phc-discussions - Re: [PHC] hash encryption

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Message-ID: <CALCETrWbbUk7y92oYQ0Ju5vvAKv1fUKZe+94GhRWP_DGAkhHkQ@mail.gmail.com>
Date: Thu, 24 Mar 2016 10:05:15 -0700
From: Andy Lutomirski <luto@...capital.net>
To: discussions <discussions@...sword-hashing.net>
Subject: Re: [PHC] hash encryption

On Thu, Mar 24, 2016 at 7:34 AM, Solar Designer <solar@...nwall.com> wrote:
> 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).

As a totally different option, you could hash the key and then plug
the key's hash into the round function hash.

--Andy

>
> Alexander



-- 
Andy Lutomirski
AMA Capital Management, LLC