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

Karthikeyan Bhargavan <karthik.bhargavan@gmail.com> Tue, 29 December 2015 20:10 UTC

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From: Karthikeyan Bhargavan <karthik.bhargavan@gmail.com>
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Date: Tue, 29 Dec 2015 22:10:47 +0200
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To: Brian Smith <brian@briansmith.org>
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Subject: Re: [TLS] draft-ietf-tls-curve25519-01: Is public key validation necessary or helpful?
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As mentioned before, validating Curve25519 public values is necessary in TLS 1.2 without session hash.
Otherwise, as we pointed out in [1], the triple handshake attack returns.

[1] http://www.internetsociety.org/doc/verified-contributive-channel-bindings-compound-authentication <http://www.internetsociety.org/doc/verified-contributive-channel-bindings-compound-authentication>

> On 29 Dec 2015, at 21:05, Brian Smith <brian@briansmith.org> wrote:
> 
> On Tue, Dec 22, 2015 at 2:09 PM, Brian Smith <brian@briansmith.org <mailto:brian@briansmith.org>> wrote:
> If an implementation only implements ECDHE cipher suites then implementing the session hash extension is not necessary, according to RFC 7627. I believe there are also a few other factors that would implementing the session hash extension to be unnecessary.
> 
> If checking that the shared value isn't zero is sufficient, and/or blacklisting the public values that DJB mentions in [1] is sufficient, either would be better than mandating the implementation of the session hash extension just for this purpose.
> 
> Actually, the check for a result of zero is already required in the current CFRG draft; see [1]. So, I think that the easiest way to fix the TLS draft is to just delete the misleading text.
> 
> [1] https://tools.ietf.org/html/draft-irtf-cfrg-curves-11#section-6.1 <https://tools.ietf.org/html/draft-irtf-cfrg-curves-11#section-6.1>
> Cheers,
> Brian
> --
> https://briansmith.org/ <https://briansmith.org/>
> 
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