Re: [TLS] I-D Action: draft-ietf-tls-curve25519-00.txt

Simon Josefsson <simon@josefsson.org> Mon, 15 June 2015 21:06 UTC

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From: Simon Josefsson <simon@josefsson.org>
To: Martin Thomson <martin.thomson@gmail.com>
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Date: Mon, 15 Jun 2015 23:06:07 +0200
In-Reply-To: <CABkgnnWk=3r9pix_5CZGRyTyx4q=F5GHb8-pktaVQ0huUd1u0g@mail.gmail.com> (Martin Thomson's message of "Mon, 15 Jun 2015 09:59:35 -0700")
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Subject: Re: [TLS] I-D Action: draft-ietf-tls-curve25519-00.txt
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Martin Thomson <martin.thomson@gmail.com> writes:

> On 15 June 2015 at 06:47, Eric Rescorla <ekr@rtfm.com> wrote:
>> In the latter case, implementors of this draft would just advertise
>> Curve25519_kex
>> and future implementors of the signature draft would  advertise
>> Curve25519_sig.
>> Note that we don't have to decide this questions now, since the issue does
>> not yet
>> arise. We can just state that advertising Curve25519 doesn't mean that you
>> should do ECDSA with it.
>
> And I'd be happy with this outcome too.  That is, we might decide
> later to reuse the code point.
>
> Of course, I'd still argue not to do the overload, based on the
> potential for implementations to support one and not t'other.

I agree.  I changed the document to say:

  Servers MUST NOT select an ECDSA certificate if there are no common
  curves suitable for ECDSA.

https://gitlab.com/jas/ietf-tls-newcurves/commit/e9bf1cbfe136d5b53cc9e52681b0a98ccaabcb44#diff-1

> On 15 June 2015 at 06:03, Viktor Dukhovni <ietf-dane@dukhovni.org> wrote:
>> It seems that provided there's also a named_curve for ECDSA
>> that matches the certificate, then one might use 25519 for a key
>> exchange that is signed with ECDSA.
>
> Perhaps I wasn't clear about this.
>
> I'm saying that we need one named_curve for 25519 + ECDH and another
> for Ec25519 + "ECDSA".

I am currently thinking the same.  That's what
draft-josefsson-tls-eddsa2 does.

/Simon