US Patent 9137658 Electronic key registration system

An offline immobilizer ECU reads an encryption key generation code from an offline additional electronic key and generates an electronic key encryption key for the offline additional electronic key using the encryption key generation code and a communication subject key encryption key held by the immobilizer ECU. The immobilizer ECU stores, in a memory, the generated electronic key encryption key and a key ID code that is read from the offline additional electronic key.