phc-discussions - Re: [PHC] multiply latency reduction via table lookups

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Date: Tue, 11 Mar 2014 08:34:12 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] multiply latency reduction via table lookups

On Mon, Mar 10, 2014 at 08:57:58PM -0400, Bill Cox wrote:
> This thread has me thinking that we can compute-time harden our PHSs
> (for Password Hashing Schemes) with L1 cache access times, as an
> alternative to multiplication, if we want to.  There's a reason we
> don't use small RAMs to accelerate multiplication: they're too slow!
> 
> Doing 32-byte random lookups in L1 cache and hashing it with one other
> 32 byte value, and writing it back to yet another random lookup, would
> run really fast on Haswell, and I do not believe it would be easy for
> an ASIC attacker to do better.

Is the "writing back" part essential?  What if the writes are many times
or even orders of magnitude less frequent than reads?  Isn't it
sufficient to ensure the data is dependent on the password and salt (so
must be stored in a RAM rather than a ROM)?  Frequent writes would hurt
performance on CPUs with write-through L1 cache, including Bulldozer.
Another drawback of frequent writes is that some of them may eventually
propagate to main memory even on systems with write-back caches, which
would waste some memory bandwidth.

> It would have to be done carefully, since we also want to write to as
> much memory as possible to maximize time*memory cost for an attacker,
> so somehow we'd need to figure out a strategy for hammering L1 cache
> without cache miss penalties while at the same time filling external
> DRAM at the bandwidth it can support.  The multiplications seem to
> happen in parallel on the scalar unit, while memory hashing can happen
> in the SIMD unit.  Is it possible to do something similar with cache
> vs external DRAM hashing?

Yes, escrypt tries to do that.  I thought TigerKDF did too.

> I don't know how to do both at the same time... Alexander?  This seems
> like your kind of thing.

The L1 cache reads may be made from the sub-block processing function.
This may be happening while further sub-blocks are being prefetched (the
prefetch instructions are issued when processing of a new block starts).
This is what escrypt does.

BTW, with blocks as large as what you use (multiple KiB rather than up
to 1 KiB as I normally test escrypt with), the number of pending
prefetches would definitely exceed what's supported in hardware.
I don't know what exactly happens then (are some prefetch instructions
simply ignored? or are there CPUs where they'd block?)  I tested this
briefly, and somehow didn't run into any significant performance
penalty, but I think there must be room for optimization there (this may
be a reason why I am not getting much of a speedup for larger r) - e.g.,
for r > 8, we could be doing the prefetches in groups of 16 (or even
fewer?) 64-byte sub-blocks, just a few sub-blocks before they're
processed.  I haven't tried that yet, focusing on r <= 8 for other
reasons I mentioned (in other postings in here).

Alexander