[Standards] [Fwd: I-D Action:draft-melnikov-digest-to-historic-00.txt]

Ian Paterson ian.paterson at clientside.co.uk
Tue Sep 11 16:10:39 UTC 2007


Dave Cridland wrote:
> On Tue Sep 11 11:55:35 2007, Jonathan Chayce Dickinson wrote:
>> Interesting because most clients used Digest-MD5, so what do we use now?
>> Cram-MD5? Or is there some other newfangled method out there?
>>
>>
> DIGEST-MD5 is still more secure than CRAM-MD5, and this won't change 
> because of that draft. :-)

I strongly agree. Despite its imperfections, RFC 3920bis should continue 
recommending and requiring DIGEST-MD5 until something better has been 
adopted by the IETF.

Furthermore I disagree with the following "problem with the DIGEST-MD5 
mechanism" included in the Internet-Draft:

"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."

In practice inner hashes may be stored relatively rarely, however that 
does not necessarily make the optional feature into a "problem". It is, 
IMHO, a critical security feature that should be employed whenever 
practical. Perhaps RFC 3920bis could encourage this practice - while 
mentioning the potential impracticalities.

Note, a change in the SASL mechanisms supported by future versions of an 
XMPP server is probably a more likely cause of "invalidating the inner 
hash" than changing XMPP usernames (which are generally fixed).

- Ian




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