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Date: Thu, 23 Jan 2014 11:44:57 +0100
From: Christian Forler <christian.forler@...-weimar.de>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Computational comparison of 3 schemes...

On 23.01.2014 00:54, Bill Cox wrote:

> There are very interesting trade-offs.  First, all three algorithms
> fail completely in timing-attack-resistant mode for an attacker using
> 1/4 of memory.  All three required no significant recalculations, and
> an attacker gets to use 1/4 of memory basically for free.  Catena-3
> only uses 1/4 of the memory, because it reuses the row memory, and
> there are 4 rows.  Both attacker and defender benefit from the 1/4th
> memory.

This observation is a little bit unfair. We claim: with (at least) 2^g
memory unites Catena-3 can be computed with (approx.) 4*2^g hash
function calls. It is obvious that you can compute Catena-3 in the same
time when you have 4 times the memory.

> However, with memory timing data, a NoelKDF attacker can probably use 1/8th or
> possibly less of memory, as it relatively easy to pebble.  Escrypt
> would probably still require closer to 1/4th, while I suspect
> attackers wont even bother trying to save even one memory location
> when attacking Catena-3.

This observation complies with my claims.. :-)


> In short, Catena-3 is the natural timing-resistant choice, but it only
> uses 1/4 of the memory of an equivalent sequential-memory-hard KDF.

Your strategy is
1. allocate a huge amount of memory and
2. overwrite the majority of it

The Catena strategy is:
1. allocate a reasonable amount of memory
2. overwrite (all of) it multiple times.


The former strategy implies that the cost for time and memory are alike
since we have S(g) * T(g) = G^2. Let assume that S(g) is not equal
T(g). Then an adversary might be willing to trade some memory units
for some additional CPU time. Thus, an adversary can  exploit linear
optimization techniques to compute a sweet spot.

In the latter approach, we assume that memory is much more expensive
then CPU time, \ie S(g)^3 * T(g) = G^4. Here exchange rate is not fair
anymore. Usually, an adversary will not exchange some memory for a lot
of CPU time.


You have multiple times elaborated that you do not like the latter
approach. Fine by me. :-)

But I do not really understand why the former strategy is superior to
the latter one.


> 1 Put all the initial pebbles down over the first N locations if there
> are N pebbles
> 2 Add all non-fixed, non-in-use pebbles that have no edges pointing at
> them from beyond the current runner pebble position to the "pebble
> group"
>     2.5 If there are no such pebbles, use a heuristic to add the
> "best" pebble to the group.
> 3 When pebbling, take virtual pebbles from the pebble group.  Reduce
> the "num available" pebbles on the group by 1, but alloc a new pebble
> rather than removing any from the group
> 4 When num available reaches 0, delete all the pebbles in the group, and goto 2

Thanks you very much for the evaluation of the three password schemes.
Great job! Have you planed to publish the source code? I might be useful
for the evaluation process of the submitted PHC candidates. :-)


Best regards,
Christian













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