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Re: [TLS] 0-RTT, server Application Data, and client Finished

Andrei Popov <Andrei.Popov@microsoft.com> Wed, 27 January 2016 18:39 UTC

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From: Andrei Popov <Andrei.Popov@microsoft.com>
To: Bill Cox <waywardgeek@google.com>, Martin Thomson <martin.thomson@gmail.com>
Thread-Topic: [TLS] 0-RTT, server Application Data, and client Finished
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Subject: Re: [TLS] 0-RTT, server Application Data, and client Finished
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Ø  The CertificateVerify message is still listed as an option in the 0-RTT client's first flight at t = 0.  Is this a mistake?  I have not heard that anyone wants to do this, as there is no possibility of a traditional proof-of-possession in the first flight.
I agree with this: client auth in 0-RTT is replayable, unless the server takes extraordinary steps (QUIC-like strike registers, database of client nonces, etc.) No plans to implement, at least for now.

Cheers,

Andrei

From: TLS [mailto:tls-bounces@ietf.org] On Behalf Of Bill Cox
Sent: Wednesday, January 27, 2016 6:13 AM
To: Martin Thomson <martin.thomson@gmail.com>
Cc: tls@ietf.org
Subject: Re: [TLS] 0-RTT, server Application Data, and client Finished

On Tue, Jan 26, 2016 at 7:32 PM, Martin Thomson <martin.thomson@gmail.com<mailto:martin.thomson@gmail.com>> wrote:

I get your point, but I don't see that as a simplification.  In my
mind, post-handshake client authentication doesn't happen.  Or, I
don't see it being commonplace.

The use case described to me was for a higher level of authentication, such as when accessing a source-code repository at work, similar to proving that you still possess a security key now and then.  I am not sure why FIDO does not fit this use case better, though.  I have a feeling this is a legacy use case of client certificates, and is being included to avoid having to transition a lot of legacy client-cert based systems to FIDO.

The CertificateVerify message is still listed as an option in the 0-RTT client's first flight at t = 0.  Is this a mistake?  I have not heard that anyone wants to do this, as there is no possibility of a traditional proof-of-possession in the first flight.  However, with QUIC-like strike registers, a reasonably accurate timestamp in the handshake, and a secure clock on the client, you can approximate a proof-of-possession.  This seems like a lot of trouble for a first-flight CertificateVerify, but if someone wants to do this, it could stay in the spec.  If CertficateVerify is to remain in the 0-RTT first flight, there needs to be some way to provide a timestamp in the handshake hash.  Is there a way to do this now?  The TLS 1.2 timestamp in the client random seems to have been removed.

Bill

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