1. An encryption apparatus based on a common key encryption system in which a plurality of expanded keys are used in a predetermined order in a data randomizing process for encryption and in a reversed order in a data randomizing process for decryption, the apparatus comprising: a plurality of round processing circuits connected in series, the round processing circuit of a first stage receiving a common key and subjecting the common key to a round function to output a sub key and the round processing circuit of other stages receiving the sub key output from the round processing circuit of a previous stage and subjecting the sub key to a round function to output a sub key, the sub key output from the round processing circuit of a last stage being the common key; and a plurality of expanded key generating circuits configured to receive the sub keys output from at least a part of said round processing circuits and output expanded keys based on all or some bits of the received sub keys.