[tor-dev] SHA-3 isn't looking so hot to me (was: Draft sketch document with ideas for future crypto ops)

Zooko O'Whielacronx zooko at zooko.com
Tue Nov 1 17:46:07 UTC 2011

On Tue, Nov 1, 2011 at 9:30 AM, Marsh Ray <marsh at extendedsubset.com> wrote:
> I too have been following the development of SHA-3 and will toss in my 2c here.

Hi Marsh! You made several good points, a few of which I quoted below.
Your points make me think, speaking loosely, that SHA-3 will turn out
to be the best function to use due to reasons of popularity or
bureaucratic fiat. Perhaps someday the chips you buy will come with
SHA-3 circuits built-in but not SHA-2, and perhaps people will start
looking at you funny if you use SHA-2 when everyone else is using
SHA-3. These may be good reasons—perhaps better reasons that the
moderate performance issues I previously mentioned or the completely
indefensible vague unease I've expressed about SHA-3 being too new.

You did make one small technical mistake though:

> SHA-3 is also being developed with attention to the amount of circuitry
> ("die area") needed to implement it in hardware. So it's possible that
> hardware acceleration will appear for SHA-3 sooner/instead of SHA-2.

Although the SHA-3 designers have indeed tried to optimize for that, I
think SHA-256 is actually still better. See Fig. 17 of
http://eprint.iacr.org/2009/510.pdf .

Below my signature is just me quoting a few of the points you made. :-)



> Agreed, SHA-3 will fix some problems. Some of these things we've been
> working around so long that they seem normal.
> There's sometimes also a benefit of being with the current NIST
> recommendation. I suspect more users will migrate off of SHA-1 to SHA-3 than
> they will to SHA-2.
> NIST may eventually 'deprecate' SHA-2 in favor of SHA-3 due to just the
> length extension issue. Which is not to say that I think there's a real
> problem using SHA-2 correctly, only that you may end up having to explain
> repeatedly why it's not a problem.

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