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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Abstract
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
X'03'.
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
selected.
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.:
ALGORITHMS(<algorithms>)
The server responds with another message of the same form containing
the one algorithm to be used for this connection:
ALGORITHMS(<algorithm>)
If the server is unable or unwilling to use any of the algorithms
specified by the client, it returns a message indicating failure:
STATUS(failure)
and closes the connection.
Challenge-Response Exchange
After the algorithm is negotiated, the server sends a challenge:
CHALLENGE(<challenge>)
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The client sends an appropriate response:
USER-IDENTITY(<username>), RESPONSE(<response>)
And the server indicates success or failure:
STATUS(success|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>),
CHALLENGE(<challenge-2>)
The server should then include a RESPONSE along with the STATUS
message:
STATUS(success|failure), RESPONSE(<response-2>)
Finally, the client replies with a STATUS message of its own before
the subnegotiation terminates
STATUS(success|failure)
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.
Acknowledgements
Thanks to Dave Blob, Wei Lu, Craig Metz, and William Perry for
assistance with this document.
References
[MSCHAP] Cobb, S., "Microsoft PPP CHAP Extensions," Informational
Memo, December 1995.
[RFC 1321] Rivest, R., "The MD5 Message-Digest Algorithm," April
1992.
[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
Email: marcvh@aventail.com
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