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Date: Thu, 07 May 2015 13:08:13 -0300
From: Marcos Simplicio <mjunior@...c.usp.br>
To: discussions@...sword-hashing.net
Subject: GPU benchmarks: Lyra2+yescrypt (was: Another PHC candidates "mechanical"
 tests (ROUND2))

Hi, again.

It took some time, but we finally completed the GPU benchmarks mentioned
in the e-mail below, both for Lyra2 and yescrypt. We did not use djm34's
yescrypt GPU implementation mentioned in another thread, though, because
while Lyra2 has been in their repository for a few months, we had
already adapted the yescrypt-opt version when we learned the news a few
days ago... Some optimizations made there might apply to our code too,
so we will take a look.

Anyhow, the partial results indicate that Lyra2 is actually more
GPU-resistant than yescrypt for a memory usage of 256 kB to 2 MB, at
least for our GPU (GeForce GTX TITAN), in all test scenarios we
considered, namely:

1) Different block lengths (C=128 and C=256 for Lyra2, yescrypt's
defaults), degrees of parallelism (1, 2 or 4, for both algorithms), and
passes through memory (T=0 and 2 for yescrypt, T=1 for Lyra2, since this
corresponds both to minimal and "same number of passes through memory"
for the schemes).

2) Different attack geometries, scripting to find the best GPU
throughput (i.e., the number of parallel guesses that resulted in the
lowest time taken per test) and also testing the number of threads per
warp that would give the best results (32 threads per warp means full
warp occupation, but the best throughput is obtained with 8 threads per
warp and one warp per block).

The numbers in the GPU are equivalent for Blake2b and BlaMka, so we
included just the former here (note: since BlaMka is slightly slower
than Blake2b in the CPU, the advantage of the GPU over the CPU when
BlaMka is employed should be slightly higher too, but the GPU still
loses in most scenarios).

Since the results may change to other GPUs, we placed the code employed
in our git (https://github.com/leocalm/Lyra/tree/master/GPU_attacks ),
so anyone can confirm/refute our numbers. Also, any bug report or
optimization suggestion is very welcome! We tried a few tricks and
checked the test vectors, but we may have missed something.

BR,

Marcos.

On 26-Mar-15 15:24, Solar Designer wrote:
> On Thu, Mar 26, 2015 at 02:29:29PM -0300, Marcos Simplicio wrote:
>> On 26-Mar-15 13:27, Solar Designer wrote:
>>> On Thu, Mar 26, 2015 at 12:27:04PM -0300, Marcos Simplicio wrote:
>>>>> Lyra2 is less suitable for low sizes like this.
>>>>
>>>> Just for the sake of clarity: why exactly?
>>>
>>> Because at too low sizes it's likely weaker than bcrypt at least
>>> against GPU attacks.  What exactly is "too low" is to be determined, and
>>> will vary by likely attacker's hardware.
>>
>> Hum... That makes me think we need to include bcrypt in our GPU
>> benchmarks and see what happens.
> 
> Yes, and in your CPU benchmarks too, so you'd be comparing GPU attacks
> on Lyra2 vs. bcrypt at the same defensive running time for them (on CPU).

We did not include bcrypt to the benchmarks because we wanted to have
comparisons against a memory-hard scheme, so we did so for yescrypt.

It is still unclear if there is a point in which yescrypt becomes more
GPU-resistant than Lyra2, but it does not appear to be at 256 KB or
higher, at least for our GPU.

Alexander: is there a memory range in which the table look-ups are
expected to make a large difference? I mean, we are testing smaller
ranges now, but since the tests take quite some time due to the search
for the best attack geometry, your suggestions on where to start would
be welcome.

> 
>> I'm not a GPU specialist, but we do
>> have a person working with the GPU implementations and the results shown
>> in our report (Sec. 7.3, Figure 20) are that, in the best conditions
>> from an attacker's perspective and for a memory usage of 2.3 MB, the
>> GPU-based implementation was 4.5 times slower (in terms of throughput)
>> than in the CPU.
> 
> OK, this may suggest the threshold at ~0.5 MB, which is quite close to
> my guess of 1 MB.
> 
> (For scrypt, it's trickier due to needing to adjust its TMTO factor, but
> for Lyra2 I expect this to be almost linear.)

The only situation in which we got Lyra2 running faster on our GPU than
on our CPU was for p=1 and 8 threads per warp (for 256KB). For yescrypt,
that happened in most of our tests (see graphs in the third column:
anything below 1 means that the GPU is winning).

> 
>> Obviously, there may be some optimization missing or maybe we need have
>> tests with an even lower memory usage, but so far I cannot say I agree
>> with that impression (which does not mean you are wrong, of course).
> [...]
>> We will try going down from 2.3 in steps of ~1/2 and see what happens.
>> I'm actually very curious to know :)
> 
> Yes, please.  I am also very curious.

Well, that started to happen at 256 KB for 1 thread, but way after it
was observed for yescrypt for all parameters tested...

Actually, we did some testing with yescrypt code (e.g., commenting some
lines) and it appears that the table lookups do not make much difference
on this range, but what is hurting the GPU is rather the memory
transference from/to the X and Y buffers. We would have to execute a
runtime analyzer to be sure, though, which is in our TODO list.


Download attachment "8threadsPerWarp_C-128.png" of type "image/png" (440629 bytes)

Download attachment "32threadsPerWarp_C-128.png" of type "image/png" (438870 bytes)

Download attachment "8threadsPerWarp_C-256.png" of type "image/png" (440199 bytes)

Download attachment "32threadsPerWarp_C-256.png" of type "image/png" (441164 bytes)

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