phc-discussions - Re: [PHC] Argon2 improvement thread

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Date: Tue, 21 Jul 2015 13:14:35 -0700
From: Bill Cox <waywardgeek@...il.com>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] Argon2 improvement thread

On Tue, Jul 21, 2015 at 12:37 PM, Solar Designer <solar@...nwall.com> wrote:

> > The designers proposed an optional update:
> > * "smarter non-linear indexing (...) in order to flatten the memory usage
> > over time"
>
> I support this tweak, although I haven't reviewed it yet - I have only
> read the relevant announcement in here.  In fact, Bill and I were
> advocating making a change in this area.
>

I also support this tweak.  Their proposal of using a distance-squared
distribution was my second-choice after a distance-cubed distribution, but
I do not have strong feelings either way.  If you get too extreme, for
example using an exponential decay distribution, then you wind up with
graph-cuts too narrow to avoid strong TMTO atacks.  With a distance-cubed
indexing distribution, the min-cut is still about 1/8th N for a N node
graph, IIRC.  The last back-edges have a 1/8th chance of crossing the
mid-point, so that makes sense to me.  With a distance-squared
distribution, those last edges will have a 1/4 chance of crossing the
mid-point.  This is better for defending against min-cut TMTO attacks, but
worse for defending against attackers who only drop later memory.  Also
it's simpler and faster to compute the distance squared distribution.

Both distributions did extremely well in my automated TMTO attacks.  I went
with distance-cubed because it did slightly better.

Bill

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