phc-discussions - Re: [PHC] Pretty cool: first sequential memory hard KDF secure against timing attacks

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Date: Wed, 08 Jan 2014 19:33:37 -0800 (PST)
From: "Stephen Touset" <stephen@...set.org>
To: discussions@...sword-hashing.net
Cc: discussions@...sword-hashing.net
Subject: Re: [PHC] Pretty cool: first sequential memory hard KDF secure
 against timing attacks

To be fair, I would argue that my blakerypt proposal and implementation were first. Order of memory access is based on a unique "session key" and not the password. :)


Still, this is an excellent result if it holds up to scrutiny!
-- 
Stephen Touset
stephen@...set.org

On Wed, Jan 8, 2014 at 3:35 PM, Bill Cox <waywardgeek@...il.com> wrote:

> If I'm not mistaken, the "cheat killer round" combined with "smoke" that I
> mentioned this morning, turns Catena (and also NeolKDF) into a sequential
> memory hard algorithm while avoiding timing attacks.
> Even better, Catena-N has to compute N rows of data, but only two are
> required to be in memory at any time.  The cheat killer round forces an
> attacker to prove he still remembers a significant portion of all of the
> computed values.  With a password-derived but smoke-randomized set of
> memory location access, is there any timing leak?  The sequential memory
> hardness is a simple matter to prove, as it becomes similar to scrypt in
> the way it accesses memory in a second pass.  The only issue how much
> memory do we really need to access?
> NoelKDF minimizes the needed memory accesses in the cheat killer round,
> because each memory access that is not saved by an attacker trying to use
> less than half the memory results in a sub-DAG of values requiring
> recomputation that is a significant fraction of the size of the whole DAG
> (random DAG with degree 2 and < 1/2 of nodes covered results in a big
> recomputation DAG).  1000 smoke covered memory accesses should do the job.
> Any weakness here?
> Bill
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