[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-melnikov-digest-to-historic) 00 draft-ietf-kitten-digest-to-historic

SASL Working Group                                           A. Melnikov
Internet-Draft                                             Isode Limited
Intended status: Informational                             July 29, 2008
Expires: January 30, 2009


                     Moving DIGEST-MD5 to Historic
                 draft-ietf-sasl-digest-to-historic-00

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   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.

   This Internet-Draft will expire on January 30, 2009.

Abstract

   This memo describes problems with the DIGEST-MD5 Simple
   Authentication and Security Layer (SASL) mechanism as specified in
   RFC 2831.  It recommends that DIGEST-MD5 to be marked as OBSOLETE in
   the IANA Registry of SASL mechanisms, and that RFC 2831 be moved to
   Historic status.

Note

   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, and should be sent to
   ietf-sasl@imc.org.



Melnikov                Expires January 30, 2009                [Page 1]


Internet-Draft        Moving DIGEST-MD5 to Historic            July 2008


Table of Contents

   1.    Overview  . . . . . . . . . . . . . . . . . . . . . . . . . . 3

   2.    Security Considerations . . . . . . . . . . . . . . . . . . . 5

   3.    IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5

   4.    Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . 5

   5.    References  . . . . . . . . . . . . . . . . . . . . . . . . . 5
   5.1.  Normative References  . . . . . . . . . . . . . . . . . . . . 5
   5.2.  Informative References  . . . . . . . . . . . . . . . . . . . 6

         Author's Address  . . . . . . . . . . . . . . . . . . . . . . 6
         Intellectual Property and Copyright Statements  . . . . . . . 7



































Melnikov                Expires January 30, 2009                [Page 2]


Internet-Draft        Moving DIGEST-MD5 to Historic            July 2008


1.  Overview

   [RFC2831] defined how HTTP Digest Authentication [RFC2617] can be
   used as a Simple Authentication and Security Layer (SASL) [RFC4422]
   mechanism for any protocol that has a SASL profile.  It was intended
   both as an improvement over CRAM-MD5 [RFC2195] and as a convenient
   way to support a single authentication mechanism for web, mail, LDAP,
   and other protocols.  While it can be argued that it was an
   improvement over CRAM-MD5, many implementors commented that the
   additional complexity of DIGEST-MD5 made it difficult to implement
   fully and securely.

   Below is an incomplete list of problems with DIGEST-MD5 mechanism as
   specified in RFC 2831:

   1.  The mechanism had too many options and modes.  Some of them were
       not well described and were not widely implemented.  For example,
       DIGEST-MD5 allowed the "qop" directive to contain multiple
       values, but it also allowed for multiple qop directives to be
       specified.  The handling of multiple options was not specified,
       which resulted in minor interoperability problems.  Some
       implementations amalgamated multiple qop values into one, while
       others treated multiple qops as an error.  Another example is the
       use of an empty authorization identity.  In SASL an empty
       authorization identity means that the client is willing to
       authorize as the authentication identity.  The document was not
       clear on whether the authzid must be omitted or can be specified
       with the empty value to convey this.  The requirement for
       backward compatibility with HTTP Digest meant that the situation
       was even worse.  For example DIGEST-MD5 required all usernames/
       passwords which can be entirely represented in ISO-8859-1 charset
       to be down converted from UTF-8 to ISO-8859-1.  Another example
       is use of quoted strings.  Handling of characters that needed
       escaping was not properly described and the DIGEST-MD5 document
       had no examples to demonstrate correct behavior.

   2.  The document used ABNF from RFC 822 [RFC0822], which allows an
       extra construct and allows for "implied folding whitespace" to be
       inserted in many places.  The difference from ABNF [RFC4234] was
       confusing for some implementors.  As a result, many
       implementations didn't accept folding whitespace in many places
       where it was allowed.

   3.  The DIGEST-MD5 document uses the concept of a "realm" to define a
       collection of accounts.  A DIGEST-MD5 server can support one or
       more realms.  The DIGEST-MD5 document didn't provide any guidance
       on how realms should be named, and, more importantly, how they
       can be entered in User Interfaces (UIs).  As the result many



Melnikov                Expires January 30, 2009                [Page 3]


Internet-Draft        Moving DIGEST-MD5 to Historic            July 2008


       DIGEST-MD5 clients had confusing UIs, didn't allow users to enter
       a realm and/or didn't allow users to pick one of the server
       supported realms.

   4.  Use of username in the inner hash.  The inner hash of DIGEST-MD5
       is an MD5 hash of colon separated username, realm and password.
       Implementations may choose to store inner hashes instead of clear
       text passwords.  While this has some useful properties, such as
       protection from compromise of authentication databases containing
       the same username and password on other servers, if a server with
       the username and password is compromised, however this was rarely
       done in practice.  Firstly, the inner hash is not compatible with
       widely deployed Unix password databases, and second, changing the
       username would invalidate the inner hash.

   5.  Description of DES/3DES and RC4 security layers are inadequate to
       produce independently-developed interoperable implementations.
       In the DES/3DES case this was partly a problem with existing DES
       APIs.

   6.  DIGEST-MD5 outer hash (the value of the "response" directive)
       didn't protect the whole authentication exchange, which made the
       mechanism vulnerable to "man in the middle" (MITM) attacks, such
       as modification of the list of supported qops or ciphers.

   7.  The following features are missing from DIGEST-MD5, which make it
       insecure or unsuitable for use in protocols:

       A.  Lack of channel bindings.

       B.  Lack of hash agility.

       C.  Lack of SASLPrep [RFC4013] support.  The original DIGEST-MD5
           document predates SASLPrep and doesn't recommend any Unicode
           character normalization.

   8.  The cryptographic primitives in DIGEST-MD5 are not up to today's
       standards, in particular:

       A.  The MD5 hash is sufficiently weak to make a brute force
           attack on DIGEST-MD5 easy with common hardware.

       B.  Using the RC4 algorithm for the security layer without
           discarding the initial key stream output is prone to attack.

   Note that most of the problems listed above are already present in
   the HTTP Digest authentication mechanism.




Melnikov                Expires January 30, 2009                [Page 4]


Internet-Draft        Moving DIGEST-MD5 to Historic            July 2008


   Because DIGEST-MD5 was defined as an extensible mechanism, it would
   be possible to fix most of the problems listed above.  However this
   would increase implementation complexity of an already complex
   mechanism even further, so the effort would not be worth the cost.
   In addition, an implementation of a "fixed" DIGEST-MD5 specification
   would likely either not interoperate with any existing implementation
   of RFC 2831, or would be vulnerable to various downgrade attacks.

   Note that despite DIGEST-MD5 seeing some deployment on the Internet,
   this specification recommends obsoleting DIGEST-MD5 because DIGEST-
   MD5, as implemented, is not a reasonable candidate for further
   standardization and should be deprecated in favor of one or more new
   password-based mechanisms currently being designed.


2.  Security Considerations

   Security issues are discussed through out this document.


3.  IANA Considerations

   IANA is requested to change the "Intended usage" of the DIGEST-MD5
   mechanism registration in the SASL mechanism registry to OBSOLETE.
   The SASL mechanism registry is specified in [RFC4422] and is
   currently available at:


      http://www.iana.org/assignments/sasl-mechanisms



4.  Acknowledgements

   The author gratefully acknowledges the feedback provided by Chris
   Newman, Simon Josefsson, Kurt Zeilenga and Abhijit Menon-Sen.
   [[anchor3: Various text was copied from other RFCs.]]


5.  References

5.1.  Normative References

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.




Melnikov                Expires January 30, 2009                [Page 5]


Internet-Draft        Moving DIGEST-MD5 to Historic            July 2008


   [RFC2831]  Leach, P. and C. Newman, "Using Digest Authentication as a
              SASL Mechanism", RFC 2831, May 2000.

5.2.  Informative References

   [RFC0822]  Crocker, D., "Standard for the format of ARPA Internet
              text messages", STD 11, RFC 822, August 1982.

   [RFC2195]  Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP
              AUTHorize Extension for Simple Challenge/Response",
              RFC 2195, September 1997.

   [RFC4013]  Zeilenga, K., "SASLprep: Stringprep Profile for User Names
              and Passwords", RFC 4013, February 2005.

   [RFC4234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, October 2005.

   [RFC4422]  Melnikov, A. and K. Zeilenga, "Simple Authentication and
              Security Layer (SASL)", RFC 4422, June 2006.


Author's Address

   Alexey Melnikov
   Isode Limited
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex  TW12 2BX
   UK

   Email: Alexey.Melnikov@isode.com
   URI:   http://www.melnikov.ca/


















Melnikov                Expires January 30, 2009                [Page 6]


Internet-Draft        Moving DIGEST-MD5 to Historic            July 2008


Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain 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; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat 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 proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.











Melnikov                Expires January 30, 2009                [Page 7]


Html markup produced by rfcmarkup 1.124, available from https://tools.ietf.org/tools/rfcmarkup/