Re: [TLS] Thoughts on Version Intolerance

mrex@sap.com (Martin Rex) Wed, 20 July 2016 10:14 UTC

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Date: Wed, 20 Jul 2016 12:14:01 +0200 (CEST)
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From: mrex@sap.com (Martin Rex)
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Subject: Re: [TLS] Thoughts on Version Intolerance
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Hanno Böck wrote:

Checking application/pgp-signature: FAILURE
> Hubert Kario <hkario@redhat.com>; wrote:
> 
>> so it looks to me like while we may gain a bit of compatibility by
>> using extension based mechanism to indicate TLSv1.3,

Forget TLS extensions, forget ClientHello.client_version.
Both in fundamentally broken, and led to Web Browsers coming up
with the "downgrade dance" that is target&victim of the POODLE attack.

We know fairly reliably what kind of negotiation works just fine:
TLS cipher suite codepoints.

Please define *ALL* TLSv1.3-specific cipher suites to
  a) indicate that the client offering it supports (at least) TLSv1.3
  b) that indication (a) will override any lower ClientHello.client_version
     that may have been used for backwards compatibility.

> 
> I'm now also collecting some data and have some preliminary
> suspicion on affected devices. My numbers roughly match yours that we
> are in the more or less 3% area of 1.3 intolerance.

The TLSv1.2 version intolerance is already a huge problem,
and I'm not seeing it go away.  Acutally Microsoft created an
awfully large installed base of TLSv1.2-intolerant servers
(the entire installed base of Win7 through Win8.1 aka 2008R2, 2012, 2012R2).


I would really like to see the TLS WG improving the situation
rather than keep sitting on its hands.  The problem has been well-known
since 2005.  And the "downgrade dance" was a predictably lame approach
to deal with the situation, because it completely subverts/evades the
cryptographic protection of the TLS handshake.


-Martin