2.1 Immobilizer Block

The immobilizer is considered the system foundation because it must always work, even if the fob battery is dead, and secures a vehicle against unauthorized engine starts. It consists of a base station, placed in the vehicle, that provides the LF (125 kHz) magnetic field enabling a wireless link with the transponder in the fob to be established. This LF immobilizer link is used to exchange the power supply and digital data between the vehicle and the passive transponder.

The implemented immobilizer system supports Microchip’s OIP, which consists of an open/unlicensed protocol stack based on AES-128 encryption. First, the ReadUID command is sent to the fob. The fob has to decode the ReadUID command and respond with its Unique ID (UID) value. If the received UID value is correct (matches the stored UID), the start authentication command is issued and challenge data is sent to the fob based on the authentication type (unilateral or bilateral). The fob receives the challenge, performs the encryption and sends a ciphered response back. This response is received by the base station and verified to complete the authentication process.