I received a notification from a large security auditing firm that of the ciphers currently available, only RC4 ciphers will be considered PCI compliant. My assumption based on the notification is that this move is intended as a rejection of CBC (Cipher Block Chaining). Well, that's fine as far as I am concerned. CBC has some serious issues as implemented in SSL v3 / TLS v1.0. In a nutshell, you can time responses for applications using the block cipher to get ranges of possible data in SSLv3 and partial payload decryption in TLS. So-called "stream" ciphers like RC4 are immune to this particular attack vector. You don't get private keys from the attack, its by no means a fast attack (minimum of three hours), and you need access to monitor the session . Further, patches for CBC exist to over-ride the timing exploit (for example the NSS libraries used by Mozilla have been patched). I will save debunking the man in the middle hysteria for a later post. What frustrate...