On November 19, 2003 07:16 pm, Swaminathan P wrote: > I have a question anout the use of sequence number as a part of the > input to the hash function during the MAC calculation. Does that > security concerns? Would the security aspects of theSSL be affected if > the sequence number is not used as a part of the input to the hash > funtion for MAC calculation?
As Les pointed out, this helps detect replay (or removal) attacks. It also forces the underlying transport to be reliable and ordered, so that the SSL/TLS encapsulation in turn presents what appears to be a reliable and ordered channel to the user of the protocol. For example, it doesn't stop you trying to run SSL/TLS over something like UDP that does not these sorts of guarantees, but this aspect of the SSL/TLS protocol will allow you to detect (in theory) if the packets arrive out of order or don't turn up at all. It will not *recover* from such problems though, so this sequence number trick is simply to verify reliability of the transport, without providing any recovery mechanism if the ordering and reliability assumptions aren't met. But there's two far more fundamental problems with trying to bypass this ordering issue anyway. (1) the ciphering used to en/decrypt the payloads of SSL/TLS records (after the initial handshake has agreed parameters) is typically stateful, so if you get records duplicated, removed, or out-of-order, then en/decryption will start misreading all subsequent records as senseless noise. (2) during a handshake or a renegotiation, the "finished" messages use MAC calculations over *all* the previous handshake messages *in order*. Note that "all" and "in order" mean that if you lose reliability during the handshake or renegotiation phase, you get the SSL/TLS equivalent of a "carrier disconnect" - no amount of sequencing logic in the application protocol will matter as this corruption in the handshake protocol will prevent application protocol data from being exchanged anyway. Cheers, Geoff -- Geoff Thorpe [EMAIL PROTECTED] http://www.geoffthorpe.net/ ______________________________________________________________________ OpenSSL Project http://www.openssl.org Development Mailing List [EMAIL PROTECTED] Automated List Manager [EMAIL PROTECTED]
