Re: [TLS] A la carte handshake negotiation

Dave Garrett <davemgarrett@gmail.com> Sun, 14 June 2015 01:15 UTC

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From: Dave Garrett <davemgarrett@gmail.com>
To: Eric Rescorla <ekr@rtfm.com>
Date: Sat, 13 Jun 2015 21:15:35 -0400
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Subject: Re: [TLS] A la carte handshake negotiation
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On Saturday, June 13, 2015 08:54:10 pm Eric Rescorla wrote:
> On Sat, Jun 13, 2015 at 8:44 PM, Dave Garrett <davemgarrett@gmail.com>;
> wrote:
> > PSK suites could be replaced with a PSK SignatureAlgorithm codepoint in
> > “signature_algorithms” extension. (this was suggested by someone at some
> > point on this list, but I don't remember where that discussion was, offhand
> 
> I don't see how this is going to work. All of the PSK cipher suites use the
> PSK as a source of keying material

A client proposing the PSK SignatureAlgorithm codepoint and a PSK identity would be offering to negotiate PSK using that key.

Anon would be proposed the same way, just with a null PSK identity. All security comes from the FS from (EC)DHE. (just like (EC)DH_anon)

The server could simply state acceptance of negotiation by echoing back the same PSK extension.

> > With the above, ECDHE_ECDSA becomes the one-true-prefix.
> 
> Maybe I'm misunderstanding what you're saying here, but I don't understand
> this point. Certainly I don't see any significant support whatsoever for
> deprecating RSA signatures, and given that we just standardized FFDHE, I don't see much
> evidence of consensus for deprecating DHE either.

This is all in the context of the proposal discussed in this thread. RSA signatures would be negotiated using the "signatures_algorithms" extension. DHE would be negotiated via the "named_groups" extension. (pick an FFDH group for FFDHE or an ECDH curve for ECDHE) The point is to negotiate any of the following via those two extensions and the ECDHE_ECDSA prefix:
DHE, ECDHE, RSA, DSS/DSA, ECDSA (or EdDSA), PSK (or anon)

This would give us fewer suites with no new extensions beyond those already proposed. Just need to use the already proposed extension additions and add a codepoint for PSK/anon.


Dave