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Date: Thu, 26 Jul 2012 03:09:43 +0400
From: Alexander Cherepanov <>
Subject: Re: mscash2 / hmac-md5 ambiguity

On 2012-07-24 03:57, magnum wrote:
> On 2012-07-23 23:19, Alexander Cherepanov wrote:
>> On 2012-07-23 14:46, magnum wrote:
>>> On 2012-07-23 11:47, Alexander Cherepanov wrote:
>>>> mscash2 hashes in their canonical form are nevertheless accepted as
>>>> hmac-md5:
>>>> $ cat mscash2.john
>>>> chatelain:$DCC2$10240#chatelain#e4e15fdfafc8e715da9edec3611bfbff
>>>> $ john mscash2.john
>>>> Warning: detected hash type "mscash2", but the string is also recognized
>>>> as "hmac-md5"
>>>> Use the "--format=hmac-md5" option to force loading these as that type
>>>> instead
>>>> Loaded 1 password hash (M$ Cache Hash 2 (DCC2) PBKDF2-HMAC-SHA-1
>>>> [128/128 SSE2 intrinsics 8x])
>>>> guesses: 0  time: 0:00:00:02 0.00% (2)  c/s: 339  trying: 123456 -
>>>> abc123
>>>> Session aborted
>>>> $ john --format=hmac-md5 mscash2.john
>>>> Loaded 1 password hash (HMAC MD5 [128/128 SSE2 intrinsics 12x])
>>>> guesses: 0  time: 0:00:00:02 0.00% (3)  c/s: 1120K  trying: 123man -
>>>> 123mah
>>>> Session aborted
>>>> IMHO that's not very good.
>>> It was much worse until we forced hmac-md5 to lower precedence than
>>> mscash. Now it is just cosmetic. That hash *is* a valid hmac-md5 hash,
>>> with a salt of "$DCC2$10240#chatelain".
>> Were these forms chosen for compatibility with other tools? I mean it's
>> a pity to have a special, canonical form for a hash which clashes with
>> other formats.
> It does not really clash, it just warns. It picks mscash2, emits the
> notice and all is fine. Is it that bad? I loved when core got this feature.

Problem is not in the warning itself. The warning is helpful. The 
problem is what it warns about.

Suppose that Korelogic will include hmac-md5 in the upcoming contest. 
Then you cannot load these hashes without --format.

Second problem: you cannot put both types of hashes in one file. 
Wouldn't it be better to have only one file with all hashes? I mean, to 
have one file instead of 20 is very convenient. Then you can select 
desirable part of it with --format. But this is impossible when there 
are collisions in canonical forms of given types of hashes.

>>> We can stop this by
>>> black-listing certain format salts. That's OK with me but in some way
>>> it's a flawed path.
>> Agreed.
> Just thinking out loud here: Let's say we could teach the hmac-MD5
> format that "it" should not emit that warning if the salt starts with
> $DCC2$10240# - as opposed to always reject it in valid(). In the
> (unlikely) case this was really a hmac salt, we could still use the hmac
> format using --format. If we don't, it will pick mscash2 and not
> complain. The only problem with this approach is it's not supported by
> the current core and I'm not sure how we could implement that... or...
> perhaps hmac-md5's valid() could take a peek at options.format (the
> --format argument) and behave differently if unset... maybe this is
> possible. I might try that some time.

You mean to just silence the warning? I don't think it should be done 
without solving underlying problem.

Alexander Cherepanov

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