Date: Fri, 28 Mar 2014 01:16:19 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] pufferfish

On Thu, Mar 27, 2014 at 08:55:25PM +0400, Solar Designer wrote:
> On Tue, Mar 25, 2014 at 01:39:58AM +0400, Solar Designer wrote:
> > OK, maybe it makes sense to apply this approach to L2, despite of the
> > inefficiency.  Or maybe not.  Typical L2 is 256 KB/core (it has actually
> > decreased in size when L3 was introduced), meaning 64 KB/thread may be
> > "safe" to use.  At this size, my guess is you give advantage to some GPU
> > attackers vs. your typical defender, for the reason explained above.
> 
> I did some testing with escrypt's S-boxes, and things are reasonably
> good.  For the first few doublings of S-box size, beyond L1 cache size
> but still fitting in L2 cache, there's only a ~10% performance hit per
> doubling on Sandy Bridge-EP.  It's not worse than that in part because
> escrypt also does some L2/L3/RAM accesses anyway, so there's some L1
> cache thrashing going on anyway.
[...]
> This is for AVX builds of current development escrypt.  The random
> lookups are 128-bit wide, made with AVX instructions.  They're uniformly
> distributed over the S-boxes of the sizes given above.

I forgot to mention: this is with 8x parallelism of the lookups in each
thread (4 parallel groups of 2 parallel lookups).  This is maybe ~2x
more parallelism than these CPUs I tested actually need when I run two
threads/core.  This is easily tested by running 1 thread/core.
Performance is above 80% of what 2 threads achieve.  This percentage is
higher than what it is e.g. with original scrypt or bcrypt, which have
less parallelism and thus benefit from 2 threads/core more (for two
independent instances, indeed).

I included this extra parallelism in this combination of settings (which
I have specialized code for) because I recognize that some otherwise
similar CPUs will only run 1 thread/core (e.g. some(?) Core i5's lack
HT) and to better support newer CPUs (not-yet-written specialized code
for these settings can partially take advantage of AVX2 and even AVX-512
for the computation, albeit not for the 128-bit lookups themselves).

The inclusion of extra parallelism must have been very helpful to reduce
the performance impact from higher latency accesses in these tests (that
go from L2).  Unfortunately, it also helps some attacks, such as with
local memory on GPUs (as compared to those attacks on same S-box size
but with less parallelism per instance).  For attacks with global memory
on GPUs, parallelism doesn't matter (not at these low sizes).

Another drawback of including extra parallelism is that non-SIMD builds,
which some people will inevitably happen to be making and using and
having other people use, will then have even more parallelism than those
crippled builds can use themselves - thereby leaving more room for
relative speedup of attacks even on similar hardware (same CPU types).
This makes me appreciate the original bcrypt even more: it's only about
2x slower for defense than for attack on current CPUs simply because it
can't use SIMD - not even for attack on CPUs until we gain fast gather
loads there.

If pufferfish has 4 parallel lookups rather than 8, it'll probably be
impacted by the size increases (beyond L1 cache size) more.  However,
going to 8x parallelism has major drawbacks (mentioned above).  It's
unclear what to do, other than to make the parallelism tunable and
thereby postpone/move the tough decision onto those configuring our
schemes for specific uses... which is obviously not great either.

Alexander

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