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Re: MS Chap v2 analysis
From: paulle () MICROSOFT COM (Paul Leach)
Date: Wed, 7 Jul 1999 16:20:09 -0700


-----Original Message-----
From: Burton Rosenberg [mailto:burtonr () citrix com]
Sent: Wednesday, July 07, 1999 3:16 PM
To: Paul Leach; BUGTRAQ () SECURITYFOCUS COM
Cc: 'schneier () counterpane com'; 'mudge () l0pht com'
Subject: RE: MS Chap v2 analysis



the parallel structure of generating the challenge response (function
ChallengeResponse() in
www.ietf.org/internet-drafts/draft-ietf-pppext-mschap-v2-03.te
x) cuts down
the strength of
the PasswordHash from 16 to 14 bytes.

this should have been addressed in version 2.

given challenge C of 8 bytes (or the "hidden challenge" of version 2),
password hash P of 16 bytes,
the response is:
  < DES_{P1} ( C ) | DES_{P2}(C) | DES_{P3}( C ) >
where, P1 is the first 7 bytes of P, P2 is the second 7 bytes
of P, and P3
is the last 2 bytes
of P followed by 5 bytes of zeros.

Break P3 by solving C' = DES_X( C ) for X given known C and
C' by brute
force over small number
( 2^16 ) of possibilities for X. This gives the last two bytes of P.

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.

Don't get me wrong -- I'm not going to claim that the MASCHAPv2 is the best
password based challenge/response protocol in the world.

Paul


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