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Date: Sun, 21 Sep 2014 06:39:52 +0000
From: Brandon Enright <bmenrigh@...ndonenright.net>
To: Steve Thomas <steve@...tu.com>
Cc: discussions@...sword-hashing.net, bmenrigh@...ndonenright.net
Subject: Re: [PHC] omegacrypt and timing

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On Wed, 17 Sep 2014 15:35:45 -0500 (CDT)
Steve Thomas <steve@...tu.com> wrote:

> > > 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.  
> >
> > Interesting. So let me make sure I understand what this attack would
> > look like.
> >
> > You'd N instances of OmegaCrypt on the GPU by allocating N ChaCha
> > states and N large regions of memory.[...]
> >  
> 
> Ah, I get what the misunderstanding is. Each thread is running a
> different password guess and there are several threads running at the
> same time. They all have their own ChaCha state and memory. When they
> hit a branch some threads are set to not do anything while the other
> ones run.

My apologies for a delayed response, this attack gave me a lot to think
about.

As currently designed, each round OmegaCrypt picks 1 of 4 branches to
run.  As you describe in your proposed attack, with a GPU capable of
data-dependently NOPing out instructions you could simply run the
branches in order and NOP out the three wrong ones.  In this way you
could keep all parallel instances of OmegaCrypt in sync with each
other.  Even though they'd be less efficient, it would defeat intended
GPU resistance of the branching.

This attack works because the worst case (running all 4 beaches) is
bounded and isn't *that* much worse that just running the correct
branch.  If the worst case wasn't known or if the worst case was
totally unbounded it would no longer be possible to run all parallel
instances through the worst case having them selectively NOP out
useless instructions.

Instead of having 4 (or N) fixed branch paths each with a fixed number
of instructions for each path, I suggest the following logic would
defeat the selective NOPing ability by making the worst case too bad:

while (coinflip()) {
   do_some_work();

   while (coinflip()) {
      do_some_more_work() {

       [...]
          [...]

Using a structure like this ends up being even simpler than N different
fixed branches and the worst case needed to keep threads in sync is
unbounded.  Even if an implementer were to cap each while (coinflip())
loop to 20 iterations (for example) 20 * 20 * [...] is far, far more
costly than the average case actually taken by the code.

I will explore this area of branching more and if OmegaCrypt makes it
to the next round I will replace the current 4 branches with something
more robust like above.

Of course, I'd love to hear any thoughts / analysis on this idea.

Regards,

Brandon

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