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Date: Fri, 13 Feb 2015 11:36:58 -0800
From: Bill Cox <waywardgeek@...il.com>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] Tradeoff cryptanalysis of Catena, Lyra2, and generic
 memory-hard functions

Interesting analysis.  I actually wrote code yesterday almost the same as
your generic attack.  It's a good algorithm.  I have some comments:

- Yescrypt is the fastest remaining entry in the PHC, not Lyra2.  Lyra2 is
the second fastest.  They are the only 2 entries that are faster than
Scrypt.

What data do you have showing Lyra2 is faster?  In particular, for a given
amount of memory hashed, Lyra2 is always slower than Yescrypt, even on one
thread, and when Yescrypt's multi-threading option is used as intended,
Yescrypt can be several times faster.  Also, while Lyra2 uses one Blake2
reduced round (a good choice, IMO), Yescrypt uses 6 of it's new round
function.  When using Yescrypt in production, I can easily change 1 line
and improve it's performance greatly for single-threaded applications.
Lyra2 is already using only one round, and I will not have that flexibility.

- You're notion of "memory*time" cost in this paper is wrong, leading to
bizaar and wrong conclusions about Lyra2

You claim Lyra2 has a time*memory cost of 0.28 when using a 1/4 memory
TMTO.  Here's what GPU attackers think that means:  they use 1/4 the
memory, and they will only have to do 12% more computations.  Yet you say
the wandering phase for a 1/4 TMTO requires 1876X more computation.  There
is simply no way you found a 0.28X memory*time attack against Lyra2 by any
normal use of the memory*time metric.

What you found is that recomputations can be done in parallel.  While this
parallelism can help an attacker, time is not equal to the computation tree
depth.  This depth is simply the theoretical minimum latency for
recomputing a value, assuming infinite computation resources and power.

Your generic attack is a good algorithm.  I think I can improve it, but I
have not yet tried this.  At some point, we may discover that there are
several rows referring to a previous row R which we keep having to
recompute.  Instead of making all those new rows have high recomputation
cost, leading to several of them being kept in memory, we can decide
instead to store R.  The point where we should do that should not be too
hard to track.

In your generic attack, for TMTO > 1/2, for each new value computed, a
significant percentage of all prior values must be recomputed, leading to a
computation effort proportional to N^2, where N is the number of original
computations without a TMTO.  I like that your Table 6 does not claim
improved memory*time based on tree depth, and instead simply reports both
recomputations and depth.  However, Table 6 is heavily dependent on N,
which is not stated.  For very large N, even the 1/3 TMTO attack will have
very high recomputations.  Maybe state N?

Bill

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