Subject: [TLS] Fwd: Clarification on interleaving app data and handshake records

From: Matt Caswell <[email-masked]>
Date: Wed, 30 Sep 2015 11:06:14 +0100

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-------- Forwarded Message --------
Subject: Clarification on interleaving app data and handshake records
Date: Wed, 30 Sep 2015 11:02:55 +0100
From: Matt Caswell <[email-masked]>
To: [email-masked] <[email-masked]>

Hi all

I have a question on how to interpret RFC 5246 with regards to the
interleaving of app data and handshake records.

RFC 5246 (and RFC 4346 before it) contains these words:

   Note: Data of different TLS Record layer content types MAY be
   interleaved.  Application data is generally of lower precedence for
   transmission than other content types.  However, records MUST be
   delivered to the network in the same order as they are protected by
   the record layer.  Recipients MUST receive and process interleaved
   application layer traffic during handshakes subsequent to the first
   one on a connection.

This wording seems to place no limits whatsoever on when it is valid to
receive app data in the handshake. By the wording in the RFC it would be
valid for app data to be received *after* the ChangeCipherSpec has been
received but *before* the Finished has been processed.

There is also this wording:

   Note: If a rehandshake occurs while data is flowing on a connection,
   the communicating parties may continue to send data using the old
   CipherSpec.  However, once the ChangeCipherSpec has been sent, the
   new CipherSpec MUST be used.  The first side to send the
   ChangeCipherSpec does not know that the other side has finished
   computing the new keying material (e.g., if it has to perform a
   time-consuming public key operation).  Thus, a small window of time,
   during which the recipient must buffer the data, MAY exist.  In
   practice, with modern machines this interval is likely to be fairly
   short.

I think this means that as soon as the first party sends a CCS, they
must not send any app data until they have received a CCS back - they
must buffer it until the CCS is seen - but on reading it again I'm not
sure! If that were the case then the second party should never expect to
see app data between CCS and Finished. It doesn't tell you anything
about what the first party can expect though, i.e. is the second party
allowed to send app data between the CCS and Finished?

Finally there is also this:

   A Finished message is always sent immediately after a change
   cipher spec message to verify that the key exchange and
   authentication processes were successful.

I suppose this trumps everything else, although as this section is
specifically talking about handshakes you could interpret "immediately"
as applying to the handshake sequence only without saying anything about
interleaved app data records.

I believe the intention is that app data should not be sent between the
CCS and Finished but the RFC wording is not exactly crystal clear. This
is the interpretation I have taken whilst fixing this OpenSSL bug:

https://rt.openssl.org/Ticket/Display.html?id=3712&user=guest&pass=guest

Can some confirm my interpretation is correct?

Thanks

Matt