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Date: Wed, 1 Apr 2015 13:04:42 +0300
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] OMG we have benchmarks

On Wed, Apr 01, 2015 at 12:24:34PM +0300, Dmitry Khovratovich wrote:
> Argon and Argon2 do not have other parameters that affect the
> performance crucially (apart from multithreading).  However, the other
> schemes do, and they must be listed (number of rounds in the internal
> function, specific flags, etc.). For fair benchmark comparison, the
> designers must choose some recommended set of parameters, explicitly
> claim its security, and yield it for measurement.

I think Milan benchmarks these schemes via the PHS() interface, which
has parameters other than t_cost and m_cost at values determined by the
designers.

The problem with this is that those parameter values chosen by the
designers may achieve different security levels for the different
schemes, in their different dimensions.  For example, as we discussed
yesterday there may be substantial differences in lowest latency (e.g.,
per memory byte processed) that an attacker might achieve, which may
affect the time factor in the area-time product, even for e.g. two
schemes that both reach 1 GB in 1 second on a defender's CPU.  Of
course, there may also be differences in TMTO resistance and in many
other dimensions (such as performance of GPU attacks).

For example, the chart above shows Argon2d-SSE and yescrypt-SSE as being
close, but yesterday we came up with a 5x+ difference in ASIC attacker's
compute latency for these for PWXrounds=2.  (I'm not sure if you agreed,
though.)  Milan's benchmarks are for PWXrounds=6 (the default), so the
difference is 15x+.  (BTW, it is puzzling that Bill found PWXrounds=2
achieving performance parity between Argon2d and yescrypt, whereas in
Milan's benchmarks yescrypt's default PWXrounds=6 appears to work almost
as well.  Are the machines so different?  Maybe there's some error here.)

Thus, these schemes are not trivially comparable, although we should try
to compare them at settings that achieve similar security, or normalize
the results accordingly.

Alexander

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