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Date: Thu, 03 Apr 2014 00:38:52 -0400
From: Daniel Franke <dfoxfranke@...il.com>
To: discussions@...sword-hashing.net
Subject: Deliberately GPU-friendly password hashes?

I think I've at least glanced at all 24 entries now and it doesn't look
like there are any which are intended to be friendly to defensive GPU
use. I think this is an unfortunate omission.

I just realized there's a pretty straightforward variation on EARWORM
which would serve this purpose well. Just replace AESRound with some
Salsa20 rounds (or some other ARX round function) interleaved with
multiplies by values read from the arena. Tune the ratio of Salsa20
rounds to multiplies such that you're making nearly full use both of
computation cores and of memory bandwidth at the same time. Increase the
chunk length in order to cover the higher memory latency and you're in
business.

Even though this wouldn't change EARWORM's specification very much, it
would be enough of a shift in its security goals that I feel it would be
dishonest to try to pass this off as a "tweak"; it would really be a
different entry. If the panel decides to reopen the call for submissions
as was discussed back in December, then I'll take advantage of the
opportunity. Otherwise, well, I guess there's not going to be any law
that says the password hashes in the PHC portfolio are the only the ones
you're allowed to use. I think this is a situation that the BLAKE2 folks
would find familiar :-).

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