phc-discussions - Re: [PHC] scrypt like hash, but with AES and HMAC-SHA-2

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Message-ID: <CAGZ8ZG1j0HTgm0zsJgYEmgpH1+fs4t-+w8ANB-4TKZByk9CRMw@mail.gmail.com>
Date: Thu, 8 Aug 2013 21:40:25 -0700
From: Trevor Perrin <trevp@...vp.net>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] scrypt like hash, but with AES and HMAC-SHA-2

On Wed, Aug 7, 2013 at 4:07 AM, CodesInChaos <codesinchaos@...il.com> wrote:
> * Random access to elements could enable cache timing attacks, but I don't
> think those are a big issue in practice, especially with unknown salts. But
> all schemes with predictable memory access I know are much weaker than
> scrypt.

You may be right, but I can imagine counter-examples:

 * File or disk encryption, where the salt is in the header.

 * An attacker who can read a site's password database or some of its
hashes, but doesn't have full control of the system.

To such a "known-salt" attacker, the memory-access pattern becomes
essentially a new password hash which he can try to crack by
simulating the memory access of candidate guesses.

This might give a known-salt side-channel attacker a faster
alternative to cracking a full password hash.

Trevor