phc-discussions - scrypt like hash, but with AES and HMAC-SHA-2

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Date: Wed, 7 Aug 2013 13:07:54 +0200
From: CodesInChaos <codesinchaos@...il.com>
To: discussions@...sword-hashing.net
Subject: scrypt like hash, but with AES and HMAC-SHA-2

An hash I designed a couple of months ago:

* Time/memory trade-offs are harmless as long as Time*Memory can't be
decreased
* Time/parallelism trade-offs are bad once parallelism exceeds defender's
maximum parallelism
* The basic construction is similar to scrypt, but the primitives are
different
* Reuse existing schemes, like AES-CBC, SHA-256, HMAC and HKDF
* AES is fast is software when AES-NI is present
* ~128 bit security is enough, you typically only apply stretching to low
entropy inputs like passwords. But hashing the original key together with
the output of the strengthening operation avoids reducing entropy to 128
bits for strong passwords.
* Well defined character encoding: UTF-8
* When used as a KDF large outputs doesn't cause multiple invocations of
the expensive part (unlike PBKDF2)
* Like in scrypt, only the query phase is supposed to be memory-hard. The
expansion phase suffers from the same attacks as scrypt's expansion
* Random access to elements could enable cache timing attacks, but I don't
think those are a big issue in practice, especially with unknown salts. But
all schemes with predictable memory access I know are much weaker than
scrypt.
* I'm not happy with how it treats parallelism and the attackable first
phase isn't nice either. I hope to improve on those in a future version.

https://gist.github.com/CodesInChaos/6173041

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