The data processing system is realizable by executing a step of ciphering contents keys used for decoding ciphered contents data by applying mutually different ciphering keys before storing ciphered contents keys in memory as header data of the corresponding contents data. One of the ciphered contents keys comprises ciphered data ciphered by a ciphering key provided for by enabling key block comprising such data composition which is solely decodable by specific device by way of disposing related keys in such corresponding nodes on the path ranging from roots to leaves of a key tree structure for distributing keys. The other ciphered contents key comprises such data ciphered by a specific key proper to a corresponding storage device to enable the device for reproducing contents data to properly and selectively utilize data of ciphered key, whereby enabling the data processing system to properly reproduce decoded contents data.