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Date: Wed, 15 Jan 2014 21:27:06 -0600 (CST)
From: Steve Thomas <steve@...tu.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Scripting memory (not so) high vs Catena in PHP (with
 optimizations)

> On January 12, 2014 at 9:12 AM Solar Designer <solar@...nwall.com> wrote:
>
> I've attached a new revision of smhkdf (still highly experimental, work
> in progress). It is about twice faster than the first revision, mainly
> due to reduced iteration count in the last loop. With $k > 1, that loop
> doesn't need as many iterations; rather, to maximize ASIC area*time, we
> want the memory filling and memory using loops to run for roughly the
> same amount of real time. (Another aspect, though, is that for attacks
> with e.g. CPUs we may want to optimize for greater SHA-512 efficiency,
> even if it reduces the AT cost somewhat. For this reason, maybe the
> optimal iteration count for the last loop needs to be somewhere inbetween
> of what I had in the first revision and in this new one.)
>
> This new revision also discourages TMTO to a slightly greater extent.

You might want to change:
        $v .= $x = hash('sha512', $x . substr($v, $j, $blocksize1), TRUE);
and
        $x = hash('sha512', $x . substr($v, $j, $blocksize2), TRUE);
to:
        $v .= $x = hash('sha512', substr($v, $j, $blocksize1) . $x, TRUE);
and
        $x = hash('sha512', substr($v, $j, $blocksize2) . $x, TRUE);

This way the attacker can't do anything until you get the data from $v. Well
the attacker could precalculate s0 and s1 from $x for the last block:
s0 = ("x" rightrotate 7) xor ("x" rightrotate 18) xor ("x" rightshift 3)
s1 = ("x" rightrotate 17) xor ("x" rightrotate 19) xor ("x" rightshift 10)

This has an added benefit of forcing the attacker to shift $x by 7 bytes
instead of using $x as is.
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