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Re: MS Chap v2 analysis
From: daw () CS BERKELEY EDU (David Wagner)
Date: Mon, 12 Jul 1999 20:34:36 -0700


In article <In article <CB6657D3A5E0D111A97700805FFE65870B48E463 () RED-MSG-51>,
Paul Leach  <paulle () MICROSOFT COM> wrote:
From: Burton Rosenberg [mailto:burtonr () citrix com]

the parallel structure of generating the challenge response [...]
cuts down the strength of the PasswordHash from 16 to 14 bytes.

Correct. But since the best attack is against the passwords themselves, the
reduction from 16 bytes to 14 bytes of strength from the password hash isn't
the primary issue.

I disagree strongly!  This property greatly increases the performance
of a dictionary attack---by a factor of about 65536, to be precise.

Suppose we hash all the entries in a dictionary containing N words.
Sort the results by the last two bytes in their hash, and burn this on
a CD-ROM.  Then, when we see a MS Chap v2 exchange, we recover the last
two bytes of the PasswordHash (using the method outlined by B Rosenburg)
and look at the appropriate entries on the CD-ROM.  We will only need
to examine N/65536 dictionary entries, and each of those can be tested
by brute force.

This reduces the cost of a dictionary attack by a factor of 65536,
which is devastating, especially when you consider that most passwords
contain relatively low entropy.

I think this alone is enough to consider MS Chap v2 seriously broken...


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