phc-discussions - Re: [PHC] A must read...

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Date: Wed, 15 Jan 2014 19:05:52 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] A must read...

On Wed, Jan 15, 2014 at 06:42:40AM -0500, Bill Cox wrote:
> I don't see how reading from unpredictable RAM locations helps.  Maybe
> my pseudo-code is hiding the loop dependencies.  Here's the actual
> current first loop (I switched from using the word "page" to "block"):
[...]
>     uint32 hash = 1;
[...]
>     for(i = 1; i < c->numBlocks; i++) {
[...]
>         for(j = 0; j < c->blockLength; j++) {
[...]
>             hash = hash*(fromVal | 1) + prevFromVal;

Oh, you're right: you do have the dependency via "hash" across
iterations of the outer loop, so the attacker shouldn't be able to
precompute the multiplies for another iteration of the outer loop
despite of the lookup indices being predictable.

The same applies to scrypt (even if its access pattern were made
predictable), where X stays across loop iterations.  In fact, the same
happens inside scrypt's BlockMix: the next 64-byte block to read is
known, but Salsa20/8 can't be precomputed because of the data dependency.

I think the same applies to Catena, too (although I have yet to take a
look at the new Catena).  I was wrong about it depending on only one of
three constraints, then.  It depends on two of three.  Not that bad.

Alexander