Re: [TLS] draft-ietf-tls-curve25519-01: Is public key validation necessary or helpful?

Eric Rescorla <ekr@rtfm.com> Thu, 31 December 2015 17:51 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Thu, 31 Dec 2015 09:50:37 -0800
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To: Adam Langley <agl@imperialviolet.org>
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Cc: Karthikeyan Bhargavan <karthik.bhargavan@gmail.com>, "<tls@ietf.org>" <tls@ietf.org>
Subject: Re: [TLS] draft-ietf-tls-curve25519-01: Is public key validation necessary or helpful?
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On Thu, Dec 31, 2015 at 9:43 AM, Adam Langley <agl@imperialviolet.org>
wrote:

> On Wed, Dec 30, 2015 at 7:40 PM, Brian Smith <brian@briansmith.org> wrote:
> > When you say "the plan," whose plan are you referring to? If you read
> that
> > whole thread, there was a lot of well-founded opposition to that plan.
> And,
> > that plan was never carried out. That is plain to see, as there was
> never a
> > draft submitted with such a change.
>
> I'm no expert on IETF processes but the draft was already in a late
> stage at the time when that came up and I think it was last the point
> for a -12? The revision that's currently in AUTH48 with the RFC Editor
> contains the change from MUST to MAY.
>
> > Not if the implementation doesn't implement RSA or finite-field DH.
>
> I think ekr's post, just previous, reflects my understanding here. For
> the vast majority of implementations, session-hash is needed because
> non-contributory key-exchange mechanisms will be included. If you do
> happen to have an implementation that only implements ECDHE with
> cofactor 1 curves or X25519 or X448, then I guess you could get away
> without implementing session hash, but I'd still implement session
> hash anyway.
>

FWIW, I don't believe that it's significantly harder to implement TLS w/
session
hash than TLS w/o session hash. You have to do the extension but since
you can just fail any handshake in which it's not offered, this isn't that
bad.
What's expensive is supporting *both* session hash and non session hash
(at least that's what we found in NSS).


Having said that, I think I'd be fine with a TLS draft that said that
> the zero check should (or must) be done because I think that it should
> be done in general.
>

As would I.

-Ekr


>
> Cheers
>
> AGL
>
> --
> Adam Langley agl@imperialviolet.org https://www.imperialviolet.org
>
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