Add.

[gsasl.git] / doc / specification / draft-nerenberg-sasl-crammd5-02.txt

Network Working Group                                         J. Klensin
Internet Draft: The CRAM-MD5 SASL Mechanism                 P. Krumviede
Document: draft-nerenberg-sasl-crammd5-02.txt                   R. Catoe
                                                      L. Nerenberg (Ed.)
                                                               June 2002



                      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
     documents 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.

     A revised version of this draft document will be submitted to the
     RFC editor as a Proposed Standard for the Internet Community.
     Discussion and suggestions for improvement are requested.
     Distribution of this draft is unlimited.

1.  How to Read This Document

     The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD
     NOT", "RECOMMENDED" and "MAY" in this document are to be inter-
     preted as defined in [KEYWORDS].

2.  Introduction

     This document defines a simple challenge-response [SASL] authenti-
     cation mechanism, using a [KEYED-MD5] digest.

3.  CRAM-MD5 Authentication Mechanism

     The mechanism name associated with CRAM-MD5 is 'CRAM-MD5'.

     This mechanism does not provide a security layer.



Klensin, et al     draft-nerenberg-sasl-crammd5-02.txt          [Page 1]
\f
Internet Draft           CRAM-MD5 SASL Mechanism               June 2002


     The data encoded in the challenge contains a presumptively arbi-
     trary string of random digits, a timestamp, and the fully-qualified
     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 user name and shared secret MUST be represented in the Unicode
     character set [UNICODE], and MUST be normalised using the Unicode
     Normalisation Form KC [NFKC]. The resulting values MUST be encoded
     as UTF-8 [UTF8].


3.1.  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


3.2.  Examples

     These examples show the use of the CRAM-MD5 mechanism with the
     IMAP4 AUTHENTICATE command [IMAP4].  The base64 encoding of the
     challenges and responses is part of the IMAP4 AUTHENTICATE command,
     not part of the CRAM-MD5 specification itself.




Klensin, et al     draft-nerenberg-sasl-crammd5-02.txt          [Page 2]
\f
Internet Draft           CRAM-MD5 SASL Mechanism               June 2002


          S: * OK IMAP4rev1 Server
          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.reston.mci.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


4.  References

4.1.  Normative References


     [ABNF]
          Crocker, D., P. Overell, "Augmented BNF for Syntax Specifica-
          tions: 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.






Klensin, et al     draft-nerenberg-sasl-crammd5-02.txt          [Page 3]
\f
Internet Draft           CRAM-MD5 SASL Mechanism               June 2002


     [KEYWORDS]
          Bradner, S., "Key words for use in RFCs to Indicate Require-
          ment Levels", BCP 9, RFC2119, Harvard University, March 1997.


     [MD5]
          Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, MIT
          Laboratory for Computer Science and RSA Data Security, Inc.,
          April 1992.


     [NFKC]
          Davis, M., M. Durst, "Unicode Standard Annex #15: Unicode Nor-
          malisation Forms", An integral part of The Unicode Standard,
          Version 3.1.1 (http://www.uni-
          code.org/reports/tr15/tr15-21.html).


     [SASL]
          Myers, J., "Simple Authentication and Security Layer (SASL),"
          RFC 2222, Netscape Communications, October 1997.


     [UNICODE]
          The Unicode Consortium, "The Unicode Standard, Version 3.1.1",
          defined by: The Unicode Standard, Version 3.0 (Reading, MA,
          Addison-Wesley, 2000.  ISBN 0-201-61633-5), as amended by the
          Unicode Standard Annex #27: Unicode 3.1 (http://www.uni-
          code.org/reports/tr27/) and the Unicode 3.1.1 Update Notice
          (http://www.unicode.org/versions/ Unicode3.1.1.html).


     [UTF8]
          Yergeau, F., "UTF-8, a transformation format of ISO 10646",
          RFC 2279, Alis Technologies, January 1998.

4.2.  Informative References

     [IMAP4] Crispin, M., "Internet Message Access Protocol - Version
     4rev1," RFC 2060, University of Washington, December 1996.

5.  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 cleartext 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



Klensin, et al     draft-nerenberg-sasl-crammd5-02.txt          [Page 4]
\f
Internet Draft           CRAM-MD5 SASL Mechanism               June 2002


     precomputation of intermediate results make it possible to avoid
     explicit cleartext storage of the shared secret on the server sys-
     tem by instead storing the intermediate results which are known as
     "contexts."

     While the saving, on the server, of the MD5 "context" is marginally
     better than saving the shared secrets in cleartext, it is not suf-
     ficient to protect the secrets if the server itself is compromised.
     Consequently, servers that store the secrets or contexts must both
     be protected to a level appropriate to the potential information
     value in the data and services protected by this mechanism.  In
     other words, techniques like this one involve a tradeoff between
     vulnerability to network sniffing and I/O buffer snooping and vul-
     nerability of the server host's databases.  If one believes that
     the host and its databases are subject to compromise, and the net-
     work is not, this technique (and all others like it) is unattrac-
     tive.  It is perhaps even less attractive than cleartext 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 network intercep-
     tion of the passwords (secrets), then this (and similar) 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 cleartext passwords in many protocols.  This doc-
     ument 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.


















Klensin, et al     draft-nerenberg-sasl-crammd5-02.txt          [Page 5]
\f
Internet Draft           CRAM-MD5 SASL Mechanism               June 2002


6.  Authors' Addresses

     John C. Klensin
     MCI Telecommunications
     800 Boylston St, 7th floor
     Boston, MA 02199
     USA

     EMail: klensin@mci.net
     Phone: +1 617 960 1011

     Paul Krumviede
     MCI Telecommunications
     2100 Reston Parkway
     Reston, VA 22091
     USA

     EMail: paul@mci.net
     Phone: +1 703 715 7251

     Randy Catoe
     MCI Telecommunications
     2100 Reston Parkway
     Reston, VA 22091
     USA

     EMail: randy@mci.net
     Phone: +1 703 715 7366

     Lyndon Nerenberg (Ed.)
     ACI Worldwide
     Suite 900
     10117 Jasper Avenue
     Edmonton, AB
     Canada  T5J 1W8

     Email: lyndon@atg.aciworldwide.com
     Phone: +1 780 424 4922



















Klensin, et al     draft-nerenberg-sasl-crammd5-02.txt          [Page 6]
\f