1. An information processing method of receiving encoded image data compression-coded for each tile and encrypting the encoded image data, characterized by comprising: repeatedly forming one tile group from a plurality of adjacent tiles and another tile group from a plurality of adjacent tile groups so as to define a hierarchical structure of the tiles and tile groups, and for a plurality of partial encoded data that constitute encoded data of a tile located at a terminal of the hierarchical structure, arranging the partial encoded data toward the terminal in ascending order of priority in decryption so as to define a tree structure that nodes the respective tile groups, the respective tiles, and the respective partial encoded data; generating encryption key information for a node located at an uppermost layer of the tree structure for an entire image expressed by the encoded image data; executing, up to a node located at the terminal, processing for generating encryption key information for a node of interest on the basis of encryption key information generated for a node located at an upper layer; when a designation input is given to define, as an object to be encrypted, a desired node position in nodes of the partial encoded data in the tree structure, setting, as an object to be encrypted, partial encoded data which is located at a higher layer and contains partial encoded data at the node position that is defined by the designation input; and executing encryption processing for each partial encoded data, which is set as an object to be encrypted, by using an encryption key generated for the partial encoded data and outputting the encrypted partial encoded data and unencrypted partial encoded data.