Cellular telephony authentication arrangement

A secure cellular telephony arrangement where the mobile unit maintains a secret that is assigned to it by the service provider, and which is known to the provider (home cellular geographic service are -- CGSA) but not to any other base station. A shared secret datum is generated by the home CGSA with the aid of the secret and some other data. That data is transmitted to the mobile unit to enable it to also generate the shared secret datum. A mobile unit wishing to communicate with a base station creates an authentication string with the aid of the shared secret datum and sends it and the unit's identity to the base station. A base station which does not have the shared secret datum is unable to immediately authenticate the mobile unit. It therefore contacts the home CGSA, receives the shared secret datum and the other data, and proceeds to authenticate the mobile unit's authentication string. With the information received from the home CGSA the base station can direct the mobile unit to regenerate the shared secret datum or even create a new one.