phc-discussions - Re: [PHC] Missed opportunity re: unpredictable addressing?

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Message-ID: <CA+hr98G86cKwn4eCa0D7Si65E2Jaa-jp=EHzmnZU99t94qSqyQ@mail.gmail.com>
Date: Fri, 19 Sep 2014 16:08:05 +0200
From: Krisztián Pintér <pinterkr@...il.com>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] Missed opportunity re: unpredictable addressing?

On Fri, Sep 19, 2014 at 5:59 AM, Alex Elsayed <eternaleye@...il.com> wrote:
> In particular, a salt is defined as 1.) public and 2.) random. I suspect
> that salt-dependent, password-independent addressing might well prove a
> useful trick.

the problem is exactly that salt should be treated as public. this
concept would be fine for secret salts, which is an option, but
unusual. for public salts, it does not help, but hinder. why it does
not help, was explained by Dmitry. let me explain why it hurts.

predictable but wildly irregular pattern is harder to optimize, but
can be optimized. it is typically a situation which urges the good
guys to omit optimization (to avoid complexity), but urges attackers
to do the optimization, and thus gain advantage. we want optimization
options to either be not there, or so easy that everybody will
implement them. difficult optimizations mean advantage to attackers,
and disadvantage to defenders.