A controller and a device generate a shared key by performing mutual authentication using a public key certificate of the controller and a public key certificate of the device. The controller and the device set an expiry for the shared key to one of the expiry of the public key certificate of the controller and the expiry of the public key certificate of the device. The controller and the device perform the mutual authentication using neither the public key certificate of the controller nor the public key certificate of the device, but the shared key, if the expiry set for the shared key has not passed.