Abstract: A method and apparatus for performing authentication in a communication system. The method includes the steps of detecting an unexpected authentication message, updating a counter value, comparing the counter value to a threshold value, and performing an encryption key update operation in response to the comparison between the counter value and the threshold value. The apparatus includes a base station (152), a switching center (128) coupled to the base station (152), and an authentication center (149) responsive to the switching center (128). The switching center (128) detects an unexpected authentication message received by the base station (152). The authentication center (149) updates a counter when the switching center (128) detects the unexpected authentication message and compares the updated counter with a threshold value. The authentication center (149) initiates an encryption key update operation when the updated counter value exceeds the threshold value.