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Date: Wed, 13 May 2015 19:53:30 +0300
From: Solar Designer <solar@...nwall.com>
To: john-dev@...ts.openwall.com
Subject: Re: get_binary_*() and get_hash_*() methods
Aleksey,
On Wed, May 13, 2015 at 07:37:21PM +0300, Aleksey Cherepanov wrote:
> On Thu, May 07, 2015 at 05:39:45PM +0300, Solar Designer wrote:
> > You can see a dirty hack like this here:
> >
> > git show 9a6f4f6f69903763e664f03d2adee97486eca9de DES_bs_b.c
>
> I tried to perform lookup in crypt_all() to exit earlier not finishing
> the computations.
The same(?) can be achieved by exiting crypt_all() early all the time
and completing the computation in cmp_one() when needed. IIRC, myrice's
raw SHA-512 on GPU formats did that, completing the computation in
cmp_one() on CPU only when needed.
> I tried it against raw-sha512, scalar implementation
> with 2 instructions reversed (endianity altering and addition of the
> state), I compared h variable (it should be the closest result). Also
> I replaced salt->index(index) with var & 0xfff (salt->hash_size == 2).
OK, but raw SHA-512 on one core isn't slow enough for this to matter.
These tricks are for faster hashes or/and for use with parallelization,
to keep the checks parallelized as well and performed in the same
threads that computed the corresponding hashes (thus, to avoid data
transfers between different cores).
> salt argument is 0 in self tests, so I tested it with
> --skip-self-tests:
> 5g 0:00:01:27 3/3 0.05721g/s 1351Kp/s 1351Kc/s 20274KC/s gok3sb
> 5g 0:00:01:27 3/3 0.05747g/s 1295Kp/s 1295Kc/s 19426KC/s 159518*
> #1 is with early exit, #2 without that additional check.
>
> The difference (3%) may be a fluctuation.
Yes, may be just a fluctuation, or may be for real, but either way it's
quite small.
> Is there any solution to not skip self tests?
Yes. crypt_all() tells the caller whether there _might_ be matches.
You simply return that, yes, there _might_ be matches every time you're
called with parameters that you have no optimized code for. For
example, if you only support salt->hash_size == 2, you can return
*count + 1 whenever you're called with salt->hash_size != 2. Of course,
you'll do this after computing the hashes sufficiently for your
get_hash*() and cmp_*() to work properly.
> There is in opencl_rawmd4_fmt_plug.c :
>
> if (salt != NULL && num_loaded_hashes != salt->count)
> load_hash(salt);
>
> How does it pass self tests?
>
>
> Also there is
>
> pw = salt -> list;
> i = 0;
> do {
> bin = (unsigned int *)pw -> binary;
> // Potential segfault if removed
> if(bin != NULL) {
> [...]
> }
> } while ((pw = pw -> next)) ;
>
> I had such problem too. Why is there 0 in binary field? Should not
> such entries be removed? Is it due to multithreading?
I'm not familiar with Sayantan's code, but "binary" may be NULL in
"single crack" mode. See single.c:
/*
* "binary" is set to NULL on entries marked for removal (so we remove them
* here) or already removed (yet we might hit them once in some obscure cases).
*/
Note that single.c is testing cracked passwords against all salts, even
though it normally tests candidate passwords against their target salts
only. There's 1 level of recursion here. This is what causes those
"obscure cases".
Alexander
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