Re: [openpgp] Ed25519 and digest choices (issue 31)

Daniel Kahn Gillmor <dkg@fifthhorseman.net> Wed, 26 May 2021 15:56 UTC

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From: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
To: Daniel Huigens <d.huigens@protonmail.com>
Cc: openpgp@ietf.org
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Date: Wed, 26 May 2021 11:55:52 -0400
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Subject: Re: [openpgp] Ed25519 and digest choices (issue 31)
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Hi Daniel--

On Tue 2021-05-25 10:59:39 +0000, Daniel Huigens wrote:
> This is the function that the specification should (and I believe
> tries to) reference. The OpenPGP specification indeed pre-hashes the
> message as well, but this is irrelevant to (and comes before) RFC8032,
> PH(x) is still x. There should be no need to reference PureEdDSA.

https://datatracker.ietf.org/doc/html/rfc8032#section-4 says:

   This document specifies parameters resulting in the HashEdDSA
   variants Ed25519ph and Ed448ph and the PureEdDSA variants Ed25519 and
   Ed448.

So the non-prehash variant named Ed25519 is indeed "PureEdDSA" -- but
it's being applied to something other than the user-visible "message" in
the OpenPGP context.  Maybe we don't need to reference PureEdDSA
explicitly, but the fact that "Ed25519" is often used as a shorthand for
the whole family of EdDSA mechanisms over curve 25519 introduces some
amount of ambiguity in the spec as it's currently written.

I welcome suggestions for text that clarifies this subtlety in the
draft.

        --dkg