A semiconductor device that functions as a key to control access to a software program resident in a computer. The device includes a continuously running pulse generator that produces an output representative of real time, a shift register permanently storing a unique number and circuitry for executing an algorithm that combines real time and the permanently stored unique number to produce a password. The password is input to the computer. The computer is coded to execute an equivalent algorithm to produce a password within the computer. The two passwords are compared and access to the computer program is afforded only if they bear a prescribed relationship. The computer can be coded to produce on the video display thereof a timespace pattern on the computer video display, circuitry for deriving the stimulus number therefrom, and circuitry for processing the stimulus number so that the password displayed by the key is a function of the value of the stimulus number. The computer executes a similar procedure on the stimulus number so that access to the software program is afforded only if correspondence exists between the user input password and the password generated in the computer.