INTERNET-DRAFT                                           M. VanHeyningen
<draft-ietf-aft-socks-chap-01>                      Aventail Corporation
Expires six months from -->                               6 January 1998

        Challenge-Handshake Authentication Protocol for SOCKS V5

Status of this Memo

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   This document specifies the integration of authentication based on
   Challenge-Handshake Authenticaton Protocol into SOCKS Version 5.  The
   primary algorithm to be used is HMAC-MD5, although the framework is
   general enough to permit use of MD5 or other keyed hash algorithms.
   This document describes the message formats and protocol details of
   incorporating CHAP into the SOCKS V5 authentication ''subnegotiation.''
   Support is included for authentication of server to client as well as
   client to server.

CHAP Method Identifier

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

The HMAC-MD5 Algorithm

   HMAC-MD5 is defined as a new CHAP algorithm with algorithm identifier
   0x85.  It uses the MD5 algorithm is defined in [RFC 1321] with the
   HMAC construct defined in [RFC 2104] to generate responses to given
   challenges; unlike in the standard MD5 CHAP, the identifier octet is

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   ignored.  Compliant implementations MUST support the HMAC-MD5
   algorithm, and MAY support others.

CHAP Exchange

   Subnegotiation begins after the server has selected the CHAP
   authentication method.

 Message Format

   In general, messages exchanged consist of a version identifier and a
   set 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 set of n assertions shall be
   represented as:

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

   Note that no ordering is assigned to the set of assertions: compliant
   implementations must accept them in any order.

  Attribute Definitions

   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   Contains CHAP NAME
         X'03'   CHALLENGE
         X'04'   RESPONSE
         X'05'   CHARSET         Optional character set

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         X'10'   IDENTIFIER      CHAP identifier
         X'11'   ALGORITHMS      Supported CHAP algorithms

   The TEXT-MESSAGE attribute may always be included in any message.
   Implementations should display its value to the user if applicable;
   it may be used for advisory information (e.g. warnings of pending
   password expiration, explanations accompanying a failure.)  If
   presenting the message to a user is not possible or not applicable,
   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 may use it to guide prompting and presentation of
   usernames, challenges, responses and text messages.  The semantics
   are those defined for charset parameter in MIME [RFC 1521]; if
   absent, a default of US-ASCII (or a superset) must be assumed.

   The IDENTIFIER is used to transport the CHAP identifier when using
   algorithms such as MD5 which require an identifier; it is included
   with all messages after the algorithm negotiation when MD5 is

 Algorithm Negotiation

   The CHAP subnegotiation begins with the client sending a message
   containing the CHAP algorithms it is willing to use (e.g. HMAC-MD5,
   MD5.)  Note that compliant implementations MUST support HMAC-MD5.:


   The server responds with another message of the same form containing
   the one algorithm to be used for this connection:


   If the server is unable or unwilling to use any of the algorithms
   specified by the client, it returns a message indicating failure:


   and closes the connection.

  Challenge-Response Exchange

   After the algorithm is negotiated, the server sends a challenge:


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   The client sends an appropriate response:

         USER-IDENTITY(<username>), RESPONSE(<response>)

   And the server indicates success or failure:


   after which the subnegotiation terminates and, upon success, the
   client should proceed with its request.  Upon failure, the server
   must close the connection.

  Mutual Authentication

   Implementations MAY support mutual authentication of client and
   server.  A client may request mutual authentication by including
   another CHALLENGE along with its response:

         USER-IDENTITY(<username>), RESPONSE(<response>),

   The server should then include a RESPONSE along with the STATUS

         STATUS(success|failure), RESPONSE(<response-2>)

   Finally, the client replies with a STATUS message of its own before
   the subnegotiation terminates


   Note that both negotiations employ the same identifier.  Whether the
   same shared secret is used in both directions is outside the scope of
   this specification, although use of a single secret does not create
   the same security considerations with SOCKS5 as are present in PPP.
   Since servers unable or unwilling to do mutual authentication will
   ignore the client's CHALLENGE, clients should handle a lack of
   RESPONSE gracefully and either continue or terminate the connection
   in accordance with their security policy.

Security Considerations

   Challenge-response protocols are generally designed to provide
   protection from passive attacks such as sniffing passwords.  CHAP
   offers limited protection from real-time active attacks.

   Algorithms other than HMAC-MD5, such as MD5 as originally specified
   in [RFC 1994], were created without the benefit of much subsequent

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   research into the area of keyed hash construction.  Their design is
   now considered weak.  A variant of CHAP called MS-CHAP [MSCHAP] is
   known to be particularly weak.

   As in all 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
   considere insufficiently secure

   In the context of a PPP connection, a CHAP challenge may be issued at
   any time to reconfirm the authentication.  This integration permits
   challenges only during connection establishment and has no provision
   for reconfirmation.


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


   [MSCHAP] Cobb, S., "Microsoft PPP CHAP Extensions," Informational
   Memo, December 1995.

   [RFC 1321] Rivest, R., "The MD5 Message-Digest Algorithm," April

   [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 1994] Simpson, W., "PPP Challenge Handshake Authentication
   Protocol (CHAP)," August 1996.

   [RFC 2104] Krawczyk, H., Bellare, M., & Canetti, R., "HMAC: Keyed-
   Hashing for Message Authentication," February 1997.

   Author's Address

      Marc VanHeyningen
      Aventail Corporation
      117 S. Main St, Suite 400

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      Seattle, WA  98104

      Phone: +1 206 777-5600
      Fax:   +1 206 777-5656

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