Electronic key registration system

An immobilizer ECU (6) transmits a vehicle ID code (VID-A) and a SEED code (SC-2h), which is read from an electronic key (2b), to a data center (10) online. The data center (10) generates an encryption key (K-2 from the received SEED code (SC-2h) and a first logic (h), and generates a further SEED code (SC-2Ag) from the encryption key (K-2), the vehicle ID code (VID-A), and a second logic (g). The immobilizer ECU (6) obtains the further SEED code (SC-2Ag) online from the data center (10), generates the encryption key (K-2) from the obtained further SEED code (SC-2Ag), the vehicle ID code (VID-A), and the second logic (g), and stores the encryption key (K-2).