A data protection system includes terminals, and an encryption device that encrypts distribution data distributed to each terminal. Each terminal corresponds with one node on a lowest level of a tree structure having hierarchies. A data protection system excludes nodes on the lowest level, determines a plurality of combination patterns that include combinations of two or more of all four nodes that are reached one level below the node, decides an individual decryption key for each determined combination pattern, and decides an individual decryption key for each node on the lowest level. The data protection system prescribes nodes that are reached from the node on the lowest level and a terminal to the node on the highest level that is an invalid node. For invalid nodes, the data protection system designates an encryption key that corresponds to the decryption key determined in correspondence with the combination pattern that combines all nodes, excluding invalid nodes, and encrypts distribution data that uses each of the designated encryption keys.