Lawful interception of end-to-end encrypted data traffic

A method of facilitating the lawful interception of an IP session between two or more terminals 12,13, wherein said session uses encryption to secure traffic. The method comprises storing a key allocated to at least one of said terminals 12,13 or to at least one of the subscribers using one of the terminals 12,13, at the terminal 12,13 and at a node 5,8 within a network 1,6 through which said session is conducted, or a node coupled to that network. Prior to the creation of said session, a seed value is exchanged between the terminal 12,13 at which the key is stored and said node 5,8. The key and the seed value are used at both the terminal 12,13 and the node 5,8 to generate a pre-master key. The pre-master key becomes known to each of the terminals 12,13 involved in the IP session and to the network node 5,8. The pre-master key is used, directly or indirectly, to encrypt and decrypt traffic associated with said IP session.