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Date: Wed, 24 Jun 2015 09:49:28 -0700
From: Bill Cox <>
To: "" <>
Subject: Re: [PHC] Anonymous comments about Argon2 and Catena

On Wed, Jun 24, 2015 at 1:00 AM, Jean-Philippe Aumasson <> wrote:

> This is to share comments from a PHC submitter, who wishes to remain
> anonymous:
> ------------------------
> Including Argon2 in the competition is good for a fair competition;
> otherwise Catena can win in that category without meaningful competition
> since its competitor RIG was kicked out already.

I agree.  Including Argon2 was the right decision, IMO.  It enhances

> It has been mentioned repeatedly that password information is leaked
> through side channel, but no experiment has been performed to measure how
> many hashes are needed to leak one bit password information in those
> candidates.

SFAIK, we don't have any remaining entries that if implemented properly
would leak any bits of the password to a cache-timing attack.

The remaining entries perform what should be a side-channel resistant
initial hash of the password, salt, and other key material to create an
initial derived key.  It may be possible for bits of this key to be
extracted through cache-timing, but without the salt, the attacker has no
ability to derive any bits of the password.  All that the attacker gains is
knowledge that a given user (who's identity is not revealed by this side
channel) is authenticating again.  This meta-data is difficult to protect
in any case, given the overall environment where we embed the password
hashing algorithm.

The main reason for defending against cache-timing attacks is to prevent an
attacker who also has the password-hash/salt database from rapidly aborting
incorrect password guesses.  It's a bit of an odd case, IMO, but the
Lyra2-style hybrid cache-timing-resistant loop followed by an unpredictable
addressing loop defends well in this case.  We don't need a purely
cache-timing-resistant algorithm as a "main" winner, IMO.  They hybrid
architecture is good enough for real-world use cases.

> A password may not get hashed many times in practice (KDF is different).
> I checked a cache-timing attack on AES, the number of samples is quite
> large in order to remove the measurement noise ( 17,720 samples according
> to the paper
> ) The number of samples may be reduced for password hashing due to the
> larger state size and longer processing time.  Anyway, experiments are
> necessary if we want to know/claim how important the side-channel attack is
> for this competition.

An attacker only needs to see the cache timing once to make use of it to
accelerate an off-line password guessing attack.  Beyond that, I don't know
what good seeing the cache timing a million times would do for the
attacker.  It wont reveal anything about the password.


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