An onboard device transfers an encrypted message encrypted outside a vehicle to one or more vehicle controllers connected to a vehicle network. When the encrypted message is an individual message to one of the vehicle controllers, the onboard device transmits the encrypted message to the one of the vehicle controllers via the vehicle network. When the encrypted message is a common message to the one or more vehicle controllers, the onboard device decrypts the encrypted message using an encryption key owned by the onboard device and then transmits the decrypted message to the one or more vehicle controllers via the vehicle network.