Internet DRAFT - draft-vanheyningen-socks-chap
draft-vanheyningen-socks-chap
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INTERNET-DRAFT M. VanHeyningen
<draft-vanheyningen-socks-chap-00> Aventail
Expires March 14, 1997 14 September 1996
Challenge-Handshake Authentication Protocol 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
areas, and its working groups. Note that other groups may also
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Abstract
This document specifies the integration of the Challenge-Handshake
Authenticaton Protocol (CHAP) [RFC 1334] into SOCKS Version 5 [RFC
1928]. It is intended to provide a simple, lightweight
authentication method which is more secure than cleartext passwords
but simpler than GSSAPI-based methods. 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'.
Subnegotiation
Subnegotiation begins after the client has selected the CHAP
authentication method.
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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, 1 = failure
X'01' TEXT-MESSAGE Informational text
X'02' USER-IDENTITY Contains CHAP NAME
X'03' CHALLENGE
X'04' RESPONSE
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 there is
no user, implementations may log its contents.
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Algorithm Negotiation
The CHAP subnegotiation begins with the client sending a message
containing the CHAP algorithms it is willing to use (e.g. MD5, MS-CHAP
[MS-CHAP]):
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>), IDENTIFIER(<ident>)
The client sends an appropriate response:
USER-IDENTITY(<username>), RESPONSE(<response>),
IDENTIFIER(<ident>)
And the server indicates success or failure:
STATUS(success|failure), IDENTIFER(<ident>)
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
Optionally, a client may request mutual authentication by including
another CHALLENGE along with its response:
USER-IDENTITY(<username>), RESPONSE(<response>),
CHALLENGE(<challenge-2>), IDENTIFIER(<ident>)
The server should then
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include a RESPONSE along with the STATUS message:
STATUS(success|failure), IDENTIFIER(<ident>),
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 and are
computed with the same shared secret. 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 in accordance with 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.
CHAP's integration of hash functions is somewhat behind the current
state of the art in MAC design. Servers should change the identifier
field with each connection even though it is not required for matching
connections, preferably in an unpredictable fashion. Implementations
should refuse to respond to too-short challenges, particularly
challenges 0 bytes long, as they may give away information about the
secret useful to an attacker. Servers should refuse to respond to
challenges until verifying the correctness of the client's response.
Adding stronger MAC designs, such as HMAC [HMAC-MD5], to CHAP's
algorithm suite is a matter for further research.
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 considered
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.
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Acknowledgements
Thanks to Dave Blob, Wei Lu, Craig Metz, and William Perry for
assistance with this document.
References
[HMAC-MD5] Krawczyk, H., Bellare, M., & Canetti, R., "HMAC-MD5:
Keyed-MD5 for Message Authentication," Internet-Draft, work in
progress.
[MS-CHAP] Cobb, S., "Microsoft PPP CHAP Extensions," Informational
Memo, December 1995.
[RFC 1344] Lloyd, B. & Simpson, W., "PPP Authentication Protocols,"
October 1992.
[RFC 1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., &
Jones, L., "SOCKS Protocol V5," April 1996.
Author's Address
Marc VanHeyningen
Aventail Corporation
401 Second Avenue South, Suite 501
Seattle, WA 98104
Phone: +1 206 777-5613
Fax: +1 206 777-5656
Email: marcvh@aventail.com
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