Re: [TLS] Application layer interactions and API guidance

Eric Rescorla <ekr@rtfm.com> Wed, 12 October 2016 19:57 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Wed, 12 Oct 2016 12:57:02 -0700
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To: Kyle Rose <krose@krose.org>
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Subject: Re: [TLS] Application layer interactions and API guidance
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On Wed, Oct 12, 2016 at 12:55 PM, Kyle Rose <krose@krose.org>; wrote:

> 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?
>

The 0-RTT traffic key incorporates the ClientHello.Random which is tied
into the full handshake.

-Ekr


>
> Kyle
>
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