Date: Mon, 30 Mar 2015 14:06:08 +0300
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Cc: Agnieszka Bielec <bielecagnieszka8@...il.com>
Subject: POMELO v2

Hi,

Agnieszka, one of our prospective GSoC students (CC'ed on this message)
is currently working to add support for PHC finalists into JtR -jumbo,
including OpenCL implementations.  She already has an initial
implementation of POMELO v2 in OpenCL, ported from the reference C code.
While this is just an initial hack, I thought I'd let this list know.
Discussion is currently in progress on the john-dev mailing list.

While at it, Agnieszka pointed out this inconsistency in the POMELO v2
reference code:

    //check the size of password, salt and output. Password is at most 256 bytes; the salt is at most 32 bytes.
    if (inlen > 256 || saltlen > 64 || outlen > 256 || inlen < 0 || saltlen < 0 || outlen < 0) return 1;

Note how the comment says "the salt is at most 32 bytes", but the check
is for "saltlen > 64".  This inconsistency is present in pomelo_x64.c
and pomelo_sse2.c, but not in pomelo_avx2.c (which says 64 in both the
comment and the code).  Which is right?

Some other observations on POMELO v2:

Unlike in v1, there's 256-bit SIMD parallelism in v2.  However, POMELO
v2 still tries to discourage use of even wider SIMD.

The random table lookups are more frequent than v1's, maybe bringing in
some bcrypt-like GPU unfriendliness.  However, with the 256-bit SIMD
parallelism, non-SIMD defensive implementations (such as plain 64-bit
and especially 32-bit builds) are now at a disadvantage in this respect.
(It's the same tradeoff I faced in yescrypt, where it can be dealt with
by tuning pwxform settings.)

The "& mask" and subsequent S[] lookups can be made more efficient at no
security loss by redefining which bits are being masked and thus
pre-shifting the (constant for each invocation) mask by 5 (for 32-byte
S[] elements), thereby directly obtaining the byte offsets.  As it is,
POMELO v2 requires either indexed addressing (with the index being
implicitly shifted left by 5 by the CPU) or explicit extra shift
instructions, either of which (and especially the latter) may have
performance cost.  On x86, indexed access is only available for up to
8-byte quantities, so we get explicit shift instructions in the
generated code.

solar@...l:~/pomelo/POMELO-v2/POMELO/pomelo_code_avx2$ objdump -d pomelo_avx2.o | fgrep -c 'shl    $0x5,'
24

yescrypt includes the optimization I mentioned above (optimally
pre-shifted mask, to avoid these shifts).  Unfortunately, getting this
optimization into POMELO now would be a tweak (it affects which hashes
are computed, since different bits would be used for index).  If we
introduce a standardization & tweaks-by-panel phase (which I think we
should) and POMELO is chosen as a winner, perhaps this is something we
should fix (in coordination with the author, indeed).

Finally, in case some of us haven't noticed yet, POMELO v2 gives very
competitive speeds in its specification document - e.g., 1.1 seconds on
i7-4770K (when running one thread, I guess) for 1 GB.  This is on par
with Lyra2 and yescrypt.  I haven't run my own benchmarks yet, but if
this is true then POMELO v2 has moved from competing with Pufferfish
only (like POMELO v1 did before) to also competing with Lyra2 and
yescrypt (and continuing to compete with Pufferfish as well).  Nice!

Alexander

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