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Date: Mon, 22 Jun 2015 14:49:12 -0700
From: Bill Cox <waywardgeek@...il.com>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Cc: Alex Biryukov - UNI <alex.biryukov@....lu>, Dmitry Khovratovich <khovratovich@...il.com>
Subject: Re: [PHC] Argon2: optional update

On Mon, Jun 22, 2015 at 1:01 PM, Dmitry Khovratovich <khovratovich@...il.com
> wrote:

> Dear PHC community,
>  following discussions in the PHC forum we decided to further increase the
> circuit depth and improve the tradeoff resistance of Argon2. We propose an
> optional update with two main changes:
>
> 1) The "random" memory block is not  uniformly selected but with a
> distribution skewed towards later blocks. This gives both higher tradeoff
> resilience and more uniform memory access. The distribution is similar to
> that in TwoCats, though for the moment we slightly prefer quadratic (rather
> than cubic) power function as it minimizes the AT gain for a tradeoff
> attacker. The detailed design rationale is provided in the update
> description.
>

Excellent.  This should significantly reduce the options for an attacker.


> 2) The round function of Blake2b is replaced with BlaMka (each addition is
> now accompanied with a 32x32 multiplication) in order to increase the
> circuit depth. In total we add 512 MULs per 1-KB block with shortest chain
> having 12 MULs.The structure of the compression function remains the same
> for the moment (thus some extra parallelism is still in place), since such
> a change apparently requires a very involved analysis.
>
> Both modifications affect the performance (the first one increases the
> speed, whereas the second one decreases it). In total, Argon2d (and
> Argon2i) would run slower by 15% with multiple threads, and by 45% with 1
> thread.
>
> This update should be considered as a post-PHC modification, i.e. the one
> that is deployed after the winner is declared.
>

I like them both!

Bill

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