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Date: Wed, 4 Jul 2007 13:47:18 +0400
From: Solar Designer <>
Subject: Re: Incremental mode limited to 8 character words?

On Mon, Jul 02, 2007 at 04:22:31PM +0100, Tom Earp wrote:
> I understand it is possible to modify src/params.h before compiling to allow jtr to create words greater than 8 characters in length when using it in incremental mode.

Correct, but you will have to generate a new .chr file - and you need
statistical information on such longer passwords (that is, many long
passwords already in your john.pot) in order for the new .chr file to
produce candidate passwords in an optimal order.

> I have attempted this with no success...there is a comment in the params.h file:
> "be careful if you change these, ((SIZE ** LENGTH) * SCALE) should fit into 64 bits. you can reduce SCALE if required"
> what does that actually mean?

It means precisely what it says ("**" denotes "raised to the power of"),
but that's a bit complicated for many people.  (The actual requirement
is a bit less strict, but it's even more complicated, so I chose to keep
the safe simplification in that comment.)

> i would like to be able to generate words between 8 and 30 characters in length

Trying length 30 likely doesn't make sense, unless each of your
characters can take no more than 4 different "values".  For example, if
you know that your passwords are nothing more than long strings of ASCII
1's and 0's, then it makes sense.  In most other cases, it doesn't - the
keyspace would be way too large.

Trying lengths that are just beyond 8 does make sense if you have
statistical info (see above) and/or the number of different characters
is reasonably small.  Here's a specific example (including new params.h
settings) for trying candidate passwords consisting of 16 digits:

Here's another example of how the same may be done with an external mode
(no need to change params.h then, but the order of tries is even less

I hope this helps.

Alexander Peslyak <solar at>
GPG key ID: 5B341F15  fp: B3FB 63F4 D7A3 BCCC 6F6E  FC55 A2FC 027C 5B34 1F15 - bringing security into open computing environments

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