In a composite system that includes a main system that operates with a main program and a plurality of sub-systems that operate both with sub-programs and under the control of the main system attachably and detachably connected with each other via a predefined bus, the main system transfers each fragment of divided target data to the sub-system, and the sub-system includes a receiving buffer that can read and write the fragment of data received from the main system temporarily. An encrypting process can be executed with the main system and the sub-system regardless of the size of target data to be encrypted, the size of memory in the sub-system, and data transfer capability between the main system and the sub-system even if the size of the target data in the main system is bigger than the size of the receiving buffer in the sub-system.