| 摘要 |
A data protection system is provided that reduces, to a degree, the amount of encrypted data that is distributed to a plurality of terminals. In the data protection system a terminal whose decryption keys are exposed by a dishonest party is made to be unable to decrypt the data correctly, while other terminals are able to decrypt the data correctly. The data protection system includes a plurality of terminals, and an encryption device that encrypts distribution data distributed to each terminal. Each terminal is corresponded with one node on a lowest level of a 4-ary tree structure or the like having a plurality of hierarchies. The data protection system, for each node in the tree structure, excluding those 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. Further, the data protection system has each terminal store all decryption keys decided for the nodes on the path from the node on the lowest level that corresponds to the terminal through to the node on the highest level. The data protection system prescribes nodes that are reached from the node on the lowest level that corresponds to the terminal through to the node on the highest level that correspond to a terminal that has been dishonestly analyzed as invalid nodes. For invalid nodes, other than invalid nodes on the lowest level, the data protection system designates an encryption key that corresponds to the decryption key decided in correspondence with the combination pattern that combines all nodes, excluding invalid nodes, of the four nodes that are reached one level below the node, and has the encryption device encrypt distribution data that uses each of the designated encryption keys.
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