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Date: Thu, 17 Apr 2014 16:36:55 -0400
From: Bill Cox <waywardgeek@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Lyra2 initial review

On Thu, Apr 17, 2014 at 4:10 PM, Solar Designer <solar@...nwall.com> wrote:

> > I tried r=32, and here are the results:
>
> Thanks!  Did you also try r=16?  In some of my tests, r=16 was actually
> faster than r=32, although that was with 2 threads/core.  r=32 needs
> about 16 KiB of L1 cache per thread, because it's 4 KiB for current
> block (X or V_i), 4 KiB for random block (V_j), and 8 KiB for pwxform
> S-boxes, so we're bumping into 32 KiB L1 cache size with 2 threads/core.
> This is not yet the case with 1 thread/core, though.


I just tried it: r=32 is faster than 16 (0.6s vs 0.644s).  r=64 is also
slower, since it begins to have cache misses.


> > Lyra2:
> > Peak memory/second: 1.52 GiB/s
> > Average memory/second: 1.14 GiB/s
>
> These look correct to me, given the Lyra2 algorithm on page 11 in
> Lyra2ReferenceGuide.pdf for T=1.
>
> > Memory bandwidth: 10.64 GiB/s
>
> This assumes a total of 7 reads plus writes per location, which I'm not
> sure is correct.  Looking at the algorithm, it might be even more, but
> that would push the memory bandwidth usage even higher, which would be
> surprisingly high.  It might be 4 r/w per iteration in Filling Loop, and
> 5 r/w per iteration in Visitation Loop, so 9 total.  If this is correct,
> we'd see 13.68 GiB/s here.  This might be realistic because, if I
> understand correctly, the same rand is reused with and without rotW(),
> which halves the amount of crypto-like processing per memory access even
> compared to reduced round TwoCats and yescrypt.  (If one round of
> pwxform is crypto-like, whereas one bit-rotate is not.)
>

I think for DRAM bandwidth, 7 is the right number.  The first loop calls
this for each memory location sequentially:

    reducedDuplexRowSetup(state, memMatrix[prev], memMatrix[rowa],
memMatrix[row]);

memMatrix[prev] is still in cache, but this function does a read and write
to memMatrix[rowa], and a write to memMatrix[row].  This loop causes 3 DRAM
accesses.  The second loop calls:

    reducedDuplexRow(state, memMatrix[prev], memMatrix[rowa],
memMatrix[row]);

This function reads from all three and writes to two, but memMatrix[prev]
is in cache, so I only count two reads, for a total of 4 accesses.  If
Lyra2 were running in cache, I'd count the cached reads and use a 9X
multiplier.

BTW, yescrypt at zero rounds of pwxform might well be a reasonable thing
> to use, if we're to maximize memory bandwidth usage while not even
> trying to keep any compute time hardening (besides the occasional Salsa20).
> I just haven't studied it enough to recommend it as an option, and I am
> not aware of a use case where zero compute time hardening would be good.
> When making the rounds count tunable (and encoding it in the flags
> field), I am not sure if I should make zero a supported value.


I think having a mode that is not explicitly compute-time hardened would be
a good thing.  My SkinnyCat subset of TwoCats has no multiplication chains,
and only has memory bandwidth to force attackers to take a while.  For
machines without a dedicated multiplier, this might be a better mode for
them to run.

I don't know what skipping pwxform calls does to security, though.  I'm
sure you can deal with that.

Bill

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