Abstract: A vehicle anti-theft system in the form of a mechanically interengagable electronic key and lock includes stored key and vehicle identifiers in both the key and in the lock. A random number is generated in the lock each time the vehicle ignition switch is closed and that random number is used to encode one of the identifiers for transmission to the key. The key uses the corresponding stored identifier to recover the random number. The other stored identifier is then encoded using the recovered random number and the encoded other identifier is returned to the lock. The lock uses the generated random number to recover the second identifier for comparison to its stored value. If the comparison fails, vehicle operation is prevented. The system may effect several trials, each using a new random number before refusing further attempts. The lock electronics is preferably disposed in the vehicle fuel tank and controls operation of the vehicle fuel pump.