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

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Date: Fri, 19 Sep 2014 16:07:07 -0400
From: Bill Cox <waywardgeek@...hershed.org>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Missed opportunity re: unpredictable addressing?

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On 09/19/2014 10:08 AM, Krisztián Pintér wrote:
> 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.
> 

I agree.  Here's one particular optimization I might use.  Since for a
given salt, all the memory access patterns will be the same,
independent of the password guess, I can interleave the memory for
several guesses together, so that when I run them in lock-step, I
reduce the cache-miss penalty for reading small blocks of memory.
That's just one example, but there are a number of them.

There are several optimizations that can be made when predictable
addressing is used.  For example, a TMTO generally improves because
you only need to keep memory you know you will use.  You can mount a
pebbling attack and minimize memory for any given TMTO using my
auto-pebbler, for example :-)

Also, some timing information can be leaked.  It might be possible for
an attacker to detect who has logged in based on a cache timing
signature, for example.

I thought a lot about how to use salt and not the password, but I
couldn't come up with anything worth pursuing.

Bill
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