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Date: Mon, 20 Apr 2015 23:08:42 +0200
From: Dmitry Khovratovich <khovratovich@...il.com>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] enhancing Argon2 (was: Competition process)

Hi Alexander,

thank you for investing so much effort into improving Argon2. Your
contributions are sound, and they will indeed do the job of latency
increase.

First, I would like to better understand your reasoning of having
S-boxes. For me, S-boxes in such ARX schemes look quite ad-hoc, and I
want to get the right reason before introducing them. So my questions
are:
1) Are S-boxes only an anti-GPU countermeasure? As I understand, they
should not be problem for FPGA and ASICs where the small-size S-boxes
can be fit into the low-latency memory. Glanced at your recent
bcrypt-FPGA paper, I suppose that FPGA would be a reasonable choice
for any attacker and almost any scheme. If yes, then why bothering
with S-boxes?
2) If my logic is wrong, then how many S-box lookups per memory block
(or per CPU cycle?) would you want? Maybe there can be an elegant
solution then?

Apart from S-boxes, the most natural way to increase the latency would
be chaining the Blake permutations and integrating high-latency
operations into them. It is a good question how to chain properly, and
we are still working on it.

Regarding your benchmarks, the impact of extra operations on 8 threads
is so low, I guess, because the bottleneck is the memory bandwidth
rather than the CPU.

We thank you again for the patches, but just want to think a bit more
on the most optimal update.


On Mon, Apr 20, 2015 at 8:41 PM, Solar Designer <solar@...nwall.com> wrote:
> Dmitry, all -
>
>
> I now put the high 32 bits of the final output of the multiplication
> chain into state[0]'s low 32 bits, which I think are then used for phi:
>
>                 phi = _mm_extract_epi32(prev_block[0], 0);
>
> BTW, this SSE4.1 intrinsic (found in original argon2d-opt-sse.cpp) may
> be replaced with SSE2's _mm_cvtsi128_si32().

Thanks for the advice (and the previous ones).

>
> Since we have 8 parallel BLAKE2b's, it may be elegant to split our
> 64-bit chain as 8 one-byte inputs/outputs into each BLAKE2b (perhaps in
> addition to using 32-bit values in state[0] and state[63]).  We could
> also merge them in-between the two groups of 8 BLAKE2b's.  Would this
> help increase the latency of tradeoff attacks?  Without thinking of this
> much yet, my gut feeling is that it won't, but it'd be some additional
> state information that is normally not stored, and that the tradeoff
> attacks will need to store (or they'd in fact incur a latency increase).
> Very little of it, though.  So probably not worth the overhead.  Yet I
> thought I'd bring this up for discussion.

This discussion needs drawing and thinking. Ideally I would want a
tradeoff-resistant solution
be understandable just in mind, without pen and paper.

>
> To increase the latency of tradeoff attacks, I think BlaMka may be used
> (along with an un-pwxform chain like this, which serves its different
> purpose - hardening non-tradeoff latency and providing some anti-GPU).
>
If using BlaMka, why extra xform chain?



-- 
Best regards,
Dmitry Khovratovich

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