phc-discussions - Re: [PHC] A review per day

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Date: Fri, 29 Aug 2014 05:27:28 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] A review per day - TwoCats

On Thu, Aug 28, 2014 at 07:38:42PM -0400, Bill Cox wrote:
> Therefore, I decided to hash memory as fast as possible.  In
> single-thread mode, TwoCats I believe is the world's fastest hashing
> algorithm.

It probably is.  yescrypt may achieve similar single-threaded speed with
reduced round counts, but I chose its defaults to be optimal for
multi-threaded rather than single-threaded use.  (Note that this does
not necessarily mean use of multiple threads for computation of one
hash, but rather use cases - which I think many of them are - where the
cost settings are limited by total capacity of the machine, including
its ability to run multiple tasks in parallel.)  Perhaps I should make
this configurable.

Another quality TwoCats might win at is multiplication latency hardening.
By using scalar multiplication only, TwoCats probably squeezes more
_sequential_ MULs per byte written than yescrypt does, at least with
yescrypt's defaults that try to keep the SIMD multipliers busy on every
clock cycle (meaning there must be much parallelism).  Again, I should
make yescrypt's instruction-level parallelism configurable (right now,
it's #define in -ref and fully hard-coded in -simd), which yescrypt's
design deliberately allows for.

> One thing I screwed up was how to use t_cost.

You can probably fix that during the tweaks period, if TwoCats becomes a
PHC finalist.

> That's all I have for now!  Please feel free to dump on TwoCats!  It's
> my turn :-)

OK, you asked for it.  I don't recall all of my complaints about TwoCats
anymore, but I think its small random lookups, which are meant to be
bcrypt-like, might not actually provide as much GPU resistance as
bcrypt's did.  I think TwoCats makes them 256-bit only, meaning that on
pre-AVX2 CPUs they might not be frequent enough, and you might not be
making enough of them per byte read/written to the large arena.  Have
you actually checked TwoCats vs. defense-optimized bcrypt running on the
same CPU when exhausting the machine's capacity (e.g., 8 independent
concurrent hash computations on your i7-3770), in terms of frequency of
those small random lookups?  And then you need to factor in how soon
their results are needed (the available parallelism, compare it against
bcrypt's four S-box lookups) and the total size of your equivalent of
bcrypt S-boxes (if 4x larger, then you probably have 4x more room for
parallelism while staying the same as bcrypt in terms of GPU resistance).

As I had mentioned, the results of such testing were one of my reasons
to keep yescrypt's default pwxform rounds count no lower than 6, or
alternatively I'd need to reduce instruction-level parallelism.  I think
you did not include such criteria when deciding on the amount of work to
do along with the main arena reads/writes in TwoCats.  You simply wanted
to maximize your memory bandwidth usage, well and the number of
sequential MULs, which is related, but not exactly it.

That said, I think TwoCats is a very good PHC candidate!

Alexander