A method of operating a node for performing handover between access networks wherein a user has authenticated for network access in a first access network. The method comprises receiving from a home network a first session key and a temporary identifier allocated to the user for the duration of a communication session. The identifier is mapped to the first session key, and the mapped identifier and key are stored at the node. A second session key is derived from the first session key and the second session key is sent to an access network, and the identifier sent to a user terminal. When the user subsequently moves to a second access network, the node receives the identifier from the user terminal. The node then retrieves the first session key mapped to the received identifier, derives a third session key and sends the third session key to the second access network.