phc-discussions - Re: [PHC] Transforming hash to different cost setting

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Message-ID: <20140322125339.GA23493@openwall.com>
Date: Sat, 22 Mar 2014 16:53:39 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Transforming hash to different cost setting

On Sat, Mar 22, 2014 at 08:39:21AM -0400, Bill Cox wrote:
> I use Christian's solution for this.  It loses up to 2X in terms of
> time*memory security, so users would not use this up-front.

Actually, up to 3x - that is, Catena's area-time cost to attacker
converges to 1/3 of what's optimal, after many upgrades.  With
(acceptably) higher granularity of upgrades, we can improve this to 3/5
of optimal.  (I posted these numbers in here a few months ago.)

> Gary has a similar idea, but he wants to store the new rehashing
> parameters in a chain with each password.  That would allow the admin
> to take that 50MiB hash and turn it into a 2-step hash of 50 MiB and
> 400MiB.  It's more efficient, but adds complexity.

I've been thinking of this too, even considered how the info would be
encoded - but I think I'll simply go for the 3/5 option.

Alexander