On Wed, Oct 12, 2016 at 2:03 PM, Ilari Liusvaara <ilariliusva...@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
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