john-users - Re: Some problems to build chr

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Date: Sun, 21 Aug 2011 15:41:37 +0200
From: "madfran" <madfran@...-ezine.org>
To: john-users@...ts.openwall.com
Subject: Re: Some problems to build chr

Hi,

> Well, this is definitely becoming a FAQ. :-(  We can't bring the same
> topic up every 3 days, can we?..  It's like the third time this week.

Perhaps the explanation is not very clear,... or perhaps the students 

are not so intelligent :-)

> > # ./john --make-charset=lanman.chr
> > Self test failed (pow64of32() overflow)
> 
> This means that you set the params.h settings too high, beyond what's
> supported.  Please read the lengthy comment right before your edits.
> Also, see this older posting for sample valid settings:
> 
> http://www.openwall.com/lists/john-users/2007/07/04/6
> 

In my poor opinion, the referenced posting not give a so clear 
explanation. Perhaps better information is in the comments of 
params.h

 * Please note that certain intermediate values computed in charset.c 

while
 * generating a new charset file should fit in 64 bits.  As long as
 * ((SIZE ** LENGTH) * SCALE) fits in 64 bits you're definitely safe, 

although
 * the exact requirement, which you can see in charset.c: 
charset_self_test(),
 * is a bit less strict.  John will refuse to generate a charset file 

if the
 * values would overflow, so rather than do the math yourself you can 

simply
 * let John test the values for you.  You can reduce the SCALE if 
required.

If I understood well, to avoid a overflow:
((SIZE ** LENGTH) * SCALE) must fit in 64 bits.
That means ((SIZE ** LENGTH) * SCALE) < 2**64
So, if I want to cover the complete range of characters (FF) the max. 

size must be 8 and the scale 1.
256**8*1 = 2**64

I am right?