[tor-dev] SHA-3 isn't looking so hot to me (was: Draft sketch document with ideas for future crypto ops)

Watson Ladd watsonbladd at gmail.com
Tue Nov 1 11:50:11 UTC 2011

Turns out that almost everything you said about SHA3 vs SHA256 performance
is wrong:
Blake256 performs better except on the Cortex A. On the ARM v6 it
outperforms SHA256. This includes
the ppc32, hardly anyones idea of a server powerhouse.
Furthermore, crypto efficiency is less likely to be a bottleneck on a
client then a node: server architectures matter
much more because we do a lot more crypto on them. (This isn't true for
each connection but servers handle more
connections then clients.)

Secondly SHA256 is already weaker then an ideal hash function. Joux's
multicollision attack works on all Merkel-Damgard constructions, and gives
multicollisions faster then is possible for an ideal hash. Length extension
attacks make HMAC use 2 hashes instead of 1, something that any
speed comparison should remember. (HMAC is a bad idea anyway: quadratic
security bounds are not the best possible, we have to use nonces anyway to
prevent replay attacks, so Wegman-Carter is a better idea for better
in{faster, more secure}. GCM would be an example of this.)

As a KDF none of this really matters, and for signatures collision
resistance is still the most important thing. But sometimes we do depend on
random oracle assumptions in proofs, and SHA3 is designed to be a better
approximation to a random oracle then SHA2.

Watson Ladd

"Those who would give up Essential Liberty to purchase a little Temporary
Safety deserve neither  Liberty nor Safety."
-- Benjamin Franklin
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