phc-discussions - Re: [PHC] Compute time hardness

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Message-ID: <20150401153632.GA14369@openwall.com>
Date: Wed, 1 Apr 2015 18:36:32 +0300
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Compute time hardness

On Wed, Apr 01, 2015 at 08:20:29AM -0700, Bill Cox wrote:
> Alexander's technique of counting sequential operations, and giving
> multiply more weight seems reasonable to me, though he's once again
> under-cutting himself by only claiming a multiply is worth 3 additions.
> Use 8 and you'll be closer to reality, IMO.

I agree.  All we know is it's somewhere between 3 and 32.

> The algorithms also need careful analysis to be  sure an attacker can't
> start computing the next operation before the previous one is done, or at
> least take into account the potential speedup.  A chain of additions, for
> example, let's you propagate the low bits very fast compared to the high
> bits.  I feel good about both the reduced Blake2b round and Yescrypt's
> PWXFORM round here.

I thought of this yesterday, and my gut feeling is that there might be
some minimal speedup for some of the sequences in Blake2b here.

Alexander