phc-discussions - Re: [PHC] TwoCats multiplication chain

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Message-ID: <20140626031219.GA21021@openwall.com>
Date: Thu, 26 Jun 2014 07:12:19 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] TwoCats multiplication chain

On Wed, Jun 25, 2014 at 01:05:04PM +0200, Thomas Pornin wrote:
> Since, for password hashing, we want code which is faster on a PC than
> on a FPGA, we should build up on operations that work best on a PC and
> worst on a FPGA. From this point of view, using CLMUL looks like a
> not-so-good idea.

I fully agree if we s/FPGA/ASIC/g above.  For FPGAs, I am not so sure.

In modern FPGAs, there are usually many 18x18 multipliers (and we can
build larger multipliers from those).  I don't know if it's possible and
cheap to use them in carry-less mode, or if this would mean spending
other resources.  So I wouldn't be surprised if CLMUL actually turns out
to be less FPGA-friendly than traditional MUL.

Perhaps someone with experience of implementing elliptic curves in FPGA
can comment on this.

Alexander