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Date: Tue, 19 Jun 2012 17:03:44 +0400
From: Solar Designer <solar@...nwall.com>
To: john-dev@...ts.openwall.com
Subject: Re: [patch] optional new raw sha1 implemetation
Tavis,
On Tue, Jun 19, 2012 at 09:48:06AM +0200, Tavis Ormandy wrote:
> I noticed that one of the problems with setting it too high was that
> John calls cmp_one for every hash after a successful cmp_all. cmp_one
> really hurts, so if max_keys_per_crypt is too high, I lose some of the
> benefit to the overhead when there is a partial match.
While theoretically true, this issue should be of no practical relevance.
A typical setting of max_keys_per_crypt for a CPU-only format (no GPU)
may be hundreds or at most a few thousand. (Larger values result in the
candidates not fitting in L1 data cache, which usually hurts.)
A partial binary ciphertext that you may be comparing against in
cmp_all() is typically 32-bit. This means that you have 4 billion
possible values for it, and you're only comparing up to a few thousand
of computed hashes against it. So you get false positives (resulting in
calls to cmp_one() for every computed hash) with a frequency of roughly
up to 1 per million of cmp_all() calls. When that happens, you have
to call cmp_one() up to a few thousand times, but that's equivalent to
roughly one call per one thousand of hashes computed. If cmp_one()
costs roughly the same as one hash computation (and usually cmp_one() is
cheaper), the performance impact is thus around 0.1%.
Now, with multiple hashes loaded for cracking (per salt if applicable)
and processed with the same cmp_all() approach, things get worse (times
the number of hashes with that salt - OK, no salt in this case).
However, this approach is only used for very small numbers of hashes.
magnum-jumbo has PASSWORD_HASH_THRESHOLD_0 at 3, meaning that this
approach is used for 1 or 2 hashes only (per salt, when applicable).
With 3 or more, cmp_all() should not be called at all.
> I noticed an easy way to fix it is just to check if get_hash() ==
> binary_hash() first
This sounds very wrong to me. When get_hash() is worthwhile to use (as
determined by the thresholds), John will use it instead of cmp_all() on
its own - with no magic needed on your part.
Perhaps there's some context to this that I am missing, though?
Thanks,
Alexander
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