phc-discussions - Re: [PHC] omegacrypt and timing

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Date: Wed, 17 Sep 2014 13:50:01 -0500 (CDT)
From: Steve Thomas <steve@...tu.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] omegacrypt and timing

> On September 17, 2014 at 1:12 PM Brandon Enright <bmenrigh@...ndonenright.net>
> wrote:
>
> To avoid misalignment, if you ran all 4 for round 1, and then selected
> the right one, then all 4 for round 2, then selected the right one,
> etc., you'd be doing 4x as many memory operations and you'd need a way
> of discarding the memory changes made by the 3 wrong branches. Is this
> the attack you're suggesting?
>

No, I'm saying that a GPU will waste clock cycles while not calculating the
wrong data paths. This is do to it's conditional execution of instructions. If a
thread is not suppose to run an instruction it will do a nop (no operation)
instead.