Patent ID: 7761085

Claim:
A method of operation in a mobile communication system including a mobile station, a first network capable of serving a 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: obtaining an authentication key held by the home network and the mobile station: generating a random seed, forming a modified session key by a key combining procedure which includes use of the authentication key; and sending the modified session key and the random seed to the visited network to permit authentication of the mobile station with the visited network, wherein the key combining procedure is carried out in only the home network and the mobile station to form the modified session key and the key combining procedure includes: obtaining a session modification key that is known only to the home network and the mobile station, and combining the session modification key with a key used in the key combining, procedure to form the modified session key, the key combined with the session modification key being either the authentication key or a session key derived from the authentication key.