john-users - Re: how to parse passwords with some known letters

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Message-ID: <20080807205352.GB22776@openwall.com>
Date: Fri, 8 Aug 2008 00:53:52 +0400
From: Solar Designer <solar@...nwall.com>
To: john-users@...ts.openwall.com
Subject: Re: how to parse passwords with some known letters

On Sat, Jul 26, 2008 at 07:38:00PM +0200, Helmut Hullen wrote:
> I've seen that about 10% of the restored passwords end with "oo" (7 or 8  
> characters), and about 30% contain somewhere "oo".
> 
> Can I tell "John" at least the case that many passwords may end with  
> "oo"?

There are two reasonable things you can do:

1. If you have a large number of passwords already cracked, and it
sounds like you do, then generate a custom .chr file based on those
passwords (that is, on your john.pot).  This is described in the
documentation for JtR:

	http://www.openwall.com/john/doc/EXAMPLES.shtml

currently, that's example number 7.

The .chr file will have information on relative frequencies of different
character triplets, at different character positions and for different
password lengths, embedded in it.  So it will "know" that "oo" is common,
just how common it is relative to other character combinations, after
what preceding characters, in what character positions, and for what
password lengths.

2. Force JtR to try passwords ending in or containing "oo" only.  This
can be done with an external mode - either a complete one or a filter()
to be used along with another cracking mode.  The filter() could in fact
filter or it could append or insert the "oo".  You've already found some
examples of how that is done:

> I've studied
>   http://article.gmane.org/gmane.comp.security.openwall.john.user/1662
>   http://www.openwall.com/lists/john-users/2008/05/20/2
>   http://www.openwall.com/lists/john-users/2008/03/31/1
> 
> but (sorry) I didn't understand how to make rules for this case.

Well, my recommendation is that you go with a custom .chr file, unless
the number of already-cracked passwords is too small.

Please let john-users know of your progress with this, and we might be
able to provide further advice.

Thanks,

Alexander

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