A bitstream of each tile in a code sequence is segmented into bitstreams for respective resolution levels (S70). Image key K_I is generated (S71). Bitstream key K_BS is generated from image key K_I using encoding parameters P (S72). Key K_Ti of tile Ti is generated from bitstream key K_BS using information associated with that tile (S74). Finally, keys corresponding to the segmented bitstreams indicating respective resolutions in tile Ti are generated (S76).