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Date: Fri, 25 Sep 2015 03:32:06 +0300
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] sub-block memory*depth attacks and countermeasures

On Thu, Sep 24, 2015 at 06:44:40PM +0300, Solar Designer wrote:
> My current understanding is that scrypt's even/odd shuffling and simple
> reverse order of sub-blocks written (vs. read) achieve a similar
> (limited) effect at increasing the recomputation depth (counted at
> sub-block operations level) for a given ratio of sub-blocks stored
> (assuming attacker's optimal choice of which sub-blocks to store).
> 
> It appears optimal for an attacker to store the first few sub-blocks
> that would be needed should the block happen to be referenced later, as
> well as a sparse selection of further sub-blocks,

Specifically, it appears that to fully avoid the depth increase from the
hashing scheme's use of sub-block shuffling (as compared to non-shuffled,
non-reversed sub-blocks order), around 2*sqrt(n) sub-blocks per
not-fully-stored block would need to be stored, where n is the sub-blocks
per block count (e.g., n=2r for scrypt), for large n.  This includes
sqrt(n) would-be-first-needed sub-blocks and another sqrt(n) sub-blocks
spread over the rest of the block.

For large n, this number is relatively small, meaning that e.g. storing
every other full block (and these sub-blocks for the rest) results in
slightly higher than 1/2 of non-TMTO memory usage.  In other words, a
1/2 TMTO at block level is only slightly higher than 1/2 TMTO overall.

For small n like 16 for (ye)scrypt's recommended r=8, this number is
significant, meaning that a 1/2 TMTO at block level might nevertheless
result in 3/4 overall TMTO (since 2*sqrt(16) = 8, which is half of 16,
meaning that we store half of the sub-blocks in the not-fully-stored
blocks).  However, for small n this formula is significantly inaccurate
(and its accurate replacements might vary by shuffling scheme a bit).
Also, this assumes that the attacker wants to fully avoid the depth
increase, whereas it might be optimal to accept some moderate increase.

(I use "n/m TMTO" to refer to the memory usage ratio only, compared to
non-TMTO.  The corresponding increases in computation and depth, and
thus in time, are likely not proportional to the memory reduction, nor
to each other, and they vary by the hashing scheme and its parameters.
What I am focusing on here is attacking the scheme such that the depth
is kept the same as it would have been for the same block-level TMTO
ratio for a variation of the same scheme without sub-block shuffling.)

> so that the rest of
> the (non-stored) sub-blocks for this block would be recomputed while the
> first few (stored) sub-blocks are being processed (as input for whatever
> new block is being computed).  (As discussed earlier, this is in
> addition to also using a block-level TMTO - that is, some blocks are
> stored in full, and some with some of their sub-blocks missing as
> discussed here.  e.g. every other block may be stored in full, and the
> other half of them with some/most sub-blocks missing, for a slightly
> higher than 1/2 memory usage total.)

Alexander

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