A control unit device in one disclosed embodiment includes a receiver and a memory that stores one or more operation keys and program instructions. The control unit further includes a processor coupled to the receiver and the memory. The program instructions are executable by the processor to cause the control unit device to, in response to a revocation command received by the receiver, perform a revocation process by selecting which of the one or more operation keys to retain in the memory based on, for each of the one or more operation keys, whether the control unit receives a message encrypted by the operation key during the revocation process.