phc-discussions - Re: [PHC] Is bandwidth all that counts?

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Date: Wed, 12 Feb 2014 03:01:45 +0000
From: Peter Maxwell <peter@...icient.co.uk>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] Is bandwidth all that counts?

On 12 February 2014 01:23, Marsh Ray <maray@...rosoft.com> wrote:

>  *From:* Peter Maxwell [mailto:peter@...icient.co.uk]
> > My problem is understanding why memory bandwidth is the critical
>
> > factor - is memory bandwidth inherently more expensive than the actual
> DRAM?
>
>
>
> It’s something that depends on MACROscopic hardware, which imposes
> physical limits to growth. It grows much slower that Moore’s law.
>
>
>
> Yet commodity hardware comes very highly optimized for memory bandwidth,
> so in effect defenders collectively participate in a big economy of scale.
> An attacker may be able to do get a better price/performance with custom
> hardware, but probably not by much (on the log2 scale).
>
>
>
> In short, we can expect that an attacker could fit 512 parallel instances
> of MD5 on a custom silicon chip. But it’s probably also a reasonable
> assumption that no one has an external RAM subsystem that’s 512 times more
> efficient than the latest DDR chips.
>


So essentially, a hardware attacker can:

- use compute operations much cheaper than defender;

- use memory storage at the roughly the same cost as defender;

- use memory bandwidth at roughly the same cost per hash operation as the
defender.

Have I got that correct?


>From what I understand - and I don't know how far down the road they
actually are - there are ASICs being produced to do scrypt hashes because
it's used in Litecoin, presumably with the side-effect that these
appliances can be used for cracking password hashes too.  Is it likely that
the need for significantly improved memory bandwidth in other contexts -
for example high-performance computing in the sciences - will drive an
improvement in technology such that the assumptions on memory bandwidth
limitations will fail in the near future?

And as a slight tangential: is it worth candidate algorithms for PHC having
a tunable to create ever-so-slight changes in computation such that any
adoption by a crypto currency isn't going to indirectly encourage hardware
specifically for cracking password hashes to be produced?

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