phc-discussions - Re: [PHC] (not) protecting password length from side-channels (Re: [PHC] Argon2 modulo division)

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Date: Thu, 23 Apr 2015 10:38:23 -0700
From: Bill Cox <waywardgeek@...il.com>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] (not) protecting password length from side-channels (Re:
 [PHC] Argon2 modulo division)

On Wed, Apr 22, 2015 at 5:47 PM, Solar Designer <solar@...nwall.com> wrote:

> On Wed, Apr 22, 2015 at 09:13:21AM -0700, Bill Cox wrote:
> > It turns out that there was not a single entry in the competition that is
> > power rail analysis resistant
>
> ... with respect to password length.
>
> I'm with Thomas on this.  It is futile for PHC candidates to fully
> protect the password length.  (They should, however, make their running
> time _mostly_ independent of password length, and Milan's benchmarks
> confirm that they do.)

I agree.  Hiding password length from a power rail analysis seems to be a
lost cause.  It sounds like Krisztián Pintér may not agree, and might look
into protecting the password length in Gambit.  It would be interesting to
see what his solution looks like.

I think the password length leakage is of similar importance as what Lyra2
leaks in cache timing during it's second phase, though cache timing data is
easier to get.  For off-line attacks, the password length is probably more
useful - not only can you speed up breaking passwords, but you can work on
short passwords first.  The mod operation in Argon2i is a minor issue, IMO,
because the password length leakage is already there, and the attacker is
able to do a power rail analysis.  You're already in big trouble in that
case.

Bill

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