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Date: Mon, 30 Apr 2012 11:30:10 +0200
From: Frank Dittrich <frank_dittrich@...mail.com>
To: john-dev@...ts.openwall.com
Subject: Re: Password Generation on GPU

On 04/30/2012 10:57 AM, Frank Dittrich wrote:
>> Incremental mode is potentially capable of using this interface for the
>> last character position, except when it has the last character index
>> fixed (and thus alters character indices in other positions only).  For
>> example, when trying passwords of length 8, incremental mode would thus
>> be able to use set_mask() 87.5% of the time in a long-running session.
>> (The growing and reducing c/s rate may be confusing, though.)
> 
> You could even use it for the last character as well, if you add 0x00 to
> the mask.
> This would have to be done for the last position only. Preferably, you'd
> even start with the shorter password, then compute all the others.
> I am sure how much of a problem the mixed length is for GPU, but you'd
> have length switches in in incremental mode anyway.

Only if the input is an empty password, we need to try the empty
password as well.
In all other cases, this is not necessary.
(Otherwise, all candidate passwords shorter than maximum length will be
tried twice.)
This assumes that the input mask contains all possible characters.

Of course, it should be possible to use a subset of the most likely
final characters first, and use less likely final characters as a mask
in a separate session.
(The optimal minimum size of the mask probably depends on how much of a
bottleneck the memory bandwidth is.)

When we generate chr files for (maxlen-1) anyway, it would also be
possible to compute an "optimal" mask of final characters, and may be
even include this into the chr file, to be used as the default mask.

(Of course, the new chr files will then not just be suboptimal for
regular usage without masking, but even incompatible.)

Frank

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