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Date: Thu, 24 Mar 2016 17:34:56 +0300
From: Solar Designer <>
Subject: Re: [PHC] hash encryption

On Thu, Mar 24, 2016 at 03:29:34PM +0300, Solar Designer wrote:
> I am considering hashing in round+keylen.

No, this does not fully do the trick, and is unnecessarily complex to
reason about.

> Another detail is that 6 least significant bits of keylen don't really
> need to be encoded, but this detail doesn't appear to be of any use,
> unless we were to encode (round + (keylen >> 6)) and declare keylen's
> needing more than 14 bits unsupported.

This isn't exactly right.  It would need to be "number of SHA-256
blocks" instead of just "keylen >> 6".  This gets too complex.

So not wanting to double the SHA-256 block count for keylen=32 and not
wanting to introduce too much complexity (to code or/and reasoning), we
seem to be limited to either relying on the trick I suggested earlier
(hash in the round number last) or providing a mitigation only for
keylen up to 2^24 (if we use aligned 32-bit words only) or up to 2^48
(if unaligned, fully using space up to the max of 55 bytes).


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