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Date: Tue, 13 Feb 2018 16:22:07 +0100
From: Solar Designer <solar@...nwall.com>
To: john-users@...ts.openwall.com
Subject: Re: Markov Sampling

On Tue, Feb 13, 2018 at 03:58:17PM +0100, Matlink wrote:
> For speed reasons, I would need to sample the output of JtR --markov
> mode with --stdout. For example, I would like to add a --sample=10
> option to get only 1 of 10 candidates generated by this mode.

Are you doing some research?  If so, please also include incremental
mode in it.

> Obviously I could do it by myself in a post-process, but the UNIX pipe
> slows a lot the candidates gathering. So I found it more efficient to
> skip some candidates.
> 
> Do you know where in the code I can add such an option?

The pre-defined --external=Parallel mode will do what you ask for.
You'll just need to customize the "node" and "total" numbers in its
init() in john.conf.

However, note that "every 10th" doesn't necessarily produce a
representative sample: the underlying cracking mode (in this case,
Markov) might happen to have some periodicity in its output, and one of
its period lengths might just happen to be a multiple of 10 or whatever.
So ideally you'd want to randomize the order (if the order somehow
doesn't matter for your research) over a larger number of candidate
passwords - say, pass a million of them through GNU coreutils' shuf(1) -
and then take every 10th out of that randomized list.

Alexander

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