Re: [TLS] Refactoring the negotiation syntax

Ilari Liusvaara <ilariliusvaara@welho.com> Mon, 18 July 2016 16:19 UTC

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Date: Mon, 18 Jul 2016 19:19:36 +0300
From: Ilari Liusvaara <ilariliusvaara@welho.com>
To: Eric Rescorla <ekr@rtfm.com>
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Subject: Re: [TLS] Refactoring the negotiation syntax
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On Mon, Jul 18, 2016 at 06:03:08PM +0200, Eric Rescorla wrote:
> On Mon, Jul 18, 2016 at 5:07 PM, Ilari Liusvaara <ilariliusvaara@welho.com>;
> wrote:
> 
> > I think the end result was insane. The problem that makes it so difficult
> > is that legacy signature types, group types and protection/PRFs interact,
> > so not all supported offers that server has enabled are actually possible
> > to chose.
> >
> 
> Can you say more about this?

I was referring to the scheme in TLS 1.2 and 1.3-14.

The problem is, that even if X is "locally" possible (enabled on both sides,
the algorithm and its "class" is advertised) does not guarantee that X is
"globally" possible (there is actual configuration using X that meets all
TLS constraints and algorithm enables).

> I spent some time mocking up an implementation locally and it was pretty
> clean.
> 
> Perhaps what made it easier is that I did:
> 
> 1. Define new cipher suites that were totally agnostic about the key
> exchange and signature.
> 
> 2. Assume everything was orthogonal except for the PSK/signature/key
> exchange interaction...

Both should eliminate the really nasty parts, as it would mean that
unless server wanted to, any X is "locally" valid is also "globally"
valid. Modulo key exchange types, but those aren't that bad usually.


-Ilari