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Date: Mon, 30 Mar 2015 17:16:48 +0300
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Tradeoff cryptanalysis of Catena, Lyra2, and generic memory-hard functions

Hi Dmitry,

Thank you for commenting on this so promptly.

On Mon, Mar 30, 2015 at 02:58:45PM +0200, Dmitry Khovratovich wrote:
> you're certainly right, the stored indices must be taken into account
> when counting memory for tradeoffs. For the default parameters of
> Catena and Lyra2 that we considered, these indices occupy less than
> 1/1000 of total memory, and we just (explicitly or implicitly) ignored
> this small addend in the computation.

OK, this feels realistic, and I hope your estimate is correct.

> The same happened to other
> "service" information that we use in the attacks (such as storing the
> access complexities, which accounted to 1/6000 of memory, cf. page 9
> of the report).

Thank you for mentioning this detail in the report.

> Indeed, smaller blocks would increase the proportion of indices in the
> memory. To have them occupy, say, 1/10 of memory, let us consider 1 GB
> memory size. 100 MB of 3-byte indices would imply 2^{25} blocks, i.e.

(Ignoring the 24 vs. 25 minor "rounding" discrepancy for the purpose of
this discussion.)

This would imply we could refer to a total of 2^{25} blocks and other
data structures (we'd need such indirect references for depth > 1
attacks), including possible duplicates (same indices), in whatever data
structure we'd use to mount the TMTO attack.

That's not the total number of blocks there is.

A question is then what TMTO ratio we could achieve while not needing
more than 2^{25} indices.

> 32-byte blocks.

I think not, for the reasons above.

We wouldn't be storing each block's index exactly once.  I guess we'd
need to store roughly depth (e.g., 26.6 in the example I referred to
before) indices per (pre-TMTO or post-TMTO?) block (including many
duplicate index values, but this doesn't help us much if at all).

Suppose we have an original memory size of 1 GB, a post-TMTO
without-indices size of 100 MB (the 1/10 ratio corresponding to depth
26.6), and we spend another 100 MB on indices (bringing the actual TMTO
ratio to only 1/5, so a significantly worse attack than we had estimated
without considering the indices).  What block size would achieve this?
We can fit 100 MB / (26.6*3) = ~1.25 million sets of 26.6 indices in our
100 MB allotted to them.  If the 26.6*3 = ~80 bytes are needed per
pre-TMTO block, then a block size of 1 GB / 1.25 million = ~800 bytes
achieves this.

I made many guesses and assumptions here.  These might be about right or
they might be way off.  I think this needs to be researched for real.

> However, on the x86 architecture random access to
> 32-byte blocks  would increase latency dramatically (my estimate would
> be 3-5x at least), and make the scheme much slower and thus weaker.

I agree there's a tradeoff here.

My point is that for some of the larger TMTO ratios in your report, we
might not have to go to a block size as low as this in order to
significantly affect the effective TMTO ratio.

> On the architectures where the memory latency is only  a few CPU
> cycles (AVR?), smaller blocks can be useful.

Right.  This would apply in cases where TMTO attacks would otherwise be
relevant to most frequently accessed portions of data kept in a device's
local memory even in an overall large-memory scheme (using external
memory for most of its data), to fit more instances per device.

Thanks again,

Alexander

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