Re: [TLS] A la carte handshake negotiation

Dave Garrett <davemgarrett@gmail.com> Sat, 13 June 2015 19:47 UTC

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From: Dave Garrett <davemgarrett@gmail.com>
To: David Benjamin <davidben@chromium.org>
Date: Sat, 13 Jun 2015 15:47:29 -0400
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Subject: Re: [TLS] A la carte handshake negotiation
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On Saturday, June 13, 2015 03:19:20 pm David Benjamin wrote:
> On Sat, Jun 13, 2015 at 2:59 PM Dave Garrett <davemgarrett@gmail.com>; wrote:
> > Yes, for TLS 1.3 negotiation, offering the ECDHE_ECDSA would be required
> > under this proposal.
> 
> Perhaps I'm misunderstanding your goals here then. My read of (4) above was
> that this was intended to use the ECDHE_ECDSA representative of, say,
> AES_128_GCM as the spelling of AES_128_GCM (so effectively repurposing the
> cipher list as the list of AEADs[*]), and that this wouldn't actually
> translate to offering ECDHE_ECDSA due to that really being negotiated by
> the other two extensions.
> 
> But this doesn't seem to be completely the case because the ClientHello is
> interpreted by both TLS 1.3 servers and existing servers. And so we need to
> consider the effects on both to avoid interop. issues.

This proposal could be extended to allow any cipher to negotiate any handshake based on the extensions, but that leaves us with a din of cipher suites. TLS 1.3 implementations would have to allow:

negotiate DHE/ECDHE + RSA/DSS/ECDSA from all suites with any of the following:
DHE_RSA, DHE_DSS, DHE_ECDSA, ECDHE_RSA, ECDHE_DSS, ECDHE_ECDSA (some of these combos don't exist, though)

negotiate DHE/ECDHE + PSK from all suites with any of the following:
DHE_PSK, ECDHE_PSK

negotiate DHE/ECDHE +anon from all suites with any of the following:
DH_anon, ECDH_anon (or named as: DHE_anon, ECDHE_anon)

We could do this, but it's more to keep track of.

By restricting TLS 1.3 suites to just those prefixed with ECDHE_ECDSA/PSK/anon, this means all others can be completely ignored for TLS 1.3+ and DHE suites can be deprecated in general.

DHE weak param negotiation is a problem, but negotiation via the extension uses strong groups. Requiring ECDHE prefixed suites also prevents negotiation of DHE with weak params with older implementations, as it's not even offered.

> [*] As you say below, not quite because you still premultiply the really
> exotic things like PSK and anon. But setting those aside for the sake of
> discussion.

I set them aside for the sake of discussion for the first draft of this proposal and most of the discussion ended up on being how to deal with them. They complicate things, but they need to be taken into account from the start regardless of which path we pick.


Dave