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Re: [TLS] TLS and KCI vulnerable handshakes

Daniel Kahn Gillmor <dkg@fifthhorseman.net> Wed, 12 August 2015 00:19 UTC

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From: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
To: Martin Thomson <martin.thomson@gmail.com>, Clemens Hlauschek <clemens.hlauschek@rise-world.com>
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References: <55C8CD7A.7030309@rise-world.com> <9A043F3CF02CD34C8E74AC1594475C73F4AD80F3@uxcn10-5.UoA.auckland.ac.nz> <9690882F-B794-4D1D-973F-DE7F90120CC3@gmail.com> <CABkgnnXruou6BbgZK8vWUeyb-gW5OTSZKPwPVPwZ826usNz9RA@mail.gmail.com> <20150811190544.GA13734@LK-Perkele-VII> <CABkgnnXnx7Fvt8oXNwMC3oLtXD4A4gFw6j-LWWuSkS3BasWqrg@mail.gmail.com> <55CA8783.8060201@rise-world.com> <CABkgnnX9pGxwPFvPw5VUUst7-DX7fAbPC90bAz1EztxZ7K9KpA@mail.gmail.com>
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Date: Tue, 11 Aug 2015 20:19:30 -0400
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Cc: Karthikeyan Bhargavan <karthik.bhargavan@gmail.com>, "tls@ietf.org" <tls@ietf.org>
Subject: Re: [TLS] TLS and KCI vulnerable handshakes
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On Tue 2015-08-11 19:59:35 -0400, Martin Thomson wrote:
> On 11 August 2015 at 16:38, Clemens Hlauschek <clemens.hlauschek@rise-world.com> wrote:
 [ MT wrote: ]
>>> NSS (incorrectly) adopts the posture that _ECDH_ suites are
>>> half-static: the server share is in the certificate, but the client
>>> side is fully ephemeral.  This is clearly in violation of RFC 5246,
>>> Section 7.4.7 and RFC 4492, Section 3.2. I'm not going to worry about
>>> that right now :)
>>
>> Please elaborate. I do not see how this half-static behaviour
>> constitutes any violations of the mentioned RFCs.
>
> Both the above cited sections say very clearly that for fixed (EC)DH
> cipher suites the client where the client has a fixed (EC)DH
> certificate, the client MUST send an empty ClientKeyExchange.

that's not how i'm reading 5246 ยง7.4.7  -- i see it as saying if the
client has decided to send a fixed (EC)DH cert, then it MUST send an
empty ClientKey Exchange.

I see no requirement that a client MUST send a certificate if it has one
that satisfies the server's CertificateRequest (and i would be strongly
opposed to adding such a requirement -- clients should not be forced to
reveal identity to a server just because of CertificateRequest message
in the handshake).

so i think NSS is doing the Right Thing here too.

   --dkg

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