phc-discussions - Re: [PHC] Question about saturating the memory bandwidth

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Date: Sat, 18 Jan 2014 13:47:07 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Question about saturating the memory bandwidth

On Sat, Jan 18, 2014 at 08:40:08AM +0000, Peter Maxwell wrote:
> And therein lies the rub.  Saturating the memory bandwidth will not be
> practical - or even nearly so - for the majority of use-cases.  Similarly,
> the idea of the defender using GPUs will not be reasonable in most
> instances either.  Yes, there are specific cases where it might be useful
> but one needs to address far more mundane scenarios like busy web-servers
> on current hardware.

There's no problem with saturating the memory bandwidth for password
hashing on busy web servers.

As to GPUs, I agree - currently not reasonable for most use cases.

> Most of the big password thefts have occurred on websites, and more
> recently arguably also with these fancy games services.  With the former
> we'll be lucky to convince folk to use anything more than a salted-hash let
> alone something that will hog all their processor cores and memory for each
> login attempt (whether that attempt is valid or not).

On a busy web server, given current not-so-high CPU core counts, it is
currently sufficient to use one CPU core (and whatever memory bandwidth
is accessible from one thread) per password hashed.  The t_cost limiting
factor is typically the desired throughput, not latency for end-users
(the desired maximum supported throughput is usually high enough that
the latency is low enough not to be noticed under normal conditions).
Thus, we don't have to introduce our own thread-level parallelism: when
the maximum throughput is almost reached, sufficient parallelism is
coming from concurrent authentication attempts anyway.  And yes, this
implies that we have lower than optimal area-time cost for the attacker
when the maximum throughput is not reached, but we have to accept that.

RAM is not disk.  Sharing RAM bandwidth between concurrent tasks works
about as well as sharing CPU time.

Alexander