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Network Working Group L. Nerenberg, Editor Internet Draft: The CRAM-MD5 SASL Mechanism Orthanc Systems Document: draft-ietf-sasl-crammd5-01.txt November 2003 The CRAM-MD5 SASL Mechanism Status of this Memo This document is an Internet Draft and is in full conformance with all provisions of Section 10 of RFC 2026. 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 docu- ments at any time. It is inappropriate to use Internet Drafts as reference material or to cite them other than as "work in progress." The list of current Internet Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright 2003, The Internet Society. All Rights Reserved. Please see the Copyright section near the end of this document for more information. Abstract This document defines a simple challenge-response authentication mechanism, using a keyed-hash digest, for use with the Simple Authentication and Security Layer (SASL). 1. Conventions Used in this Document The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" in this document are to be interpreted as defined in [KEYWORD]. 2. CRAM-MD5 Authentication Mechanism This document defines a simple challenge-response [SASL] authenti- cation mechanism, using a [KEYED-MD5] digest, for use with [SASL]. The mechanism name associated with CRAM-MD5 is 'CRAM-MD5'. This mechanism does not provide a security layer. Nerenberg draft-ietf-sasl-crammd5-01.txt [Page 1] Internet Draft CRAM-MD5 SASL Mechanism November 2003 The data encoded in the challenge contains a presumptively arbi- trary string of random digits, a time-stamp, and the fully-quali- fied primary host name of the server. The client makes note of the data and then responds with a string consisting of the user name, a space, and a "digest." The latter is computed by applying the keyed MD5 algorithm from [KEYED-MD5] where the key is a shared secret and the digested text is the chal- lenge (including angle-brackets). The client MUST NOT interpret or attempt to validate the contents of the challenge in any way. This shared secret is a string known only to the client and server. The "digest" parameter itself is a 16-octet value which is sent in hexadecimal format, using lower-case US-ASCII characters. When the server receives this client response, it verifies the digest provided. Since the user name may contain the space charac- ter, the server MUST scan the client response from right to left; the first space character encountered separates the digest from the user name. If the digest is correct, the server should consider the client authenticated and respond appropriately. The client MUST prepare the user name and shared secret strings using the [SASLPrep] profile of the [StringPrep] algorithm. The resulting values MUST be encoded as UTF-8 [UTF8]. 2.1. Formal Syntax The following syntax specification uses the augmented Backus-Naur Form (ABNF) as specified in [ABNF], and incorporates by reference the Core Rules defined in that document. challenge = "<" 1*DIGIT "." 1*DIGIT "@" hostname ">" digest = 32(DIGIT / %x61-66) ; A hexadecimal string using only lower-case ; letters hostname = 1*(ALPHA / DIGIT) *("." / "-" / ALPHA / DIGIT) response = user SP digest user = 1*OCTET 2.2. Examples The examples in this section do NOT form part of the specification. Where conflicts exist between the examples and the formal grammar or specification text, the latter are authoritative. These examples show the use of the CRAM-MD5 mechanism with the IMAP4 AUTHENTICATE command [IMAP4]. The base64 encoding of the Nerenberg draft-ietf-sasl-crammd5-01.txt [Page 2] Internet Draft CRAM-MD5 SASL Mechanism November 2003 challenges and responses is part of the IMAP4 AUTHENTICATE command, not part of the CRAM-MD5 specification itself. S: * OK [CAPABILITY IMAP4rev1 STARTTLS LOGINDISABLED AUTH=CRAM-MD5] C: A0001 AUTHENTICATE CRAM-MD5 S: + PDE4OTYuNjk3MTcwOTUyQHBvc3RvZmZpY2UucmVzdG9uLm1jaS5uZXQ+ C: dGltIGI5MTNhNjAyYzdlZGE3YTQ5NWI0ZTZlNzMzNGQzODkw S: A0001 OK CRAM-MD5 authentication successful In this example, the shared secret is the string tanstaaftanstaaf Hence, the Keyed MD5 digest is produced by calculating MD5((tanstaaftanstaaf XOR opad), MD5((tanstaaftanstaaf XOR ipad), <1896.697170952@postoffice.example.net>)) where ipad and opad are as defined in [KEYED-MD5] and the string shown in the challenge is the base64 encoding of <1896.697170952@postoffice.reston.mci.net>. The shared secret is null-padded to a length of 64 bytes. If the shared secret is longer than 64 bytes, the MD5 digest of the shared secret is used as a 16 byte input to the keyed MD5 calculation. This produces a digest value (in hexadecimal) of b913a602c7eda7a495b4e6e7334d3890 The user name is then prepended to it, forming tim b913a602c7eda7a495b4e6e7334d3890 Which is then base64 encoded to meet the requirements of the IMAP4 AUTHENTICATE command (or the similar POP3 AUTH command), yielding dGltIGI5MTNhNjAyYzdlZGE3YTQ5NWI0ZTZlNzMzNGQzODkw 3. References 3.1. Normative References [ABNF] Crocker, D., P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC2234, Internet Mail Consortium and Demon Internet Ltd., November 1997. [KEYED-MD5] Krawczyk, Bellare, Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, IBM and UCSD, February 1997. Nerenberg draft-ietf-sasl-crammd5-01.txt [Page 3] Internet Draft CRAM-MD5 SASL Mechanism November 2003 [KEYWORD] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC2119, Harvard University, March 1997. [MD5] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, MIT Labo- ratory for Computer Science and RSA Data Security, Inc., April 1992. [SASL] Myers, J., "Simple Authentication and Security Layer (SASL)," RFC 2222, Netscape Communications, October 1997. [SASLPrep] Zeilenga, K., "SASL String Preparation Profiles", draft-ietf-sasl- saslprep (work in progress) [StringPrep] Hoffman, P., M. Blanchet, "Preparation of Internationalized Strings (stringprep)", RFC 3454, IMC and Viagenie, December 2002. [UTF8] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 2279, Alis Technologies, January 1998. 3.2. Informative References [IMAP4] Crispin, M., "Internet Message Access Protocol - Version 4rev1," RFC 3501, University of Washington, March 2003. 4. Security Considerations It is conjectured that use of the CRAM-MD5 authentication mechanism provides replay protection for a session. This mechanism does not obscure the user name in any way. Accord- ingly, a server that implements both a clear-text password command and this authentication type should not allow both methods of access for a given user name. Keyed MD5 is chosen for this application because of the greater security imparted to authentication of short messages. In addition, the use of the techniques described in [KEYED-MD5] for pre-computa- tion of intermediate results make it possible to avoid explicit clear-text storage of the shared secret on the server system by instead storing the intermediate results which are known as "con- texts." While the saving, on the server, of the MD5 "context" is marginally better than saving the shared secrets in clear-text, it is not sufficient to protect the secrets if the server itself is compromised. Consequently, servers that store the secrets or con- texts must both be protected to a level appropriate to the poten- tial information value in the data and services protected by this Nerenberg draft-ietf-sasl-crammd5-01.txt [Page 4] Internet Draft CRAM-MD5 SASL Mechanism November 2003 mechanism. In other words, techniques like this one involve a trade-off between vulnerability to network sniffing and I/O buffer snooping and vulnerability of the server host's databases. If one believes that the host and its databases are subject to compromise, and the network is not, this technique (and all others like it) is unattractive. It is perhaps even less attractive than clear-text passwords, which are typically stored on hosts in one-way hash form. On the other hand, if the server databases are perceived as reasonably secure, and one is concerned about client-side or net- work interception of the passwords (secrets), then this (and simi- lar) techniques are preferable to clear-text passwords by a wide margin. As the length of the shared secret increases, so does the diffi- culty of deriving it. While there are now suggestions in the literature that the use of MD5 and keyed MD5 in authentication procedures probably has a lim- ited effective lifetime, the technique is now widely deployed and widely understood. It is believed that this general understanding may assist with the rapid replacement, by CRAM-MD5, of the current uses of permanent clear-text passwords in many protocols. This document has been deliberately written to permit easy upgrading to use SHA (or whatever alternatives emerge) when they are considered to be widely available and adequately safe. Even with the use of CRAM-MD5, users are still vulnerable to active attacks. An example of an increasingly common active attack is 'TCP Session Hijacking' as described in CERT Advisory CA-95:01. CRAM-MD5 does not authenticate the server and does not include a client-supplied nonce. As a result, it is possible to construct a server with a fixed challenge string that has pre-computed the hashes for all possible passwords up to a certain length (or from a dictionary). Such a server could then immediately determine the user's password if it is sufficiently short. 5. IANA Considerations The SASL Mechanism Registry entry for CRAM-MD5 must be updated to reference this specification. 6. Contributors The CRAM-MD5 mechanism was originally specified in RFC 2095, IMAP/POP AUTHorize Extension for Simple Challenge/Response. The authors of that document -- John C. Klensin, Paul Krumviede, and Randy Catoe -- are to be credited with the design and specification of CRAM-MD5. This memo serves only to re-state CRAM-MD5 within the formal context of SASL, which specification it preceded by several months. Nerenberg draft-ietf-sasl-crammd5-01.txt [Page 5] Internet Draft CRAM-MD5 SASL Mechanism November 2003 7. Intellectual Property The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to per- tain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such pro- prietary rights by implementers or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Execu- tive Director. 8. Editors' Address Lyndon Nerenberg Orthanc Systems 1606 - 10770 Winterburn Road Edmonton, Alberta Canada T5S 1T6 Email: lyndon+rfc-crammd5@orthanc.ca Nerenberg draft-ietf-sasl-crammd5-01.txt [Page 6] Internet Draft CRAM-MD5 SASL Mechanism November 2003 9. Full Copyright Statement Copyright 2003, The Internet Society. All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, pub- lished and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this para- graph are included on all such copies and derivative works. How- ever, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the proce- dures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGI- NEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WAR- RANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Nerenberg draft-ietf-sasl-crammd5-01.txt [Page 7]