Re: [TLS] MUST <x> or what?

Peter Gutmann <pgut001@cs.auckland.ac.nz> Fri, 28 August 2015 04:29 UTC

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From: Peter Gutmann <pgut001@cs.auckland.ac.nz>
To: Martin Thomson <martin.thomson@gmail.com>, EKR <ekr@rtfm.com>
Thread-Topic: [TLS] MUST <x> or what?
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Date: Fri, 28 Aug 2015 04:29:11 +0000
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Subject: Re: [TLS] MUST <x> or what?
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Martin Thomson <martin.thomson@gmail.com>; writes:

>The opposite in fact. NSS checks extensions first. That is how we filter out 
>ECC cipher suites if the named_groups extension doesn't list anything we 
>support.

I have to do the same thing, bouncing back and forth between cipher suites and
extensions in order to find something that fits.  That was the motivation for
the creation of the "Standardised ECC Cipher Suites for TLS" draft:

   TLS-ECC [3] provides an extremely flexible, and by extension
   extremely complex means of specifying a large number of options
   involving the use of ECC algorithms for TLS [2].  As such the "cipher
   suites" in TLS-ECC [3] and by extension TLS-ECC-Brainpool [4] aren't
   suites in the conventional TLS sense but more an indication of intent
   to negotiate a Chinese menu, with details to be decided on later via
   various TLS extensions and parameter settings.  This makes deciding
   on a particular suite nondeterministic, since later parameter choices
   and settings can negate the initial "cipher suite" choice, requiring
   returning to the suite list to try with another Chinese-menu suite in
   the hope that later parameter choices allow it to be used.

Peter.