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 18:30 UTC

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From: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
To: "Blumenthal\, Uri - 0553 - MITLL" <uri@ll.mit.edu>
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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 13:35:42 -0400, Blumenthal, Uri - 0553 - MITLL wrote:
> On 4/13/15, 13:31 , "Daniel Kahn Gillmor" <dkg@fifthhorseman.net> wrote:
>
>>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
>>
>>……...
>>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.
>
> Cryptographically sound algorithms and protocols should be immune to this
> concern. And we should accept only cryptographically sound algorithms &
> protocols. :-)

What do you think an implementation should do when a peer tries to
negotiate the combination above?  Require an RSA-PSS signature from the
server over some material derived in part from the PSK key agreement
mechanism?

          --dkg