Internet Engineering Task Force (IETF) A. Melnikov
Request for Comments: 6331 Isode Limited
Obsoletes: 2831 July 2011
Moving DIGEST-MD5 to Historic
This memo describes problems with the DIGEST-MD5 Simple
Authentication and Security Layer (SASL) mechanism as specified in
RFC 2831. It marks DIGEST-MD5 as OBSOLETE in the IANA Registry of
SASL mechanisms and moves RFC 2831 to Historic status.
Status of This Memo
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published for informational purposes.
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Table of Contents
1. Introduction and Overview . . . . . . . . . . . . . . . . . . 22. Security Considerations . . . . . . . . . . . . . . . . . . . 53. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 54. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 55. References . . . . . . . . . . . . . . . . . . . . . . . . . 55.1. Normative References . . . . . . . . . . . . . . . . . . . . 55.2. Informative References . . . . . . . . . . . . . . . . . . . 51. Introduction and Overview
[RFC2831] defines 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, email, the
Lightweight Directory Access Protocol (LDAP), and other protocols.
While it can be argued that it is an improvement over CRAM-MD5, many
implementors commented that the additional complexity of DIGEST-MD5
makes it difficult to implement fully and securely.
Below is an incomplete list of problems with the DIGEST-MD5 mechanism
as specified in [RFC2831]:
1. The mechanism has too many options and modes. Some of them are
not well described and are not widely implemented. For example,
DIGEST-MD5 allows the "qop" directive to contain multiple values,
but it also allows for multiple qop directives to be specified.
The handling of multiple options is not specified, which results
in minor interoperability problems. Some implementations
amalgamate multiple qop values into one, while others treat
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 is not clear on whether
the authzid must be omitted or if it can be specified with an
empty value to convey this. The requirement for backward
compatibility with HTTP Digest means that the situation is even
worse. For example, DIGEST-MD5 requires all usernames/passwords
that can be entirely represented in the ISO-8859-1 charset to be
down converted from UTF-8 [RFC3629] to ISO-8859-1 [ISO-8859-1].
Another example is the use of quoted strings. Handling of
characters that need escaping is not properly described, and the
DIGEST-MD5 document has no examples to demonstrate correct
2. The DIGEST-MD5 document uses 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 a
more common ABNF defined in [RFC5234] is confusing for some
implementors. As a result, many implementations do not accept
folding whitespace in many places where it is 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 does not provide any
guidance on how realms should be named and, more importantly, how
they can be entered in User Interfaces (UIs). As a result, many
DIGEST-MD5 clients have confusing UIs, do not allow users to
enter a realm, and/or do not allow users to pick one of the
4. Use of username in the inner hash is problematic. 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. 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 is rarely done in practice. First,
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 [DES] and RC4 security layers are
inadequate to produce independently developed interoperable
implementations. In the DES/3DES case, this is partly a problem
with existing DES APIs.
6. DIGEST-MD5 outer hash (the value of the "response" directive)
does not protect the whole authentication exchange, which makes
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, making it
insecure or unsuitable for use in protocols:
A. Channel bindings [RFC5056].
B. Hash agility (i.e., no easy way to replace the MD5 hash
function with another one).
C. Support for SASLPrep [RFC4013] or any other type of Unicode
character normalization of usernames and passwords. The
original DIGEST-MD5 document predates SASLPrep and does not
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 [RFC6151].
B. The RC4 algorithm is prone to attack when used as the
security layer without discarding the initial key stream
C. The DES cipher for the security layer is considered insecure
due to its small key space [RFC3766].
Note that most of the problems listed above are already present in
the HTTP Digest authentication mechanism.
Because DIGEST-MD5 is defined as an extensible mechanism, it is
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 is not worth the cost. In
addition, an implementation of a "fixed" DIGEST-MD5 specification
would likely either not interoperate with any existing implementation
of [RFC2831] 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.
The Salted Challenge Response Authentication Mechanism (SCRAM) family
of SASL mechanisms [RFC5802] has been developed to provide similar
features as DIGEST-MD5 but with a better design.
2. Security Considerations
Security issues are discussed throughout this document.
3. IANA Considerations
IANA has changed 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
The author gratefully acknowledges the feedback provided by Chris
Newman, Simon Josefsson, Kurt Zeilenga, Sean Turner, and Abhijit
Menon-Sen. Various text was copied from other RFCs, in particular,
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
[DES] National Institute of Standards and Technology, "Data
Encryption Standard (DES)", FIPS PUB 46-3,
[ISO-8859-1] International Organization for Standardization,
"Information technology - 8-bit single-byte coded
graphic character sets - Part 1: Latin alphabet No. 1",
ISO/IEC 8859-1, 1998.
[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.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys",
BCP 86, RFC 3766, April 2004.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User
Names and Passwords", RFC 4013, February 2005.
[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication
and Security Layer (SASL)", RFC 4422, June 2006.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5802] Newman, C., Menon-Sen, A., Melnikov, A., and N.
Williams, "Salted Challenge Response Authentication
Mechanism (SCRAM) SASL and GSS-API Mechanisms",
RFC 5802, July 2010.
[RFC6151] Turner, S. and L. Chen, "Updated Security
Considerations for the MD5 Message-Digest and the HMAC-
MD5 Algorithms", RFC 6151, March 2011.
[RFC6229] Strombergson, J. and S. Josefsson, "Test Vectors for
the Stream Cipher RC4", RFC 6229, May 2011.
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