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Date: Sun, 2 Jul 2017 22:30:43 +0200
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: [PHC] Rig team "shows that Argon2i fails to maintain the claimed memory hardness"

Hi,

I think this should be in here:

https://eprint.iacr.org/2017/603

"Cryptanalytic Time-Memory Tradeoff for Password Hashing Schemes

Donghoon Chang and Arpan Jati and Sweta Mishra and Somitra Kumar Sanadhya

Abstract: A cryptanalytic technique known as time-memory tradeoff (TMTO)
was proposed by Hellman for finding the secret key of a block cipher.
This technique allows sharing the effort of key search between the two
extremes of exhaustively enumerating all keys versus listing all
possible ciphertext mappings produced by a given plaintext (i.e. table
lookups).  The TMTO technique has also been used as an effective
cryptanalytic approach for password hashing schemes (PHS).  Increasing
threat of password leakage from compromised password hashes demands a
resource consuming algorithm to prevent the precomputation of the
password hashes.  A class of password hashing designs provide such a
defense against TMTO attack by ensuring that any reduction in the memory
leads to exponential increase in runtime.  These are called
\textit{Memory hard} designs.  However, it is generally difficult to
evaluate the ``memory hardness" of a given PHS design.  In this work,
we present a simple technique to analyze TMTO for any password hashing
schemes which can be represented as a directed acyclic graph (DAG).  The
nodes of the DAG correspond to the storage required by the algorithm and
the edges correspond to the flow of the execution.  Our proposed
technique provides expected run-times at varied levels of available
storage for the DAG.  Although our technique is generic, we show its
efficacy by applying it on three designs from the ``Password Hashing
Competition" (PHC) - Argon2i (the PHC winner), Catena and Rig.  Our
analysis shows that Argon2i fails to maintain the claimed memory
hardness.  In a recent work Corrigan-Gibbs et al. indeed showed an attack
highlighting the weak memory hardening of Argon2i.  We also analyze these
PHS for performance under various settings of time and memory
complexities."

I only skimmed the paper so far.  It looks like Argon2i 1.3 does show
prohibitively large recomputation penalties, but perhaps less than was
claimed by Argon team?  I think the paper would benefit from citing the
specific claims that it says are not met.  Also, unlike Argon team's own
approach, here we don't see the separation between computation and
depth penalties.  It would be preferable to see both metrics.

Per the paper, Rig shows much higher recomputation penalty than all
Argon2i and Catena versions and settings tested.

The paper also shows Rig as the fastest on the authors' machine (IIRC,
the same as they used before).  This is great (for Rig) and perhaps
mostly right, but there are two caveats: the machine has 50% higher
memory bandwidth than is typical for that CPU, and this is
single-threaded performance (if I get it right).  In practice, users of
these schemes will probably either use multi-threaded operation (when
deriving crypto keys at large memory usage) or have multiple instances
run in parallel (in the password hashing use case, and at lower memory
per instance than can be seen in the comparison chart).  I think we
discussed this before, and there were multi-threaded and parallel
instances zoom-in results posted as well.  I don't recall how well Rig
fared per those - I'd expect it to appear very good, but not nearly that
good.  OTOH, the paper is generous in how it benchmarks scrypt, using an
incompatible faster revision (not just a faster implementation).

Overall, it's very good to see continued research in this area!

Alexander

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