Hi Matt: I agree with your interpretation, in that there should be no records of any type between the CSS and the Handshake/Finished message, even during re-handshake. The full handshake (and subsequent key material) cannot be validated until the Handshake/Finished messages has been received and processed.
Any AppData received between a CSS and Handshake/Finished is potentially malicious data trying to sneak in before the Handshake is verified; forwarding this data to an upper layer would be potentially harmful. A MITM would hope that the data would be processed by an application before the TLS layer receives and verifies the Handshake/Finished message. -- -Todd Short // tsh...@akamai.com<mailto:tsh...@akamai.com> // "One if by land, two if by sea, three if by the Internet." On Oct 13, 2015, at 5:12 AM, Matt Caswell <fr...@baggins.org<mailto:fr...@baggins.org>> wrote: Hello, Does anyone have any views on the below? Thanks Matt 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 TLS@ietf.org<mailto:TLS@ietf.org> https://www.ietf.org/mailman/listinfo/tls
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