DATA PROTECTION SYSTEM THAT PROTECTS DATA BY ENCRYPTING THE DATA

• US 20100034388A1
• Filed: 04/16/2008
• Published: 02/11/2010
• Est. Priority Date: 03/29/2001
• Status: Active Grant

• Current Assignee: Makoto Tatebayashi, Motoji Ohmori, Natsume Matsuzaki, Toshihisa Nakano
• Original Assignee: Makoto Tatebayashi, Motoji Ohmori, Natsume Matsuzaki, Toshihisa Nakano
• Inventors: Tatebayashi, Makoto, Ohmori, Motoji, Matsuzaki, Natsume, NAKANO, Toshihisa
• Granted Patent: US 8,416,953 B2
• Time in Patent Office: Days
• US Class Current: 380/277
• CPC Class Codes: G06F 21/10 Protecting distributed programs or content, e.g. vending or licensing of copyrighted material protection in video systems or pay television H04N7/16 ; Digital rights management [DRM]
• CPC Class Codes: G06F 2221/2107 File encryption
• CPC Class Codes: G11B 20/00086 Circuits for prevention of unauthorised reproduction or copying, e.g. piracy indicating unauthorised use of record carriers in general G11B23/28; scrambling for television signal recording H04N5/913; network architectures or network protocols for network security H04L63/00; cryptographic mechanisms or cryptographic arrangements for secret or secure communication H04L9/00
• CPC Class Codes: G11B 20/0021 involving encryption or decryption of contents recorded on or reproduced from a record carrier
• CPC Class Codes: H04L 2209/60 Digital content management, e.g. content distribution
• CPC Class Codes: H04L 2209/80 Wireless network architectures or network communication protocols for wireless network security H04W12/00
• CPC Class Codes: H04L 2463/101 applying security measures for digital rights management data processing systems or methods, specially adapted for commerce, e.g. marketing, shopping, billing, auctions or e-commerce G06Q30/00
• CPC Class Codes: H04L 63/064 Hierarchical key distribution, e.g. by multi-tier trusted parties
• CPC Class Codes: H04L 63/065 for group communications cryptographic mechanisms or cryptographic arrangements for key management involving conference or group key H04L9/0833
• CPC Class Codes: H04L 9/08 Key distribution or management, e.g. generation, sharing or updating, of cryptographic keys or passwords network architectures or network communication protocols for supporting key management in a packet data network H04L63/06
• CPC Class Codes: H04L 9/0822 using key encryption key
• CPC Class Codes: H04L 9/0836 using tree structure or hierarchical structure
• CPC Class Codes: H04L 9/0891 Revocation or update of secret information, e.g. encryption key update or rekeying
• CPC Class Codes: H04N 2005/91364 the video signal being scrambled
• CPC Class Codes: H04N 21/2347 involving video stream encryption
• CPC Class Codes: H04N 21/26606 for generating or managing entitlement messages, e.g. Entitlement Control Message [ECM] or Entitlement Management Message [EMM] arrangements for conditional access to broadcast information or to broadcast-related services H04H60/14
• CPC Class Codes: H04N 21/4325 by playing back content from the storage medium reproduction of recorded television signals H04N5/76; reproduction of recorded television signals H04N9/79
• CPC Class Codes: H04N 21/4405 involving video stream decryption
• CPC Class Codes: H04N 5/913 for scrambling ; for copy protection
• CPC Class Codes: H04N 7/1675 Providing digital key or authorisation information for generation or regeneration of the scrambling sequence pseudo-random number generators in general G06F7/58

Abstract

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.

Claims

1-43. -43. (canceled)

44. A decryption key determining device that determines decryption key groups for use in decryption to be individually assigned to at least three terminals that obtain encrypted data and decrypt the obtained encrypted data, comprising:

a decryption key setting unit for

(a) determining 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, and

(b) corresponding an individual decryption key with each terminal and each determined terminal group; and

a decryption key group assignment unit for determining, for each terminal, a decryption key group based on the corresponded decryption key and the decryption keys corresponded with each terminal group in which the terminal is included, as a decryption key group to be assigned to the terminal, respectively.

45. The decryption key determining device of claim 75, wherein said decryption key setting unit is further operable to determine the terminal groups so that at least one terminal group exists that completely includes the two or more terminal groups, and so that any one of the terminal groups within the two or more terminal groups that shares a same terminal group 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.

46. The decryption key determining device of claim 45, wherein said decryption key setting unit is further operable to determine the terminal groups so that each terminal group includes three or more terminals as members, and so that a terminal group exists that contains three or more terminal groups.

47. The decryption key determining device of claim 46, wherein:

said decryption key setting unit is operable to, when each terminal is corresponded with a node on a lowest level in an N-ary tree structure having a plurality of hierarchies, where N is a natural number equal to or greater than three,

determine, for each node other than the nodes on the lowest level, a plurality of combination patterns, of each one of N nodes that are reached from the each node constituting a parent node of the one of the N nodes, that are combinations of two or more of the N nodes including the one of the N nodes, and that include a combination of all of the N nodes,

decide an individual decryption key for each determined combination pattern and store the decided decryption key in correspondence with the parent node, and

further store an individual decryption key in correspondence with each node in the lowest level;

said decryption key group assignment unit is operable to determine decryption keys to be assigned to each terminal, the decryption keys being (a) decryption keys selected from among the decryption keys stored in said decryption key setting unit in correspondence with each node on a same path, which is a path from the node corresponding to the terminal on the lowest level to a node on the highest level, excluding the node that corresponds to the terminal, and corresponding with all of the combination patterns that include a node that is one level below the each node on the same path, and (b) the decryption key stored in said decryption key setting unit in correspondence with the terminal;

the terminal groups have a one-to-one correspondence with the combination patterns, and each terminal group is a group whose members are all the terminals that correspond to all nodes on the lowest level that are reached from all the combined nodes in the corresponding combination pattern.

48. The decryption key determining device of claim 47, wherein:

said key setting unit is operable to determine the plurality of combination patterns for each node, excluding the nodes on the lowest level, in the N-ary tree structure, by prescribing combination patterns so that each combination pattern corresponds to one of all combinations of two or more of the N nodes that are one level below and are reached from the each node constituting a parent node of the two or more of the N nodes; and

said decryption key setting unit is operable to store the determined decryption keys in correspondence with the parent node.

49. The decryption key determining device of claim 47, wherein:

said key setting unit is operable to determine the plurality of combination patterns for each node, excluding the nodes on the lowest level, in the N-ary tree structure by prescribing combination patterns so that each combination pattern corresponds to one of all combinations of all N of the nodes and N−

1 of the N nodes that are one level below and are reached from the each node constituting a parent node of the N nodes; and

said key setting unit is operable to store the determined decryption keys in correspondence with the parent node.

75. The decryption key determining device of claim 44, wherein said decryption key group assignment unit is operable to determine, for each terminal, the corresponded decryption key and all decryption keys corresponded with all terminal groups that include the terminal, as the decryption key group to be assigned to the terminal.

50. A decryption key determining method that determines decryption key groups, for use in decryption, to be individually assigned to at least three terminals that obtain encrypted data and decrypt the obtained encrypted data, said method comprising:

a terminal group determination step of determining 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;

a decryption key correspondence step of corresponding an individual decryption key with each terminal and each determined terminal group; and

a decryption key group assignment step of determining, for each terminal, a decryption key group based on the corresponded decryption key and decryption keys corresponded with each terminal group in which the terminal is included, as a decryption key group to be assigned to the terminal, respectively.

51. A computer program, recorded on a computer-readable medium, for causing a computer to execute a decryption key determining processing that determines decryption key groups, for use in decryption, to be individually assigned to at least three terminals that obtain encrypted data and decrypt the obtained encrypted data, said program comprising:

a terminal group determination code segment for determining 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;

a decryption key correspondence code segment for corresponding an individual decryption key with each terminal and each determined terminal group; and

a decryption key group assignment code segment for determining, for each terminal, a decryption key group based on the corresponded decryption key and decryption keys corresponded with each terminal group in which the terminal is included, as a decryption key group to be assigned to the terminal, respectively.

52. A computer-readable recording medium having stored thereon a computer program causing a computer to execute a decryption key determining processing that determines decryption key groups, for use in decryption, to be individually assigned to at least three terminals that obtain encrypted data and decrypt the obtained encrypted data, said decryption key determining processing comprising:

a terminal group determination step of determining 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;

a decryption key correspondence step of corresponding an individual decryption key with each terminal and each determined terminal group; and

a decryption key group assignment step of determining, for each terminal, a decryption key group based on the corresponded decryption key and decryption keys corresponded with each terminal group in which the terminal is included, as a decryption key group to be assigned to the terminal, respectively.

53. A decryption terminal system including three or more terminals for obtaining encrypted data and decrypting the obtained encrypted data, each terminal comprising:

a decryption key group storage unit for storing a decryption key group that has been individually assigned to the terminal according to a predetermined key assignment method;

an encrypted data obtaining unit for obtaining encrypted data; and

a decryption unit for decrypting the obtained encrypted data using by one of the stored decryption keys,

wherein the predetermined key assignment method

(a) determines 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) decides one or more decryption keys individually in correspondence with each terminal and each determined terminal group, and

(c) assigns, 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.

54. The decryption terminal system of claim 76, wherein said encrypted data obtaining unit is operable to obtain the encrypted data by reading the encrypted data from a data recording medium.

55. The decryption terminal system of claim 54, wherein:

the data recording medium has recorded thereon encryption key designation information for designating at least one encryption key;

each terminal further includes

a random number generation unit for generating key data that is a random number,

a content storage unit for storing digital content that is a digital work,

an encryption key selection unit for reading the encryption key designation information from the data recording medium, and selecting the at least one encryption key designated by the encryption key designation information, from an encryption key group that corresponds to the stored decryption key group,

a key data encryption unit for generating an encrypted data group by encrypting the generated key data successively by using all of the at least one selected encryption keys, and recording the encrypted key data group on the data recording medium, and

a content encryption unit for generating encrypted content data by encrypting the stored content data by using the generated key data, and recording the encrypted content data on the data recording medium;

said encryption data obtaining unit is operable to obtain the recorded encrypted key data and the recorded encrypted content data;

said decryption unit is operable to obtain key data by decrypting the obtained encrypted key data by using one of the stored decryption keys; and

each terminal further includes a content decryption unit for decrypting the obtained encrypted content data by using the generated key data.

56. The decryption terminal system of claim 53, wherein:

the encrypted data has been transmitted from an external transmission device; and

said encrypted data obtaining unit is operable to obtain the encrypted data by receiving the encrypted data.

76. The decryption terminal system of claim 53, wherein the predetermined key assignment method assigns, to each terminal, the decryption key decided in correspondence with the terminal and all decryption keys decided in correspondence with all terminal groups that include the terminal.

57. An encryption key designation device that designates one or more encryption keys to be used in encrypting distribution data that is distributed to three or more terminals, said encryption key designation device comprising:

a decryption key setting unit for

(a) determining 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, and

(b) deciding one or more decryption keys individually in correspondence with each terminal and each determined terminal group;

a decryption key group correspondence unit for corresponding, with each terminal, a decryption key group 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;

an invalid terminal designation unit for designating one or more terminals as invalid terminals; and

an encryption key designation unit for, when all the decryption keys corresponded with the terminals by said decryption key group correspondence unit, 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, designating encryption keys that respectively correspond to all of the valid decryption keys that are selected as a result of the procedure.

58. The encryption key designation device of claim 77, wherein said decryption key setting unit is further operable to determine the terminal groups so that at least one terminal group exists that completely includes the two or more terminal groups, and so that any one of the terminal groups within the two or more terminal groups that shares a same terminal group 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.

59. The encryption key designation device of claim 58, wherein said decryption key setting unit is further operable to determine the terminal groups so that each terminal group includes three or more terminals as members, and so that a terminal group exists that contains three or more terminal groups.

60. The encryption key designation device of claim 59, wherein:

said decryption key setting unit is operable to, when each terminal is corresponded with a node on a lowest level in an N-ary tree structure having a plurality of hierarchies, where N is a natural number equal to or greater than three,

determine, for each node other than the nodes on the lowest level of the N-ary tree structure, a plurality of combination patterns, for each one of N nodes that are reached from the each node constituting a parent node of the one of the N nodes, that are combinations of two or more of the N nodes including the one of the N nodes, and that include a combination of all of the N nodes,

decide an individual decryption key for each determined combination pattern and store each decided decryption key in correspondence with the parent node, and

further store an individual decryption key in correspondence with each node in the lowest level;

said decryption key group correspondence unit is operable to correspond decryption keys with each terminal, the decryption keys being (a) decryption keys selected from among the decryption keys stored in said decryption key setting unit in correspondence with each node on a same path, which is a path from the node corresponding to the terminal on the lowest level to a node on the highest level, excluding the node that corresponds to the terminal, and corresponding with all of the combination patterns that include a node that is one level below the node on the same path, and (b) the decryption key stored in said decryption key setting unit in correspondence with the terminal;

the terminal groups have a one-to-one correspondence with the combination patterns, and each terminal group is a group whose members are all the terminals that correspond to all nodes on the lowest level that are reached from the combined nodes in the corresponding combination pattern;

said encryption key designation unit is operable to, for the N-ary tree structure, prescribe all nodes from which a node on the lowest level that corresponds to an invalid terminal is reached as invalid nodes, perform encryption key designation processing first with a node on the highest level as a processing target node, and repeatedly perform the encryption key designation processing until all processing target nodes have been processed; and

said encryption key designation unit is operable to perform the encryption key designation processing on one processing target node at a time that has not been processed, and being processing that

(i) when a combination pattern exists that relates to a combination that includes all nodes, other than invalid nodes, one level below the processing target node, designates an encryption key that corresponds to the decryption key stored by said decryption key setting unit in correspondence with the combination pattern, and if one or more invalid nodes exists one level below the processing target node, and the one level below the processing target node is not the lowest level, newly makes all of the one or more invalid nodes that are one level below the processing target node processing target nodes, and

(ii) when a combination pattern does not exist that relates to a combination including all nodes, other than invalid nodes, one level below the processing target node, designates encryption keys stored in correspondence with all the nodes, other than the invalid nodes, by said decryption key setting unit in the level one level below if one level below is the lowest level, and if one level below is not the lowest level, newly makes all the nodes that are one level below the processing target node processing target nodes.

61. The encryption key designation device of claim 60, wherein:

said decryption key setting unit is operable to determine the plurality of combination patterns for each node excluding the nodes on the lowest level in the N-ary tree structure by prescribing combination patterns so that each combination pattern corresponds to one of all combinations of two or more of the N nodes that are one level below and are reached from the each node constituting a parent node of the N nodes;

said decryption key setting unit is operable to store, in correspondence with the parent node, all combination patterns determined for the parent node as invalidation pattern information that is made by connecting values in a predetermined order, each value showing whether or not one of the N nodes that are reached from the parent node is a target of combination, decide an individual decryption key for each invalidation pattern information, and store the individual decryption key in correspondence with the parent node and the invalidation information for which the individual decryption key has been decided;

said encryption key designation unit is operable to, for the N-ary tree structure, prescribe all nodes that are reached from a node on the lowest level of the N-ary tree structure that corresponds to an invalid terminal as invalid nodes, and after designating, for each node, excluding the nodes on the lowest level, invalidation information that shows whether or not the N nodes that are one level below and are reached from the node are invalid nodes, perform the encryption key designation processing;

said encryption key designation unit is operable to perform the encryption key designation processing on one processing target node at a time that has not been processed, and being processing that

(a) when invalidation pattern information exists that matches invalidation information designated for the processing target node, designates an encryption key that corresponds to the decryption key stored by said decryption key setting unit in correspondence with the invalidation pattern information, and if an invalid node exists one level below the processing target node, and the one level below the processing target node is not the lowest level, newly makes all invalid nodes that are one level below the processing target node processing target nodes, and

(b) when invalidation pattern information does not exist that matches invalidation information designated for the processing target node, designates encryption keys corresponding to the decryption keys stored by said decryption key setting unit in correspondence with all the nodes, other than the invalid nodes, in the level that is one level below if the one level below is the lowest level, and if the one level below is not the lowest level, newly makes all the nodes that are one level below the processing target node processing target nodes.

62. The encryption key designation device of claim 60, wherein:

said decryption key setting unit is operable to determine the plurality of combination patterns for each node, excluding the nodes on the lowest level, for the N-ary tree structure, by prescribing combination patterns so that each combination pattern corresponds to one of all combinations of all N of the nodes and N−

1 of the N nodes that are one level below and are reached from the each node constituting a parent node of the N nodes; and

said decryption key setting unit is operable to store the determined decryption keys in correspondence with the parent node.

77. The encryption key designation device of claim 57, wherein said decryption key group correspondence unit is operable to correspond, with each terminal, the decryption key decided in correspondence with the terminal and all decryption keys decided in correspondence with all terminal groups that include the terminal.

63. An encryption device for encrypting distribution data that is to be distributed to three or more terminals, said encryption device comprising:

a decryption key setting unit for

(a) determining 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, and

(b) deciding one or more decryption keys individually in correspondence with each terminal and each determined terminal group;

a decryption key group correspondence unit for corresponding, with each terminal, a decryption key group 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;

an invalid terminal designation unit for designating one or more terminals as invalid terminals;

an encryption key designation unit for, when all the decryption keys corresponded with the terminals by said decryption key group correspondence unit, other than decryption keys assigned to the one or more invalid terminals, are prescribed as valid decryption keys, and 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, designating encryption keys that respectively correspond to all of the valid decryption keys that are selected as a result of the procedure;

an encryption unit for encrypting the distribution data successively by using all designated encryption keys, so as to generate an encrypted distribution data group; and

an output unit for outputting the generated encrypted distribution data externally from said encryption device.

64. The encryption device of claim 78, wherein said output unit is operable to externally output key designation information for identifying each encryption key designated by said encryption key designation unit, together with the encrypted distribution data group.

65. The encryption device of claim 78, further comprising:

a content storage unit for storing content data that is a digital work;

a random number generation unit for generating the distribution data, the distribution data being a random number; and

a content encryption unit for encrypting the content data by using the generated distribution data as a key, so as to generate encrypted content data,

wherein said output unit is operable to externally output the generated encrypted content data together with the encrypted distribution data group.

66. The encryption device of claim 78, wherein said output unit is operable to record the encrypted distribution data group to a data recording medium.

67. The encryption device of claim 78, wherein said output unit is operable to transmit the encrypted distribution data group to each terminal.

78. The encryption device of claim 63, wherein said decryption key group correspondence unit is operable to correspond, with each terminal, the decryption key decided in correspondence with the terminal and all decryption keys decided in correspondence with all terminal groups that include the terminal.

68. An encryption key designation method that designates an encryption key to be used in encrypting distribution data that is distributed to three or more terminals, said comprising:

a terminal group determination step of determining 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;

a decryption key correspondence step of corresponding one or more decryption keys individually in correspondence with each terminal and each determined terminal group;

a decryption key group correspondence step for corresponding, with each terminal, a decryption key group based on, in addition to the decryption key corresponded with the terminal, decryption keys prescribed in correspondence with all terminal groups that include the terminal, respectively;

an invalid terminal designation step of designating one or more terminals as invalid terminals; and

an encryption key designation step of, when all the decryption keys corresponded with the terminals in said decryption key group correspondence step, other than decryption keys assigned to the one or more invalid terminals, are prescribed as valid decryption keys, and 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, designating encryption keys that respectively correspond to all of the valid decryption keys that are selected as a result of the procedure.

69. A computer program, recorded on a computer-readable medium, for causing a computer to execute a designation processing that designates one or more encryption keys to be used in encrypting distribution data that is distributed to three or more terminals, said program comprising:

a terminal group determination code segment for determining 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;

a decryption key correspondence code segment for corresponding one or more decryption keys individually in correspondence with each terminal and each determined terminal group;

a decryption key group correspondence code segment for corresponding, with each terminal, a decryption key group based on, in addition to the decryption key corresponded with the terminal, decryption keys prescribed in correspondence with all terminal groups that include the terminal, respectively;

an invalid terminal designation code segment for designating one or more terminals as invalid terminals; and

an encryption key designation code segment for designating, when all the decryption keys corresponded with the terminals in said decryption key group correspondence code segment, other than decryption keys assigned to the one or more invalid terminals, are prescribed as valid decryption keys, and 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 all of the valid decryption keys that are selected as a result of the procedure.

70. A program recording medium having stored thereon a computer program for causing a computer to execute a designation processing that designates one or more encryption keys to be used in encrypting distribution data that is distributed to three or more terminals, said designation processing comprising:

a terminal group determination step of determining 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;

a decryption key correspondence step of corresponding one or more decryption keys individually in correspondence with each terminal and each determined terminal group;

a decryption key group correspondence step of corresponding, with each terminal, a decryption key group based on, in addition to the decryption key corresponded with the terminal, decryption keys prescribed in correspondence with all terminal groups that include the terminal, respectively;

an invalid terminal designation step of designating one or more terminals as invalid terminals; and

an encryption key designation step of, when all the decryption keys corresponded with the terminals in said decryption key group correspondence step, other than decryption keys assigned to the one or more invalid terminals, are prescribed as valid decryption keys, and 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, designating encryption keys that respectively correspond to all of the valid decryption keys that are selected as a result of the procedure.

71. A computer-readable recording medium having stored thereon a plurality of pieces of encrypted distribution data that have been generated by using each of a plurality of encryption keys to encrypt distribution data that is distributed to three or more terminals, the plurality of encryption keys having been designated by a computer using a designation processing, said designation processing comprising:

a terminal group determination step of determining 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;

a decryption key correspondence step of corresponding one or more decryption keys individually in correspondence with each terminal and each determined terminal group;

a decryption key group correspondence step of corresponding, with each terminal, a decryption key group based on, in addition to the decryption key corresponded with the terminal, decryption keys prescribed in correspondence with all terminal groups that include the terminal, respectively;

an invalid terminal designation step of designating one or more terminals as invalid terminals; and

an encryption key designation step of, when all the decryption keys corresponded with the terminals in said decryption key group correspondence step, other than decryption keys assigned to the one or more invalid terminals, are prescribed as valid decryption keys, and 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, designating encryption keys that respectively correspond to all of the valid decryption keys that are selected as a result of the procedure.

72. A decryption terminal that obtains encrypted data and decrypts the obtained encrypted data, said decryption terminal comprising:

a decryption key group storage unit for storing a decryption key group that has been individually assigned to said decryption terminal according to a predetermined key assignment method;

an encrypted data obtaining unit for obtaining encrypted data; and

a decryption unit for decrypting the obtained encrypted data by using one of the stored decryption keys,

wherein the predetermined key assignment method, when there are three or more terminals including said decryption terminal,

(a) determines two or more terminal groups so that said decryption terminal belongs to a plurality of terminal groups, where each of the plurality of terminal groups includes two or more terminals as members, and the plurality of terminal groups exist so that a terminal group that includes said decryption terminal as a member does not completely include another one of the plurality of terminal groups and is not completely included in another one of the plurality of terminal groups,

(b) decides one or more individual decryption keys in correspondence with said decryption terminal and in correspondence with each determined terminal group, and

(c) assigns, to said decryption terminal, a decryption key group based on the one or more individual decryption keys decided in correspondence with said decryption terminal and one or more individual decryption keys decided in correspondence with all the terminal groups that include said decryption terminal.

73. A decryption terminal that obtains encrypted data and decrypts the obtained encrypted data, said decryption terminal comprising:

a decryption key group storage unit for storing a decryption key group that has been individually assigned to said decryption terminal according to a predetermined key assignment method;

an encrypted data obtaining unit for obtaining encrypted data; and

a decryption unit for decrypting the obtained encrypted data by using one of the stored decryption keys,

wherein the predetermined key assignment method

(a) performs a first procedure of determining, for each node other than leaves in an N-ary tree structure that has a plurality of hierarchies and whose leaves are in one-to-one correspondence with a plurality of decryption terminals that includes said decryption terminal, a plurality of combination patterns that each combine at least two of N nodes that are one level below and are reached from the each node constituting a parent node of the N nodes, deciding an individual decryption key for each determined combination pattern, and corresponding the decided decryption keys with the parent node, where N is a natural number equal to or greater than three,

(b) performs a second procedure of, for each node, other than a leaf, on a path from the leaf corresponding to said decryption terminal to a root of the N-ary tree structure, designating, from among the decryption keys corresponded with the node by the first procedure, decryption keys that correspond to all the combination patterns that relate to combinations that include a node that is one level below the node on the path, and

(c) assigns the designated decryption keys to said decryption terminal.

74. A computer-readable recording medium having stored thereon a plurality of pieces of encrypted distribution data that have been generated by using each of a plurality of encryption keys to encrypt distribution data that is distributed to three or more terminals, the plurality of encryption keys having been designated by designation processing, said designation processing comprising:

a decryption key correspondence step of determining, for each node, other than leaves, in an N-ary tree structure that has a plurality of hierarchies and whose leaves are in one-to-one correspondence with the terminals, a plurality of combination patterns that each combine at least two of N nodes that are one level below and are reached from the each node constituting a parent node of the N nodes, deciding an individual decryption key for each determined combination pattern, and corresponding the decided decryption keys with the parent node, where N is a natural number equal to or greater than three;

a decryption key group correspondence step of designating, for each terminal, decryption keys to each node, other than a leaf, on a path from the leaf corresponding to the terminal to a root of the N-ary tree structure, the decryption keys being from among the decryption keys corresponded with the node by said decryption key correspondence step and corresponding to all the combination patterns that relate to combinations that include a node that is one level below the node on the path, and corresponding all the designated decryption keys with the terminal;

an invalid terminal designation step of designating one or more terminals as invalid terminals; and

an encryption key designation step of, 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, repeating a procedure for selecting an assigned valid decryption key that is in correspondence with the most terminals not designated as invalid terminals until all terminals not designated as invalid terminals have been assigned a selected valid decryption key, and designating encryption keys that correspond to all of the valid decryption keys that are selected as a result of the procedure.