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Date: Thu, 13 Feb 2014 22:54:57 +0400
From: Solar Designer <>
Subject: Re: [PHC] multiply-hardening (Re: NoelKDF ready for submission)

On Thu, Feb 13, 2014 at 01:02:49PM -0500, Bill Cox wrote:
> Running the scalar unit in parallel with the SIMD unit, and having
> both doing what they are best at (multiply in scalar unit, memory r/w
> and parallel ADD/XOR/SHIFT/AND/OR in SIMD unit) seems like the best
> solution for processors that support it.

That's not exactly what I meant.  It sounds like you're suggesting
moving data between scalar and vector registers, which is overhead.

What I meant is that you could do all sorts of operations, including
multiply, on both scalar and vector registers, in separate chains.
The scalar chain would be optimized for optimal latency - doing the
multiplies as frequently as you can.  It will provide the latency
hardening, not letting ASICs compute a block quicker even if it has a
higher memory bandwidth.  The vector chain would be optimized for high
throughput - doing as many multiplies per real time as you can - and for
high memory bandwidth.  You don't even have to access memory from the
scalar chain, nor mix its computation with the vector chain's more
frequently than before and after each block.

> How can we add parallel SIMD computation to the scalar processing
> without having trouble on processors with weak SIMD?

I think we need to have tunable SIMD and instruction-level parallelism
settings.  For ancient CPUs, we can set both to 1.  Yes, this means that
hashes initially computed on newer CPUs and with these settings tuned
for newer CPUs will be slower to compute on older CPUs - more so than
e.g. bcrypt, which leaves most execution units idle on modern CPUs.

> Are they an
> important case, and if so, are they already so slow that
> multiplication hardening is not useful?

Yes, I was also thinking that requiring multiplies at all may hurt
ancient machines too much, making it difficult/unreasonable to come up
with OS distro default cost settings for password hashing that would be
usable on machines ranging from VAX to modern x86-64 and beyond.  With
bcrypt's use of only 32-bit ADDs and XORs, older machines are not that
much slower than modern ones; with multiplies, the gap will be wider.

This is why you see non-multiply "round function" examples for SN=4 and
SN=2 in sim-mix.c.  Maybe we should support something like that as a
tunable setting, to make the hash function potentially usable as default
on a *BSD distro (even if with significantly weakened settings), but we
need to make sure it'd be strictly more attack resistant than bcrypt.


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