Re: [TLS] Application layer interactions and API guidance

Kyle Rose <krose@krose.org> Wed, 12 October 2016 19:55 UTC

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From: Kyle Rose <krose@krose.org>
Date: Wed, 12 Oct 2016 15:55:49 -0400
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To: Ilari Liusvaara <ilariliusvaara@welho.com>
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Subject: Re: [TLS] Application layer interactions and API guidance
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On Wed, Oct 12, 2016 at 2:03 PM, Ilari Liusvaara <ilariliusvaara@welho.com>;
wrote:

> > There's my confusion. I misinterpreted both the Zero-RTT diagram and the
> > table of handshake contexts under "Authentication Messages", specifically
> > "ClientHello ... later of EncryptedExtensions/CertificateRequest". I'm
> > guessing I should be looking at the 0-RTT row only? I.e., if 0-RTT is
> > accepted, is the second Finished message from the client ("{Finished}")
> the
> > same message encrypted differently (using the handshake traffic secret)?
>
> No, there is no difference in ClientFinished in case of 0-RTT accept or
> reject (other than the contents of the CH and EE hashed in).
>

Still confused. :-)

In the message flow for 0-RTT, there are two Finished messages sent from
client to server. One is sent right after CH, and is protected by the
client_early_traffic_secret: (Finished). The other is sent after the server
sends its Finished, and this is protected by the handshake_traffic_secret:
{Finished}.

In the table under "Authentication Messages", there are four rows, one for
each Mode: 0-RTT, 1-RTT (Server), 1-RTT (Client), and Post-Handshake.

Which handshake context is used for the (Finished) message and which is
used for the {Finished} message?

The thing that protects the 0-RTT data from substitution is the record
> protection MACs that are made using key derived from the PSK secret. So
> if the PSK secret is unknown, the key for 0-RTT can't be derived, and
> as consequence, 0-RTT data can't be altered (there's end-of-data marker
> too, preventing truncation).
>

Altered is one thing, and I agree that is prevented; I'm talking about
substitution.


> And basically, ServerFinished MAC covers everything up to that point,
> and ClientFinished MAC covers the entiere handshake (0-RTT data not
> included).
>

So client Finished doesn't protect 0-RTT data, but...


> You can't swap out 0-RTT data (without PSK keys). One can only create
> new connection attempts (that fail!) with the same 0-RTT data (and the
> same ClientHello) before or after the real connection (if any, it
> could be supressed, in which case you would get only failed handshakes
> with 0-RTT data).
>

This is exactly what I'm trying to understand. What specifically prevents
this swapping? I.e., what ties the 0-RTT data sent on a particular
connection to the rest of that connection, such that replacing that 0-RTT
data with 0-RTT data from a previous successful connection will cause a
failure?

Kyle