Apparatus and method for automatically generating an authentication key (A-Key) in a mobile station wherein the mobile station periodically receives random numbers from a base station, and automatically generates the authentication key from a received random number and from mobile station identification data stored in the mobile station. The mobile station identification data may include the electronic serial number of the mobile station, a user selected value, and a cellular system selected value. In addition, mixing registers may be utilized by an encoding algorithm, such as the CAVE algorithm, for encoding the random numbers and the identification data to automatically generate the mobile station authentication key.