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Date: Sat, 16 Nov 2013 14:39:36 -0500
From: Daniel Franke <dfoxfranke@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Some updates on EARWORM

Solar Designer <solar@...nwall.com> writes:

> On Fri, Aug 23, 2013 at 09:44:18PM -0400, Daniel Franke wrote:

>> * I've cut CHUNK_WIDTH from 4 to 2, leaving CHUNK_LENGTH at 128. Testing
>>   on my workstation seems to indicate that neither the reduction in
>>   internal parallelism nor the increased frequency of random memory
>>   accesses results in any performance penalty.

I've actually changed my mind again since I wrote this: I'm going with
CHUNK_WIDTH=4, CHUNK_LENGTH=64. The wider internal state makes certain
proofs simpler.

> This means that your memory accesses are to 4 KB regions (with
> sequential access within those regions), correct?  2*128*16 = 4096.

Either way, yes.

> Is there a performance penalty with lower CHUNK_WIDTH or/and
> CHUNK_LENGTH?  If so, how bad is it e.g. for 2 KB, 1 KB, 512 bytes?

There is. I don't have the figures handy. I'll retest this later today
once I've gotten my code pushed.

>> * I wrote a GPU implementation of EARWORM today. It computes batches of
>>   256 workunits of a single hash computation, doing the initial and
>>   final PRF computations sequentially on the host, while farming out the
>>   expensive main loops to the GPU for parallel execution.  A 25600
>>   workunit computation over a 256MiB arena takes about 3.05 seconds on
>>   my Radeon 7850. The same computation on two CPU threads takes about
>>   2.25 seconds. I was struck by how similar these numbers are.

My GPU code had a couple stupid bugs (no surprise at this point) that
make these numbers completely bogus. The 7850 actually takes about 3x
the time that the CPU does. It suffers from the same sort of bottlenecks
that bcrypt does.

> Is this a defensive or offensive kind of implementation (if it were
> finished, optimized, cleaned up, etc.)?  It sounds like you're computing
> just one instance of EARWORM, but with some parallelism in it (albeit by
> far not enough parallelism to use a GPU optimally), so I assume
> defensive?  Anyhow, this doesn't tell us much about GPU attack speeds on
> EARWORM.

Actually, this seems to be enough parallelism to use my (not very
high-end) GPU optimially; though at this phase of experimentation you
should take that claim with a grain of salt. Remember that each workunit
already has considerable internal parallelism.

It's a design goal of EARWORM that if the defender has GPUs available,
he can use them just as effectively as the attacker can (even if they're
not very effective for either side).

>> * I'll be posting reference and AES-NI-optimized implementations of
>>   EARWORM to GitHub this weekend, as soon as I'm done with some
>>   finishing touches on the test harness.  Note that these
>>   implementations are designed for benchmarking and validation only, and
>>   lack the user-level API that I plan to include in later
>>   implementations (Obviously, nobody should be using EARWORM in
>>   production right now anyway!).
>
> Of course.  And I guess "this weekend" will be now. ;-)

Yes. Thanks for giving me the necessary kick in the rear to get working
on this again :-)

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