phc-discussions - Re: [PHC] "Attack on the iterative compression function"

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Message-ID: <20150414223935.GA8300@openwall.com>
Date: Wed, 15 Apr 2015 01:39:35 +0300
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] "Attack on the iterative compression function"

On Wed, Apr 15, 2015 at 01:29:30AM +0300, Solar Designer wrote:
> Dmitry - you claim that "[...] it allows to reduce the memory by the
> factor of 12 or even higher while still reducing the area-time product."
> Since table 2.1 only goes to 1/11, for depth 32.8, and you say that "In
> concrete proposals, s can be 64, 128, 256 and even larger", I guess this
> "factor of 12" applies to the next column in the table (not shown),
> where depth would presumably be in the 32.8 to 64 range?  Do I interpret
> this correctly?

Hmm, probably not correct interpretation.  I was thinking of needing to
store occasional full blocks anyway, but then the attack doesn't make
sense.  So this probably corresponds to storage of one sub-block per
block (and then re-running the whole thing storing 2nd sub-block, etc.),
and some column in the middle of the table.  Right?

Doesn't appear to apply to yescrypt, because:

> However, the attack would appear to rely not only on the last
> sub-blocks, but also on occasional full blocks, so I don't see how it'd
> improve upon the higher-level TMTO attack alone.  The last sub-block of
> output of each BlockMix_pwxform depends on the entire input block, and
> if you never store entire blocks then you'd need to restart computation
> from block 0 in order to reach a block's last sub-block.

Alexander