Re: [TLS] supported_versions question

Brian Smith <brian@briansmith.org> Mon, 07 November 2016 22:02 UTC

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From: Brian Smith <brian@briansmith.org>
Date: Mon, 7 Nov 2016 12:02:34 -1000
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To: Kurt Roeckx <kurt@roeckx.be>
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Cc: David Benjamin <davidben@google.com>, "tls@ietf.org" <tls@ietf.org>
Subject: Re: [TLS] supported_versions question
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Kurt Roeckx <kurt@roeckx.be> wrote:

> So I guess you're also saying that a server that implements TLS
> 1.1 to TLS 1.3, but disables TLS 1.2 and TLS 1.3 support should
> ignore the supported_versions even when it knows about it?
>
> I guess I have same questions but with only TLS 1.3 disabled, to
> be sure when we need to look at it.


>From the perspective of the application developer, if I've disabled TLS 1.3
then I'd rather not  have any TLS 1.3 behavior implemented; I'd rather have
the implementation behave the same as before it supported TLS 1.2,
more-or-less. That means in particular that supported_versions should be
ignored if TLS 1.3 and later have been disabled. Put another way, if TLS
1.2 is the maximum version the application has enabled, then the TLS 1.3
specification in total is irrelevant and only RFC 5246 (and perhaps
earlier) apply, and those specifications don't specify supported_versions.

Consider in particular that an application might want to disable (or avoid
enabling) TLS 1.3 to avoid some problem (perhaps a security problem) with
supported_versions processing. It would be bad to not have any way of
disabling supported_versions processing, and it doesn't seem good from a
usability standpoint to have a separate nob to control it.

Cheers,
Brian
-- 
https://briansmith.org/