Re: [openpgp] SHA3 algorithm ids.

Peter Gutmann <pgut001@cs.auckland.ac.nz> Tue, 11 August 2015 16:39 UTC

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From: Peter Gutmann <pgut001@cs.auckland.ac.nz>
To: Daniel Kahn Gillmor <dkg@fifthhorseman.net>, Phillip Hallam-Baker <phill@hallambaker.com>, Derek Atkins <derek@ihtfp.com>
Thread-Topic: [openpgp] SHA3 algorithm ids.
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Date: Tue, 11 Aug 2015 16:39:21 +0000
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Cc: IETF OpenPGP <openpgp@ietf.org>, ianG <iang@iang.org>
Subject: Re: [openpgp] SHA3 algorithm ids.
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Daniel Kahn Gillmor <dkg@fifthhorseman.net>; writes:

>Is your concern CPU time or bandwidth (network/storage) or something else?

Yes.

>If it's CPU time: on some architectures SHA-512 implementations are faster
>than SHA-256 implementations (except for digests of very short messages):

A huge number of devices, and in particular ones with less CPU power, are
still 32-bit, and will remain so for a long time, probably more or less
indefinitely.

In addition from the original post it was unclear whether -512 referred to
SHA2 or SHA3 (which is why I qualified my reply as SHA2-256 and SHA3-512),
SHA3 will be ever worse than SHA2-512 in speed terms.

In terms of network/storage, there's the unnecessary use of 64-byte hashes
that I've already mentioned.  For TLS and SSH you really don't need more than
maybe 64 bits (not bytes), and certainly 64 bytes is nothing more than a
pointless waste of space when you're sending lots of small data quantities
back and forth.

Peter.