Re: [TLS] Version negotiation, take two

Hubert Kario <hkario@redhat.com> Wed, 14 September 2016 18:02 UTC

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From: Hubert Kario <hkario@redhat.com>
To: Andrei Popov <Andrei.Popov@microsoft.com>
Date: Wed, 14 Sep 2016 20:02:31 +0200
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Subject: Re: [TLS] Version negotiation, take two
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On Wednesday, 14 September 2016 17:39:59 CEST Andrei Popov wrote:
> Do you mean a TLS extension code point per TLS version?

yes, e.g. if extension 0x00a0 is present assume TLSv1.3 support, 0x0121, 
TLSv1.4; same way EMS and MtE works

> One argument against this was that this makes it difficult to express the
> client's prioritization of TLS versions, but IMHO arguably the server
> should not care.

I don't think we should depart from the "highest mutually supported version" 
negotiation algorithm, any kind of preference in the client list is likely to 
cause additional problems - version misnegotiation

if client will want to advertise support for TLSv1.3 and TLSv1.5 but not 
TLSv1.4 it will still be able to do that
 
> -----Original Message-----
> From: TLS [mailto:tls-bounces@ietf.org] On Behalf Of Hubert Kario
> Sent: Wednesday, September 14, 2016 9:40 AM
> To: David Benjamin <davidben@chromium.org>;
> Cc: tls@ietf.org
> Subject: Re: [TLS] Version negotiation, take two
> 
> On Wednesday, 14 September 2016 16:17:50 CEST David Benjamin wrote:
> 
> > Yes, we find list intolerance too---servers which only look at the 
> > second byte in a cipher suite, servers which forgot a default in their 
> > NamedGroup switch-case, servers which get confused on unknown 
> > HashAlgorithms, servers which require the final extension 
> > non-empty---but this is dramatically less than version intolerance. 
> > It's usually within tolerable levels that we needn't resort to fallbacks.
> > 
> > The proposal switches from something which we know does not work to 
> > something new. Perhaps this new one will break too, but it is very 
> > similar to things that have worked before, and I am hopeful that GREASE
> > will help.
> 
> Was the option to do "one extension point = specific TLS version supported"
> 
> discussed too? What arguments are there against it?
> 
> --
> Regards,
> Hubert Kario
> Senior Quality Engineer, QE BaseOS Security team
> Web: www.cz.redhat.com
> Red Hat Czech s.r.o., Purkyňova 99/71, 612 45, Brno, Czech Republic


-- 
Regards,
Hubert Kario
Senior Quality Engineer, QE BaseOS Security team
Web: www.cz.redhat.com
Red Hat Czech s.r.o., Purkyňova 99/71, 612 45, Brno, Czech Republic