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Date: Tue, 27 Feb 2018 23:11:53 +0100
From: Solar Designer <>
Subject: Re: Getting full performance out of multiple GPU's

On Tue, Feb 27, 2018 at 03:59:44PM -0600, Adam Lininger wrote:
> Thank you for the help. I believe I am properly using a GPU and the actual
> hashing is being done there. The nvidia-smi command seems to agree. Based
> on your email, I suspect the problem is that the korelogic rule generation
> is still happening on the CPU rather than the GPU. Is there an easy way to
> correct that? I've copied my command below.
> john --wordlist=rockyou.dict --format=descrypt-opencl
> --devices=1,2,3,4,5,6,7,8 --fork=8 --rules=korelogic htpasswd

Yes, it's one of those cases where you'll have to use --mask, and this
prevents you from using solely the KoreLogic rules - at best, you may
add a mask on top of them, but a mask would push many candidates beyond
the length of 8, which is the maximum for descrypt, making this approach
not entirely reasonable.

You may have better luck running the rules against your descrypt hashes
on CPU, and using your GPUs for something like:

john --incremental --mask='?w?a?a?a?a' --format=descrypt-opencl --devices=1,2,3,4,5,6,7,8 --fork=8 htpasswd

This will output something like:

Loaded 1 password hash (descrypt-opencl, traditional crypt(3) [DES OpenCL])
Warning: MaxLen = 13 is too large for the current hash type, reduced to 4

The warning is OK - it means that JtR correctly reduced incremental
mode's maximum length to 4 due to descrypt's maximum of 8 and mask of 4.

You could also want to use the --min-length and --max-length options to
split this into several invocations of JtR varying by length (perhaps
from 5 to 8), so that you keep track of what lengths have already been
exhausted vs. not yet.  You'll probably complete lengths up to 7
inclusive within a day, and then focus on length 8 for some days more
(if for one hash; otherwise multiply by the number of different salts).


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