Abstract: The method of secure data transmission between an immobilizer (1) of a motor vehicle's anti-theft device and a motor control unit (2) includes storing an internal key (20′) in both immobilizer and the motor control unit; generating a random bit sequence (21) from existing sensor data in the motor control unit in response to a wake-up signal (24) from the immobilizer (1); transmitting the random bit sequence from the motor control unit to the immobilizer and storing it therein; producing a key code (22) with the random bit sequence (21) and the internal key (20′) in both control units; encoding a message (23) including redundant data with said key code (22) in the immobilizer; sending the message (23) to the motor control unit; reading and checking the message (23) in the motor control unit by means of the redundant data contained in the message (23) to verify correct encoding and transmitting a release signal to the motor control unit when the encoding is correct.

Abstract: A method for assuring data transmission between control units in a motor vehicle is proposed, which units are connected to one another via a data bus, wherein the communication between the devices occurs in an encoded form and an encoding is dependent on secret keys stored in the control units and on a random value that changes over time.

Abstract: A system and method for transmitting enciphered data between first and second terminals over a data transmission channel. The system comprises a transmitter located at the first terminal, a receiver located at the second terminal and cipher computers located at each of the first and second terminals. At least one of the cipher computers is programmable and has a program memory. Storage means are provided for storing a program code or algorithm and a secret cipher key, and means are further provided for transmitting the algorithm to the programmable computer for storage in its program memory. The data transmitted between the first and second terminals is enciphered in accordance with the algorithm stored in the program memory of the programmable computer and determined by the secret cipher key.