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Re: [TLS] Negotiate only symmetric cipher via cipher suites (was: Ala Carte Cipher suites - was: DSA should die)

Daniel Kahn Gillmor <dkg@fifthhorseman.net> Mon, 13 April 2015 17:31 UTC

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From: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
To: Dave Garrett <davemgarrett@gmail.com>, tls@ietf.org
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Subject: Re: [TLS] Negotiate only symmetric cipher via cipher suites (was: Ala Carte Cipher suites - was: DSA should die)
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On Mon 2015-04-13 13:25:20 -0400, Dave Garrett wrote:

>> So if we have to have non-(EC)DHE PSK, what would it mean if a TLS peer
>> were to try to negotiate:
>> 
>>   key agreement: PSK
>>  authentication: RSA-PSS
>> 
>> Do we just say "don't do that"?
>
> SGTM

This, i think, is one of the serious concerns about splitting the
ciphersuite into orthogonal vectors.

Once the full cartesian explosion is available by multidimensional
enumeration, we have to mark out which corners of the space are actually
bad ideas, and we have to make sure our implementations don't stumble
into those corners by accident.

This isn't impossible to do, but it seems ripe for subtle implementation
bugs.

     --dkg

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