Abstract: All subscribers operating within a communication system initially generate and transmit a random password and their ID code to the operation (control) center of a communication system. Thereafter, in a first embodiment, subscribers are commanded to modify (in a predetermined manner) and re-transmit their password and ID code either periodically or upon requesting access to the communication system. The modified password is retained as the current (authorized) password by both the subscriber and the system. In a second embodiment, subscribers are commanded to generate and transmit a new random password and their ID code to the system, which maintains a history of each subscriber's current and previous passwords. According to the invention, some of the subscribers having an unauthorized (duplicated) ID code may be inactive (off) when the subscriber is commanded to modify or generate a password.