge-Response Authentication
   Method (CRAM) as it fits into the SOCKS Version 5 authentication
   "subnegotiation."

Initial Negotiation

   During initial SOCKS V5 negotiation, the client and server negotiate
   the authenticiation method.  The METHOD for this protocol shall be
   X'05'.




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Challenge-Reponse Framework

   Subnegotiation begins after the client has selected the CRAM
   authentication method.

 Message Format

   In general, messages exchanged consist of a version identifier and a
   list of attribute-value assertions, where attributes are single
   octets and values are sequences of 0-255 octets.

           +-----+-------+------+---------+------+------+---
           | VER | NAVAS | ATT1 | VAL1LEN | VAL1 | ATT2 | ...
           +-----+-------+------+---------+------+------+---
           |  1  |   1   |   1  |    1    | 0-255|  1   | ...
           +-----+-------+------+---------+------+------+---

   VER contains the current version of the subnegotiation, which is
   X'01'.  NAVAS contains the number of attribute-value assertions to
   follow.  Each AVA includes ATT_i, containing the attribute, VAL_iLEN,
   containing the length of VAL_i, and VAL_i.  In general, robust
   implementations should ignore assertions with attributes they do not
   understand.  This provides a powerful and general mechanism for future
   extensions while allowing backward compatibility.

   Notationally, a single message with a list of n assertions shall be
   represented as:

        ATT_1(VAL_1), ATT_2(VAL_2), ... ATT_n(VAL_n)

 Attributes

   The following attribute definitions apply to all messages:

        ATT     Label           Meaning
        ----------------------------------------------
        X'00'   STATUS          0 = success
        X'01'   TEXT-MESSAGE    Informational text
        X'02'   USER-IDENTITY
        X'03'   CHALLENGE
        X'04'   RESPONSE
        X'05'   CHARSET

   The TEXT-MESSAGE attribute may always be included in any message
   (except submethod negotiation.)  Implementations should display its
   contents to the user if applicable; it should be used for advisory
   information (e.g. warnings of pending password expiration,
   explanations accompanying a failure.) If there is no user,



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   implementations may log its contents.

   The CHARSET attribute provides advisory infomration about the
   character set in use; it, too, may be present in any message.
   Implementations should use it to guide presentation of information to
   users.  The semantics are identical to that of the charset parameter
   in MIME [RFC 1521]; if absent, a default of ISO-8859-1 should be
   assumed.

   Submethods may also define their own additional attributes, but must
   not redefine the above standard ones.

Protocol Exchange

   Generic challenge-response simply authenticates the client's identity
   by sending a textual challenge from the server which the client
   displays to the user.  The user somehow (e.g. by using an external
   application or a security token) computes the appropriate response and
   enters it.

   First, the client asserts its identity to the server:

        USER-IDENTITY(<username>)

   The server then responds with a textual challenge to be displayed to
   the user:

        CHALLENGE(<challenge>)

   The client displays this challenge, prompts for a response, and sends
   it:

        RESPONSE(<response>)

   The server may respond with a new challenge method, for which a new
response is
   required.  Arbitrarily many challenges may be issued.
   When the server finished issuing challenges, it sends a status message:

        STATUS(success|failure)

   and the subnegotiation terminates.  If the authentication did not succeed,
   the server must drop the connection.

Security Considerations

   Challenge-response protocols are generally designed to provide
   protection from passive attacks such as sniffing passwords.  The
   security mechanisms here generally offer only limited protection from



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   real-time active attacks.

   In most challenge-response security mechanisms, it is important that
   challenges be produced in a fashion an adversary cannot predict or
   duplicate.  As with all negotiation-based security, implementations
   may be vulnerable to downgrade attacks.  Clients and servers should
   refuse to operate with methods and algorithms considered
   insufficiently secure.

Acknowledgements

   Thanks to Dave Blob, Wei Lu, Craig Metz and William Perry for
   assistance with this document.

References

   [RFC 1521] Borenstein, N, & Freed, N., "MIME (Multipurpose Internet
   Mail Extensions) Part One: Mechanisms for Specifying and Describing
   the Format of Internet Message Bodies," September 1993.

   [RFC 1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., &
   Jones, L., "SOCKS Protocol V5," April 1996.

   [RFC 1938] Haller, N., & Metz, C., "A One-Time Password System," May
   1996.

   [RFC 2058] Rigney, C., Rubens, A., Simpson, W. A., & Willens, S.,
   "Remote Authentication Dial In User Service (RADIUS)", January 1997.

Author's Address

   Marc VanHeyningen
   Aventail Corporation
   117 S. Main Street, Suite 400
   Seattle, WA   98104

   Phone: +1 206 777-5600
   Fax:   +1 206 777-5656
   Email: marcvh@aventail.com












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