Re: [Cfrg] Point validation in TLS 1.3

Peter Gutmann <pgut001@cs.auckland.ac.nz> Wed, 30 November 2016 03:07 UTC

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From: Peter Gutmann <pgut001@cs.auckland.ac.nz>
To: Eric Rescorla <ekr@rtfm.com>, cfrg <cfrg@irtf.org>
Thread-Topic: [Cfrg] Point validation in TLS 1.3
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Date: Wed, 30 Nov 2016 03:06:55 +0000
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Subject: Re: [Cfrg] Point validation in TLS 1.3
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Eric Rescorla <ekr@rtfm.com> writes:

>Matt Green has submitted a pull request to require validation for ECDHE (TLS
>1.3 already requires it for FFDHE). We wanted to make sure the CFRG was aware
>of this and see if there were objections.

So this runs into the same issue I've run into in -LTS, which means it's a
convenient time to bring it up: What is it that the implementer reading the
spec needs to verify?  In other words during a TLS handshake, what data
exchanged in the handshake must be checked with:

  This process consists of three steps: (1) verify that Y is not the point at
  infinity (O), (2) verify that for Y = (x, y) both integers are in the
  correct interval, (3) ensure that (x, y) is a correct solution to the
  elliptic curve equation.

Peter.