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

Adam Langley <> Thu, 31 December 2015 17:43 UTC

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From: Adam Langley <>
To: Brian Smith <>
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Cc: Karthikeyan Bhargavan <>, "<>" <>
Subject: Re: [TLS] draft-ietf-tls-curve25519-01: Is public key validation necessary or helpful?
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On Wed, Dec 30, 2015 at 7:40 PM, Brian Smith <> 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.

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.



Adam Langley