A method of operation in a mobile communication system includes a mobile station, a first network capable of serving the mobile station as a home network and a second network capable of serving the mobile station as a visited network, including carrying out in the home network the steps of: generating a random seed (RS), modifying the random seed by combination with an authentication key (K) held by the home network and the mobile station to form session keys (KS and KS′), sending the session keys (K′S and K′S′) to the visited network to permit authentication of the mobile station, and characterized in that the following steps are carried out in the home network: providing a further modification key (SMK), and carrying out a further key modification (of KS and KS′) in the production of the session key (K′S and K′S′) using the further modification key (SMK).