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Date: Mon, 27 Apr 2015 09:33:08 +0300
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] "Attack on the iterative compression function"

On Sat, Apr 25, 2015 at 03:11:40PM -0300, Marcos Antonio Simplicio Junior wrote:
> De: "Solar Designer" <solar@...nwall.com>
> > On Sun, Apr 19, 2015 at 03:03:33AM -0300, Marcos Antonio Simplicio Junior wrote:
> > > 3) If we initialize a row in one order, and each subsequent
> > > read/update is done in the same but the columns are themselves
> > > reversed, then there is no easy pipelining (unless, of course, if
> > > some extra columns besides the first one are kept in memory to
> > > accelerate computations)
> > 
> > And of course I meant approach (3), where every BlockMix would write the
> > block in reverse order from reading. (This might be easier implemented
> > by reading in reverse order and writing in sequential order, since we're
> > already starting with the last sub-block in (ye)scrypt now. Also,
> > keeping the writes in sequential order allows to keep Integerify()
> > defined in the same way as it is in scrypt, using the last sub-block.
> > OTOH, sequential reads could be friendlier to the hardware
> > prefetcher.)
> 
> The only issue here would be that you would need an extra buffer, because the memory position from which you read is not the same on which you write (?)

Good point!  I was thinking in terms of equivalent of original scrypt's
shuffling, only changing the shuffling order to be simple reverse order.
Despite of this shuffling, optimized implementations are able to avoid
needing a write to a temporary buffer, as illustrated by the revision
that was possible in the yescrypt v0 submission to PHC, where
YESCRYPT_RW could be set without YESCRYPT_PWXFORM.  In that revision
invoked with those flags, SMix2 would proceed with XOR'ing over existing
blocks yet it would use original scrypt's BlockMix with its shuffling.
Yet yescrypt-0.5/yescrypt-simd.c: blockmix_salsa8_xor_save() updated the
block in-place.

Basically, the existing random block being XOR'ed over would not be
shuffled relative to BlockMix output.  Rather, BlockMix output would be
shuffled relative to its input, which is the previous block.

In other words, the shuffling would occur for only one of two blocks
that a newly (over)written block depends on.  So it'd extend the length
of only one out of two branches of the recomputation sub-tree.

What do you think of this approach?

> As a side note, if one does implement reversal as a separate, second operation shown our illustration, the memory bandwidth might be larger in architectures with write-through caches: the write operation would go down the memory hierarchy the first time we write, and also the second.

Right.  Somewhat similarly, Argon2 appears to suffer from this: the
writes to state[] between the first 8 and the second 8 BLAKE2b's may
waste memory bandwidth on architectures with write-through caches.

> > Also, why do you call this attack "pipelining"?
[...]

> I'm not sure if Colin Percival was thinking the same thing as we [...]

Thank you!  From these illustrations, which basically show the same
attack that Dmitry had described, I conclude that the "pipelining" here
is not the same as enabling the use of a pipelined crypto primitive or
such (this is left as an implementation detail, which may be used or not
regardless of this attack, although indeed with more parallelism it
makes more sense).

Alexander

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