Re: [TLS] PR #493: Multiple concurrent tickets

Felix Günther <guenther@cs.tu-darmstadt.de> Mon, 06 June 2016 13:11 UTC

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Subject: Re: [TLS] PR #493: Multiple concurrent tickets
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Hi,

On 06/06/2016 11:53 +0200, Antoine Delignat-lavaud wrote:
> Le 2016-06-04 16:51, Eric Rescorla a écrit :
>> I wanted to call out to cryptographers/analysts that this formalizes
>> the existing practice (going back to RFC 5077) of having multiple
>> ticket values tied to the same basic secret (though less so with 1.3
>> because tickets issued on connection N+1 don't have the same RMS as
>> those on connection N). If there is a problem with this, that would be
>> good to know.
> 
> Looking at the pull request, I don't think it will have much impact on
> the protocol analysis given that it doesn't introduce any adversarial
> capability that wasn't already present before. If anything, your change
> may enable a proof of session unlinkability for well-behaved clients
> connecting to honest servers, under a number of restrictions.

I agree with Antoine that I don't expect multiple ticket values for the
same cryptographic key to have a (negative) impact on the/our security
results. As the ticket value is, through ClientHello, part of the
handshake hash, hence flowing into the key derviation, separate ticket
values should "domain-separate" the keys established from the same RMS.

And yes, might be one can establish some form of cryptographic
session/handshake unlinkability for honest client-server communication.

Cheers,
Felix