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

[[e-mail] (Martin Rex)]

Matt Caswell wrote:
> 
> Does anyone have any views on the below?

Yup.  Interleaving application & handshake records is a
highly dangerous idea (and fortunately some TLS implementations
will abort if you try).

http://www.ietf.org/mail-archive/web/tls/current/msg07648.html

http://www.ietf.org/mail-archive/web/tls/current/msg09743.html


-Martin

> On 30/09/15 11:06, Matt Caswell wrote:
> > 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
> >
> >
> 
> _______________________________________________
> TLS mailing list
> [e-mail]
> https://www.ietf.org/mailman/listinfo/tls