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Date: Mon, 3 Jun 2013 18:16:37 +0400
From: Solar Designer <solar@...nwall.com>
To: john-dev@...ts.openwall.com
Subject: Re: cmp_all on gpu for descrypt-opencl

Hi Sayantan,

On Mon, Jun 03, 2013 at 07:12:45PM +0530, Sayantan Datta wrote:
> I have implemented a password generator which can crack all numeric(upto 8
> digit) password for descrypt. I have also tested it using a few hashes from
> the test-suite.  The generator is integrated with finalize_keys and doesn't
> use global memory. It is also very light on registers. The password are
> saved to global memory only when positive. The comparisons are done in gpu.
> 
> The code is currently in my git hub repo , bleeding-jumbo branch.
>  https://github.com/Sayantan2048/JohnTheRipper.git

I just took a look.  Overall, this is a nice experiment, but there are
two major issues with the approach taken:

1. The passwords generated by JtR are totally ignored by this
descrypt-opencl hack, which generates its own all-numeric candidate
passwords instead.  What we need is to have the GPU substitute all
possible characters (out of the currently configured charset) into a few
character positions, but leave the rest as generated by the host.

2. You use modulo division, which probably has high cost of its own.

What speeds are you getting?  After 5 minutes of running on one descrypt
hash (for other than an all-numeric password, or it would get cracked
within seconds) on bull's 7970, I get a reported speed of 83M c/s (not
sure if the calculation is correct or not, though, as it obviously
depends on correctness of your *pcount updates).  This is somewhere
inbetween of what we had before and what we should be able to achieve.

Running on "3269 password hashes with 2243 different salts", I get
reported speeds of as low as "500752c/s 684361C/s" after 2 minutes (and
not a single password is cracked yet, even though several are
all-numeric).  Where's the new bottleneck?  With password generation on
host, your descrypt-opencl was ~50 times faster at this sort of test.

Thanks,

Alexander

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