1. An information processing system in which a key tree is formed so as to include leaves, a root, and nodes existing in paths from the respective leaves to the root, wherein a plurality of devices are assigned to respective leaves and keys are assigned to the root, the leaves, and the nodes, respectively; and an enabling key block (EKB) is provided to a device, wherein the enabling key block (EKB) includes data produced by selecting a path in the key tree and encrypting an upper-level key in the selected path using a lower-level key in the selected path such that the encrypted data can be decrypted only by the device which can use a node key set corresponding to the selected path, wherein the key tree includes a plurality of subtrees serving as category trees that are grouped in accordance with categories and managed by category entities; and the enabling key block (EKB) is produced by a key distribution center (KDC) such that an EKB type definition list representing the correspondence between an EKB type identifier and one or more category tree identification data each identifying a category tree that can process an EKB of an EKB type identified by the EKB type identifier is held in the key distribution center (KDC), one or more category tree identification data corresponding to an EKB type identifier included in an EKB request are extracted from the EKB type definition list, and an EKB is produced which can be decrypted in common in the one or more category trees identified by the extracted one or more category tree identification data.