Date: Tue, 13 Mar 2018 18:22:59 +0100
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: [PHC] yescrypt vs. Argon2

Hi,

I just wrote the below for the yescrypt web page and maybe the included
documentation (later), and I think it's best reviewed by this community.
I really want to make sure I got everything right, so please correct me
if anything looks wrong to you or anything important is missing.

yescrypt's advantages:

* Greater resistance to offline attacks (increasing attacker's cost at
same defender's cost), including because:

  * yescrypt supports optional ROM for protection from use of botnet
    nodes (and other relatively small memory devices)

  * yescrypt has a dependency not only on RAM and maybe ROM, but also on
    fast on-die local memory, which provides bcrypt-like anti-GPU
    properties even at very low per-hash RAM sizes (where Argon2 might
    even lose to bcrypt in terms of GPU attack speed)

  * yescrypt currently has less low-level parallelism within processing
    of a block, yet allows for tuning it later, whereas Argon2 has a
    fixed and currently commonly excessive amount of such parallelism,
    which may be extracted to speed up e.g. GPU attacks through use of
    more computing resources per the same total memory size due to each
    hash computation's memory needs being split between 32 threads[1]
    (yescrypt currently has four 16-byte lanes that can be processed in
    parallel within a 64-byte sub-block before running into a likely
    data dependency for the next sub-block, whereas Argon2 allows for
    parallel processing of eight 128-byte chunks within a 1 KiB block
    with only two synchronization points for the entire block, as well
    as of four 32-byte parts of the 128-byte chunks with only two more
    synchronization points for the entire 1 KiB block)

  * yescrypt uses computation latency hardening based on integer
    multiplication and local memory access speed, which ties its
    per-hash RAMs up for a guaranteed minimum amount of time regardless
    of possibly much higher memory bandwidth on the attacker's hardware,
    whereas Argon2 uses only the multiplications and performs 6 times
    fewer of those sequentially (96 sequential multiplications per 1 KiB
    for yescrypt vs. 16 per 1 KiB for Argon2, providing correspondingly
    different minimum time guarantees)

* yescrypt has or will have extra optional built-in features: hash
  encryption (now), hash upgrade (temporarily removed from 1.0, to be
  re-added later), SCRAM-like client-side computation of challenge
  responses (already part of the algorithm, not yet exposed via the API)

* yescrypt's cryptographic security is provided by SHA-256, HMAC, and
  PBKDF2, which are NIST-approved and time-tested (the rest of
  yescrypt's processing, while most crucial for its offline attack
  resistance properties, provably does not affect its basic
  cryptographic hash properties), whereas Argon2 relies on the newer
  BLAKE2 (either choice is just fine for security, but use of approved
  algorithms may sometimes be required for compliance)

* yescrypt source tree also provides SHA-256, HMAC, PBKDF2, and scrypt,
  which may reduce total complexity if you need any of those anyway

yescrypt's drawbacks:

* yescrypt is more complex, which complicates design analysis and code
  review, and increases the risk of human error

* yescrypt is cache-timing unsafe (like bcrypt, scrypt, and Argon2d, but
  unlike Argon2i)

* yescrypt isn't the PHC winner (Argon2 is), but is merely a "special
  recognition"

* yescrypt isn't available in third-party libraries yet (Argon2 is)

Other observations:

* yescrypt's complexity is related to its current primary target use
  case (mass user authentication) and is relatively small compared to
  the total complexity of the authentication service, so the risk may be
  justified

* Cache-timing safety is unimportant on dedicated servers, is mitigated
  for some use cases and threat models by proper use of salts, and is
  fully achieved in Argon2 only in its 2i flavor and only through
  reduction of resistance to the usual offline attacks compared to the
  2d flavor

* yescrypt's single-threaded memory filling speed on an otherwise idle
  machine and at our currently recommended settings is lower than
  Argon2's, but that's a result of our deliberate tuning (there's a knob
  to change that, but we don't recommend doing so) preventing yescrypt
  from bumping into memory access speed prematurely, and is irrelevant
  for determining server request rate capacity and maximum response
  latency where multiple instances or threads would be run (under that
  scenario, the algorithms deliver similar speeds[2])

* yescrypt has been designed and currently configured to fit the SSE2
  and NEON instruction sets and 128-bit SIMD perfectly, not benefiting
  from AVX's 3-register instructions nor from AVX2's and AVX-512's wider
  SIMD (although it can be reconfigured for wider SIMD later), whereas
  Argon2 significantly benefits from those at least when running fewer
  threads or concurrent instances than are supported by the hardware
  (yet yescrypt's SSE2 code is competitive with Argon2's AVX2 code under
  full server load)

* yescrypt vs. Argon2 benchmarks are further complicated by these two
  schemes having different minimum amount of processing over memory, and
  thus different average memory usage, which needs to be taken into
  account (yescrypt's minimum processing is 4/3 of Argon2's, and its
  average memory usage is 5/8 of peak for yescrypt t=0 vs. 1/2 of peak
  for Argon2 t=1)

[1] https://gitlab.com/omos/argon2-gpu/blob/master/data/kernels/argon2_kernel.cl
[2] https://github.com/P-H-C/phc-winner-argon2/issues/231

Alexander

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