[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20120619135631.GD7123@cmpxchg8b.com>
Date: Tue, 19 Jun 2012 15:56:31 +0200
From: Tavis Ormandy <taviso@...xchg8b.com>
To: john-dev@...ts.openwall.com
Subject: Re: [patch] optional new raw sha1 implemetation
On Tue, Jun 19, 2012 at 05:31:39PM +0400, Solar Designer wrote:
> On Tue, Jun 19, 2012 at 03:11:30PM +0200, Tavis Ormandy wrote:
> > I don't think so, the performance impact is indeed very small, 0.1%
> > sounds right. Still, I want that 0.1% :-)
>
> Understood.
>
> 0.1% was almost a worst case estimate. How large is your actual
> max_keys_per_crypt?
At the moment, just 256. But I would like it to be much higher.
> Is your cmp_one() really as heavy as one SHA-1
> computation?
A little heavier, because I optimise for the cmp_all case. for cmp_one,
I have to unpack keys and so on.
> Are you frequently running this against exactly 1 or 2
> loaded hashes (not 3 or more)?
Yes.
> I am not sure if there's a way to reclaim that 0.1% without incurring it
> (or more) elsewhere. For example, you can do full instead of partial
> hash comparisons in cmp_all(), but this might make its code slower
> (through differences in the code and maybe through worse locality of
> reference when full rather than partial binary hashes are in memory).
Hmm, not sure I follow. When I return a possible match from cmp_all,
john calls cmp_one on all max_keys_per_crypt. As these are quite
expensive for me as max_keys_per_crypt get's higher, and get_hash() ==
get_binary() is very cheap, it seems like an easy win to test that
before I really do the cmp_one test.
Does that sound wrong? It's not a big deal, but I don't see how it can
hurt.
I mean, just this in cmp_one():
if (get_hash() != get_binary())
return 0
return full_comparison();
Tavis.
--
-------------------------------------
taviso@...xchg8b.com | pgp encrypted mail preferred
-------------------------------------------------------
Powered by blists - more mailing lists
Confused about mailing lists and their use?
Read about mailing lists on Wikipedia
and check out these
guidelines on proper formatting of your messages.
