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Date: Tue, 27 Mar 2012 18:38:11 +0400
From: Solar Designer <solar@...nwall.com>
To: john-dev@...ts.openwall.com
Subject: Re: JtR: GPU for slow hashes

On Tue, Mar 27, 2012 at 05:21:44PM +0530, SAYANTAN DATTA wrote:
>   I have a basic question regarding MSCash2.

BTW, you probably already found these, but in case not:

http://openwall.info/wiki/john/MSCash2_simple_code
http://openwall.info/wiki/john/MSCash2

These are a couple of implementations/descriptions of MSCash2 written by
contributors to JtR.  These may be easier to understand than code
currently in JtR is.

> In above algorithm two inputs namely ussrname and password are used to
> produce a certain hash.Then the generated  hash is compared with the stored
> value and if they are equal then the username and password entered are
> correct.Now we hit and try different values of password to produce a hash
> which matches the given value .So does this mean the username is some fixed
> string??

For one hash being cracked, yes.

> i.e is the username known to us beforehand and we only have to
> guess the password?

Yes.

> Right now my code assumes the username a fixed string.Is it all right??

Maybe not, depending on what you mean by assuming a fixed string -
compile time constant vs. runtime value fixed only for a given hash
currently being processed (out of many loaded for cracking).

JtR supports cracking of many hashes at once, which may correspond to
different usernames.  In MSCash2, the usernames are effectively used as
salts - so JtR formats' interfaces for salts are used to support them.
You also need to support running with an arbitrary set of salts
(usernames) and hashes loaded for cracking at once (e.g., there might be
10000 hashes with 9900 different salts).

Also note that you only need to implement the PBKDF2 step on GPU - the
rest may stay on CPU.  Lukas did the same in his CUDA code (based on my
suggestion).  This may simplify the task, and it will let us easily
reuse the PBKDF2 with SHA-1 implementation for things such as WPA.

Alexander

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