Patent ID: 7395425

Claim:
A data protection system that comprises three or more terminals, an encryption device, and an encryption key designation device, and that protects distribution data that is to be distributed to said terminals, according to said encryption device encrypting the distribution data, wherein: each terminal is operable to (i) store a decryption key group assigned individually to the terminal, respectively, according to (a) a determination of two or more terminal groups, which are groups having two or more terminals as members, so that each of the terminals is a member of at least one of the terminal groups, and so that any one of the terminal groups that shares a same terminal as a member with another one of the terminal groups does not completely include the other one of the terminal groups sharing the same terminal as a member and is not completely included in the other one of the terminal groups sharing the same terminal as a member, (b) a decision of one or more decryption keys individually in correspondence with each terminal and each determined terminal group, and (c) an assignment, to each terminal, the decryption key group, where the decryption key group is based on the decryption key decided in correspondence with the terminal, and decryption keys decided in correspondence with all terminal groups that include the terminal, respectively; (ii) obtain an encrypted distribution data group that has been output from said encryption device, and (iii) use a stored decryption key to decrypt encrypted distribution data; said encryption key designation device is operable to designate encryption keys, and includes: an invalid terminal designation unit for designating one or more terminals as invalid terminals; and an encryption key designation unit for designating, when all the decryption keys assigned to the terminals, other than decryption keys assigned to the one or more invalid terminals, are prescribed as valid decryption keys, and when a procedure for selecting an assigned valid decryption key for the most terminals not designated as invalid terminals is repeated until all terminals not designated as invalid terminals have been assigned a selected valid decryption key, encryption keys that respectively correspond to each of the valid decryption keys that are selected as a result of the procedure; and said encryption device includes an encryption unit for encrypting distribution data by successively using all the designated encryption keys to encrypt distribution data, generating the encrypted distribution data group, and outputting the generated encrypted distribution data group.