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

Adam Langley <agl@imperialviolet.org> Thu, 31 December 2015 03:30 UTC

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From: Adam Langley <agl@imperialviolet.org>
To: Brian Smith <brian@briansmith.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 Wed, Dec 30, 2015 at 4:03 PM, Brian Smith <brian@briansmith.org> wrote:

> I think it is a good idea to implement the session hash extension, in
> general. However, I think it is a bad idea to prescribe it as the solution
> for this particular problem because:
>
> 1. draft-irtf-cfrg-curves-11, in sections 6.1 and section 6.2 already
> require the check for a non-zero result, and that check is sufficient.

As discussed on the CFRG list, the plan is that the final curves RFC
will say that the zero check is a MAY. (See
https://www.ietf.org/mail-archive/web/cfrg/current/msg07611.html)

I don't mind if the integration of curve25519 in TLS requires a
zero-check or not, but what property are people hoping to gain? If one
wants to avoid triple-handshake like issues then session-hash is the
answer. A client can use an RSA key exchange to control the PMS
completely, of course, and, with finite-field DH, a value of zero or
p-1 will usually allow the same.


Cheers

AGL