INTERNET-DRAFT M. VanHeyningen Aventail Corporation Expires six months from --> 6 January 1998 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 distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as ``work in progress.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet- Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). 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 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 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() The server responds with another message of the same form containing the one algorithm to be used for this connection: ALGORITHMS() 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() The client sends an appropriate response: USER-IDENTITY(), 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(), RESPONSE(), CHALLENGE() The server should then include a RESPONSE along with the STATUS message: STATUS(success|failure), RESPONSE() 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 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 Seattle, WA 98104 Phone: +1 206 777-5600 Fax: +1 206 777-5656 Email: marcvh@aventail.com