phc-discussions - Re: [PHC] hash encryption

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Date: Thu, 24 Mar 2016 15:47:41 +0300
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] hash encryption

On Thu, Mar 24, 2016 at 03:29:34PM +0300, Solar Designer wrote:
> encoding the full length or full round+keylen as a 64-bit number.

... which has another drawback, besides code complexity: we'd be
exceeding one SHA-256 block with a likely typical key size of 32 bytes.
A maximum of 55 bytes fits in one SHA-256 block (if it's the only one,
so needs padding).  Currently, we have:

16+1+32 = 49

If encoding round+keylen as 64 bits:

16+8+32 = 56

This is one byte too much, resulting in a 2x slowdown.

So maybe we shouldn't encode the full keylen for that reason.  Maybe
encode round+keylen as 32-bit, or use the approach I had suggested (with
round number hashed in last).

Alexander