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Versions: 00 draft-ietf-sasl-digest-to-historic

SASL Working Group                                           A. Melnikov
Internet-Draft                                             Isode Limited
Intended status: Standards Track                       September 8, 2007
Expires: March 11, 2008


                     Moving DIGEST-MD5 to Historic
                  draft-melnikov-digest-to-historic-00

Status of this Memo

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

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






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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.


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  . . . . . . . . . . . . . . . . . . . 5

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



























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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 implemented.  For example,
       DIGEST-MD5 allowed the "qop" directive to contain multiple
       values.  But it also allowed for multiple qop directives to be
       specified.  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 use
       of 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 allowes for
       an extra construct and allows for "implied folding whitespace" to
       be inserted in many places.  Difference from ABNF [RFC4234] was
       confusing for some implementors.  As a result, many
       implementations didn't allow for folding whitespaces in many
       places where they were allowed.

   3.  The DIGEST-MD5 document uses a concept of "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



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       DIGEST-MD5 clients had confusing UI, 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 chose to store inner hashes instead of clear
       text passwords.  While this has some useful properties, such as
       compromise of an authentication database on one server does not
       automatically compromise an authentication database with the same
       username and password on other servers, in practice this was
       rarely done.  Firstly, the inner hash is not compatible with
       commonly deployed Unix password databases.  Secondly, change of a
       username invalidates the corresponding 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 insuitable for use in protocols:

       A.  Lack of channel bindings.

       B.  Lack of hash agility.  MD5 hash is suffuciently weak to make
           a brute force attack on DIGEST-MD5 easy with common hardware.

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

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

   Bacause DIGEST-MD5 mechanism 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.



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   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 and Kurt Zeilenga. [[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.

   [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.




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   [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/


























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Full Copyright Statement

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Acknowledgment

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   Administrative Support Activity (IASA).





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