Openwall GNU/*/Linux - a small security-enhanced Linux distro for servers
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Date: Fri, 13 Jul 2018 11:56:18 +0200
From: Solar Designer <>
Subject: Re: Need info - multiple hashes

On Fri, Jul 13, 2018 at 12:05:57PM +0530, Sudheendra S S wrote:
> Need help - If the input file to john contacts diff type of hashes (MD5
> Crypt, DES, Blowfish), would john try to crack all hash types? I am trying
> to input shadow hashes from multiple servers with diff OSes (Suse/RHEL).

Usually not.  John will tell you that there are multiple hash types and
what "--format" options you may use to request testing of those other
types.  Then you need to run one session per hash type.  It is OK to run
those multiple sessions simultaneously, including in the same directory.
Just give them different "--session" names and don't exceed your total
logical CPU count.  Limit each session's use of logical CPUs (with
OMP_NUM_THREADS environment variable and "--fork" option parameter,
which are multiplied) such that the total isn't greater than what you
have supported in hardware.  For example, on a machine with N logical
CPUs, you could set OMP_NUM_THREADS=1 in all invocations and split N
across the "--fork" counts for your different sessions, or you could
do it the other way around with OpenMP thread counts adding up to N (or
a bit less if the machine isn't otherwise completely idle) and then
don't use "--fork".

For those hash types you mention, there's a trick that will make John
crack them simultaneously: "--format=crypt".  But it will result in much
worse performance, both in terms of code efficiency (system-provided
not-so-optimized password hashing code will be used) and in terms of
(not) proceeding further to more complex passwords for faster ones of
the hash types (the overall progress will be limited by the slowest hash
type, so you will miss easily crackable passwords of the faster hashes).


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