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Date: Sun, 2 Jul 2017 14:44:44 +0200
From: Solar Designer <>
Cc: Denis Burykin <>
Subject: Re: DES-based crypt(3) cracking on ZTEX 1.15y FPGA boards (descrypt-ztex)

On Sat, Jul 01, 2017 at 02:08:36PM -0800, Royce Williams wrote:
> Some quick keys_per_crypt tests on the cluster.
> A value of 2 was a ~5.4% improvement. Values above 2 were worse -
> enough that I only let them run for a few minutes to verify.

Thank you!  It is unexpected to me that higher values slow things down
not only in terms of passwords cracked by a certain time (which can be
explained by the less optimal order), but also in terms of c/s.  But
that's what our testing is for.

So maybe we should just make a multiplier of 2 the default.  With one
board, it increases the speed from 806M to 819M for me.  (That's half
way to the 831M figure I obtained with a multiplier of 8.)

> Stock:
> 325g 0:00:59:59 0.49% (ETA: 2017-07-08 16:38)
>     0.09029g/s 4550Kp/s 9881Mc/s 14078MC/s nujuaaaa..nujualks
> With keys_per_crypt *= 2:
> 369g 0:01:29:18 11.76% (ETA: 08:01:28)
>     0.06886g/s 4585Kp/s 10454Mc/s 14840MC/s ayceaaaa

10454/9881 = 1.058 (or 5.8% speedup)
10454/14 = 747M per board (or about 91% of what I have with 1 board)

Regarding the bugs you reported, I've just merged Denis' pull request
today that I think should fix one of those.  It also makes JtR wait and
resume cracking if it's running with just 1 board and it experiences
intermittent failure.  I've just tested this latter property by extreme
overclocking by 15% - it worked (the board got removed and re-detected
during cracking, which resumed on its own).

At 15% overclock the error rate is probably high enough that I am
missing some crackable passwords, so it's not suitable for actual use.
Per my testing, reasonable overclocks tend to be 5% to 10%.


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