phc-discussions - Re: [PHC] hash encryption

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Date: Wed, 23 Mar 2016 04:38:18 +0000
From: Samuel Neves <sneves@....uc.pt>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] hash encryption

On 23/03/2016 03:41, Solar Designer wrote:
> Hi,
>
> I am planning to release yescrypt 1.0 later this year.  One planned
> feature that stayed on my to-do list so far is builtin support for
> (re-)encryption of hashes.
>
> One way to do it would be to use a block cipher like AES.  However,
> yescrypt does not normally use any block cipher, and bringing in an
> extra crypto primitive just for this one feature is not great.
>
> Another way would be to reuse the salt as IV when reusing an already
> available hash function as a stream cipher.  However, this increases
> reliance on salts being unique.
>
> Yet another way would be to create a block cipher out of a hash, using
> the Luby-Rackoff construction.  This is my current preference.
>
> Attached is an implementation of a 256-bit block cipher based on
> SHA-256 (which yescrypt has anyway).  I'd appreciate a review.

Underlying the SHA-256 compression function there is already a block cipher, often known as SHACAL-2. It is used in
Davies-Meyer mode (SHACAL-2(key, block) + block) to turn it into a compression function. Turning SHA-256 into a Feistel
block cipher seems like overkill, but obviously works.

Alternatively, sticking with primitives already in (ye)scrypt, one can reuse the underlying permutation of Salsa20 to
turn it into an Even-Mansour block cipher---P(block xor key) xor key.