phc-discussions - Re: [PHC] Lyra, Password Key Derivation Based On The Sponge Construction

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Date: Tue, 7 Jan 2014 09:24:18 -0500
From: Bill Cox <waywardgeek@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Lyra, Password Key Derivation Based On The Sponge Construction

Thanks for this very interesting link.  Lyra first fills a matrix with hash
data which is derived from the password, and then randomly picks a "row"
and for each location it updates the hash state from the location's value,
and then XORs into the location the next output of the hashing engine.

I only skimmed the slides, which mention a proof of exponential runtime
with no memory, while scrypt is O(N^2).  Catena and Lyra both pick
particular random-ish DAG architectures in order to prove security.  That's
really awesome, but I think that a random DAG has better security than
either, especially since both Lyra and Catena DAGs appear as sub-DAGs of
random DAGs in fairly large sizes.

The point of Lyra may be the proof.  If so, it's a good piece of work.  If
the author would like to enter the competition, he may want to "borrow"
some good ideas from other entries.  Catena's anti-timing attack stuff is
very cool, and all Lyra has to do is pick a random row that does not depend
on the password.  For decent security, Lyra needs it's outer time loop to
run more than once, so that a large majority of rows have a good chance of
impacting the values in other rows, and it does one read/write and a hash
per memory location per loop.  So, it's not as fast as scrypt, escrypt,
which do the job with one read/write and two hashes per location, or
NoelKDF, which does it with one-ish read/write and one hash per location.
 I say "one-ish" because I cheat, knowing that if I read V(i), then V(i+1)
is most likely already cached, so I added V(i+1) into the hash.  He could
also hash only the first item in each row into his hash state, allowing
user-configurable parallelism.

Bill

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