Patent ID: 8667291

Claim:
A data processing device for using a digital work recorded on a recording medium having also recorded thereon (i) a plurality of record digest values generated from a plurality of data blocks constituting the digital work and (ii) record signature data generated by applying, with use of a signature key, a signature generating algorithm to a first combination made of at least some or all of the plurality of record digest values, the data processing device comprising: a verification key storing unit configured to store a verification key corresponding to the signature key; a using unit configured to use the digital work; a selecting unit configured to, each time the digital work is used, randomly select a predetermined number of data blocks from all of the plurality of data blocks, the predetermined number being smaller than the number of all the plurality of data blocks; a calculating unit configured to calculate a plurality of calculation digest values from the selected data blocks; a reading unit configured to read remaining record digest values corresponding to unselected data blocks from among the plurality of record digest values; a generating unit configured to generate a second combination based on calculation digest values and the remaining record digest values, the second combination being the same as data which is generated from the first combination by replacing record digest values corresponding to the selected data blocks with corresponding calculation digest values; and a signature verifying unit configured to verify the record signature data by applying, with use of the verification key, a signature verification algorithm to the second combination and the record signature data.