Methods and apparatus are provided for insuring that a motor vehicle is being operated by an authorized operator. The apparatus includes an electronic control unit (ECU), an engine immobilizer unit, and a shared encryption key. The ECU generates a challenge by combining the output of a pseudo-random number generator and the output of a somewhat random number generator and cycling the combined number through a linear feedback shift register. The ECU sends the challenge to the immobilizer unit where it is encrypted with the shared key and sent back to the ECU as a response. The ECU uses the same key to encrypt the challenge and compares the encrypted challenge to the response. If the response matches the encrypted challenge, engine operation is enabled.