1. A server apparatus, comprising: a key distribution center (KDC) which manages capability information of a plurality of entities and generates an enabling key block (EKB) decodable only by the entities having common capability by using a sub-enabling key block (sub-EKB) generated by the entities having the common capability, wherein the enabling key block (EKB) corresponds to a key tree in which respective keys are corresponded to a root, nodes, and leaves on a path from the root to the leaves of a tree in which a plurality of devices are constituted as the leaves, and wherein the sub-enabling key block (sub-EKB) is generated based only on a key set corresponding to nodes or leaves included in a sub-tree of the key tree, wherein the sub-tree is classified based on data processing capability of the devices within the sub-tree.