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Date: Thu, 30 Jul 2015 08:00:36 +0300
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Finalizing Catena, Lyra2, Makwa, yescrypt

On Tue, Jul 28, 2015 at 07:26:32AM +0000, Jean-Philippe Aumasson wrote:
> Catena, Lyra2, Makwa, yescrypt are PHC's special recognitions. We'll
> deliver packages (specs+code) of their final versions in Q3. Meanwhile,
> minor changes can be made to the algorithms, but we need to agree on those.
> 
> @Submitters of C, L, M, y: Do you intend to make any changes to your
> algorithms? If so, which ones?

I am considering these tweaks to yescrypt:

1. A trivial change to how t is updated on hash upgrades.  (In v1, t is
reset to 0 on the first hash upgrade, to maximize the area-time product
growth on upgrades - but I've since realized that in some cases, such as
when t was very high initially, this strategy is not convenient.)

2. Reverse order of sub-blocks to mitigate Argon team's depth*memory
tradeoff attack (which might be practical at low yet non-negligible
memory reduction factors like 1/2 or 1/3).  I would then appreciate
review of the revised algorithm by the Argon and Lyra2 teams, if they
are willing to.

3. Re-introduction of TMTO-resistant pwxform S-box initialization
(dropped between v0 and v1 for simplicity of a potential yescrypt-lite,
while not simplifying the full yescrypt) or/and introduction of S-box
overwrites in BlockMix_pwxform.

4. A lower Salsa20 rounds count (likely 2 instead of 8) for its use in
BlockMix_pwxform.

While I am considering these, I will not necessarily make all 4.

The common theme in potential tweaks 3 and 4 (and maybe 2) is that since
yescrypt wasn't selected as the winner anyway, I don't have to focus on
allowing for a much simpler yescrypt-lite subset (hopefully, Argon2 with
MAXFORM will fill that space... although without MAXFORM, it won't).
It would still be possible to have a yescrypt-lite; it would just be
slightly more complex than what's possible with the current yescrypt v1.

We'll also need to agree on currently recommended sets of pwxform
parameters (up to 3 sets), or on one such set.

There should also be improvements to specs and code beyond what I listed
above, but those would not be changes to the algorithm.  Just clearer
specification and more robust code with improved APIs.

Alexander

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