Data protection system that protects data by encrypting the data

Technical Field

The present invention relates to a data protection system that encrypts data and distributes the encrypted data to a plurality of terminals, and in particular to a technique for determining keys for use in encryption and decryption of data.

Background Art

Against a background in recent years of developments in multimedia-related technology, emergence of large-capacity recording mediums, and so on, systems have been appearing that generate digital content of moving images, audio, and the like, store the digital content on large-capacity recording mediums such as optical disks, and distribute the recording mediums.

Digital content recorded on a distributed optical disk or the like is read by a terminal such as computer or a reproduction device, and becomes a target for reproduction, copying, etc.

Usually in such a system encryption techniques are used to protect the so-called copyright of the digital content, in other words, to prevent illegal use of the digital content such as illegal copying.

Specifically, the system encrypts the digital content using a particular encryption key, records the encrypted digital content on an optical disk or the like, and distributes the optical disk. Only a terminal that holds a decryption key that corresponds to the encryption key can obtain the original digital content by using the decryption key to decrypt data read from the optical disk, and perform reproduction and the like of the digital content.

Note that methods for encrypting digital content and recording the encrypted digital content on a recording medium include a method such as encrypting the digital content itself with an encryption key that corresponds to the decryption key held by the terminal. In another method, after digital content is encrypted with a particular key and recorded on a recording medium, a decryption key that corresponds to the encryption key is encrypted with an encryption key that corresponds to the decryption key held by the terminal.

As one example of such a system, <nplcit id="ncit0001" npl-type="b"><text>National Technical Report Vol. 43, No. 3, pp. 118-122, Engineering Administration Center, Matsushita Electric Industrial Company, June 18, 1997</text></nplcit> discloses a DVD (Digital Versatile Disk) right protection system. In this DVD right protection system each DVD reproduction terminal for reproducing digital content recorded on a distributed DVD pre-stores a master key. The master key is determined by the manufacturer of the particular reproduction terminal. The reproduction terminal, which uses this master key in the decryption process, has a function of ultimately decrypting and reproducing the digital content recorded on the DVD. Note that a key group that has been encrypted with each manufacturer's master key and that is necessary for decrypting the digital content is recorded on the DVD.

Usually, the decryption key held by the terminal is kept secret. However, it is possible that analysis of the terminal by a dishonest party will lead to the decryption key being recognized and exposed.

There is a danger that once a decryption key held by a particular terminal is exposed, the dishonest party may create a terminal, software, or the like for decrypting digital content using the exposed key, and make illegal copies of the digital content. Consequently, to protect copyright it will no longer be possible to encrypt and distribute digital content using an encryption key that corresponds to the exposed decryption key.

For example, taking into consideration a DVD reproduction terminal in the above-described DVD right protection system, it will no longer be possible to distribute digital content that has been encrypted using a particular master key once that master key has been exposed.

As a result, after the master key is exposed, DVD manufacturers must use a different master key to encrypt digital content for distribution. However, a problem arises that since the DVD reproduction terminal that has been analyzed and numerous DVD reproduction terminals made by the same manufacturer all hold the same master key, these DVD reproduction terminals are unable to reproduce digital content that is newly generated, recorded, and distributed on DVD after the exposure of the master key. In other words, if one DVD reproduction terminal is analyzed by a dishonest party, there will be numerous DVD reproduction terminals that in the future will not be able to use newly generated DVDs.

One method for solving this problem is to provide a separate decryption key for each DVD reproduction terminal, encrypt digital content or the key needed for decrypting the digital content with encryption keys that correspond to the decryption key held by all the DVD reproduction terminals, and record all the encrypted data obtained as a result on the DVD. According to this method, since it is possible to record on the DVD all encrypted data that is obtained by encrypting digital content using each encryption key that corresponds to each unexposed decryption key that is held in the group of DVD reproduction terminals, even if the decryption keys of some DVD reproduction terminals are exposed, all DVD reproduction terminals except those whose keys have been exposed will still be able to use DVDs that are newly generated in the future.

However, there is a problem with this method in that when an enormous amount of DVD reproduction terminals are expected to be a target for distribution of a DVD, an enormous amount of data must be recorded on the DVD.

Disclosure of the Invention

In view of the above-described problems, the object of the present invention is to provide a data protection system that encrypts data such as digital content and keys necessary for decrypting the data, the system using an encryption technique that reduces to a degree the amount of encrypted data that is distributed, and that, when a decryption key held by a particular terminal is exposed by analyzing or the like by a dishonest party, prevents the encrypted data from being decrypted correctly by the particular terminal, but enables the encrypted data to be decrypted by other terminals; and to provide a technique that is useful in the structure of such a data protection system. <patcit id="pcit0001" dnum="EP1051036A2"><text>EP 1 051 036 A2</text></patcit> discloses a system for restricting access to transmitted programming content, which transmits a program identifier with the encrypted programming content. A set-top terminal or similar mechanism restricts access to the transmitted multimedia information using stored decryption keys. The set-top terminal receives entitlement information periodically from the head-end, corresponding to one or more packages of programs that the customer is entitled to for a given period. Each program is encrypted by the head-end server prior to transmission, using a program key, K_P, which may be unique to the program. The set-top terminal uses the received program identifier, p, together with the stored entitlement information, to derive the decryption key necessary to decrypt the program. Each of the k-bit program keys, K_P, used to encrypt transmitted programs is obtained by applying one or more pseudo-random hash functions, H, such as a length-doubling hash function, H, to a master key, m. The illustrative hash function, H, takes a k-bit binary value and produces a binary value having a length of 2k, with H₀ being the left half of the output of the hash function, and H₁ being the right half of the output of the hash function. A program key, K_P, is obtained by recursively applying a hash function, H₀ or H₁, to the master key, m, depending on the corresponding binary value of each bit position of the program identifier, p. The hash operation is represented in terms of an n-level binary tree, T, referred to as the key tree, with the master key, m, placed at the root of the tree. The tree is generated by applying the hash functions H₀ and H₁ to each node, until the desired number of tree levels (n) have been created. The program keys, K_P, correspond to the leaf nodes at the bottom level of the tree. The program identifier, p, associated with each program key, K_P, corresponds to the path through the key tree from the root to the desired leaf node.

<nplcit id="ncit0002" npl-type="b"><text>Marcel Waldvogel et Al. in their paper entitled "The VersaKey Framework: Versatile Group Key Management", IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS; IEEE SERVICE CENTER; PISCATAWA; US; vol. 17, no. 9, 1 September 1999 (1999-09-01), XP011055017, ISSN: 0733-8716</text></nplcit> describe that middleware supporting secure applications in a distributed environment faces several challenges. Scalable security in the context of multicasting or broadcasting is especially hard when privacy and authenticity is to be assured to highly dynamic groups where the application allows participants to join and leave at any time. Unicast security is well-known and has widely advanced into production state. But proposals for multicast security solutions that have been published so far are complex, often require trust in network components, or are inefficient. In this paper, the authors propose a framework of new approaches for achieving scalable security in IP multicasting. The solutions assure that newly joining members are not able to understand past group traffic and that leaving members may not follow future communication. For versatility, the framework supports a range of closely related schemes for key management, ranging from tightly centralized to fully distributed, and even allows switching between these schemes on-the-fly with low overhead. Operations have low complexity [O(log N) for joins or leaves], thus granting scalability even for very large groups. The authors also present a novel concurrency-enabling scheme, which was devised for fully distributed key management. In this paper, the authors discuss the requirements for secure multicasting, present their flexible system, and evaluate its properties based on the existing prototype implementation.

In order to achieve the above-described object, the data protection system of the present invention is a data protection system that includes three or more terminals, an encryption device, and an encryption key designation device, and protects distribution data that is to be distributed to the terminals, according to the encryption device encrypting the distribution data, wherein each terminal stores a decryption key group assigned individually to the terminal according to a predetermined key assignment method, obtains an encrypted distribution data group that has been output from the encryption device, and uses a stored decryption key to decrypt encrypted distribution data; 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 a relationship is satisfied such that any of the terminal groups that shares a same terminal as a member with another one or more of the terminal groups does not completely_include and is not completely included in the other one or more of the terminal groups, (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 decided in correspondence with the terminal and all decryption keys decided in correspondence with all terminal groups that include the terminal; the encryption key designation device designates encryption keys, and includes: invalid terminal a designation unit for designating one or more terminals as invalid terminals; an encryption key designation unit, 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 supposing that a procedure for selecting an assigned valid decryption key for the most terminals to which a selected valid decryption key is not assigned is repeated until all terminals have been assigned a selected valid decryption key, for designating encryption keys that respectively correspond to all of the valid decryption keys that are selected as a result of the procedure; and the 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.

Here, the distribution data is either recorded on a recording medium and distributed, or distributed via a wired or wireless communication path. It is assumed that the data will ultimately reach the terminals.

Supposing that there are terminals 1, 2, and 3, the terminal groups determined by the above-described predetermined key assignment method are group A consisting of terminal 1 and terminal 2, group B consisting of terminal 1 and terminal 3, and group C consisting of terminal 2 and terminal 3. As a result of the predetermined key assignment method, the decryption keys stored by terminal 1 are terminal 1's unique decryption key, a decryption key A corresponding to group A, and a decryption key B corresponding to group B. Furthermore, the decryption keys stored by terminal 2 are terminal 2's unique decryption key, the decryption key A corresponding to group A, and the decryption key C corresponding to group C, and the decryption keys stored by terminal 3 are terminal 3's unique decryption key, the decryption key B corresponding to group B, and the decryption key C corresponding to the group C. In this example, if terminal 2 is analyzed dishonestly and all of the decryption keys that it holds are exposed, terminal 2 is designated as an invalid terminal, in other words a terminal to be invalidated, and the encryption key that is designated by the encryption key designation unit is the encryption key corresponding to the decryption key B.

Accordingly, if data is distributed to each terminal that has been encrypted using the encryption key corresponding to the decryption key B, terminal 2 is not able to correctly decrypt the data, while terminal 1 and terminal 3 are able to correctly decrypt the data. To achieve the same object, it is also possible to distribute data that has been encrypted using the encryption key corresponding to terminal 1's unique decryption key and the encryption key corresponding to terminal 3's unique decryption key, but compared to this method, the method of using the encryption key corresponding to the decryption key B uses fewer encryption keys. The effect of this method is that the less encryption keys used, the smaller the amount of the encrypted data that is distributed.

In other words, according to the present invention, it is possible, in a data protection system that distributes the same encrypted data, such as the keys necessary to decrypt encrypted digital content, to a plurality of terminals to control the amount of encrypted data to be distributed. Furthermore, when a designated terminal has its decryption keys exposed, that designated terminal cannot correctly decrypt data, whereas other terminals can correctly decrypt the data.

Furthermore, the predetermined key assignment method may further perform the determining of terminal groups so that a terminal group exists that completely includes the plurality of terminal groups, and so that a relationship is satisfied such that any plurality of terminal groups within the plurality of terminal groups that shares a same terminal group as a member with another one or more of the terminal groups does not completely include and is not completely included in the other one or more of the terminal groups.

For example, if a decryption key AB is corresponded with terminal group AB that includes a terminal group A and a terminal group B, and a decryption key BC is corresponded with a terminal group BC that includes the terminal group B and a terminal group C, terminals that belong to terminal group A but do not belong to terminal groups B and C have the decryption key AB, but do not have the decryption key BC. Accordingly, even after a terminal that belongs to the terminal group A but does not belong to the terminal groups B and C is dishonestly analyzed, by encrypting data using the encryption key corresponding to the decryption key BC, it is at least possible for the terminals included in the terminal group BC, in other words the terminals included in the terminal group B and the terminals included in the terminal group C, to correctly decrypt the data using the decryption key BC. This makes it possible to encrypt the data using few encryption keys in a way that many terminals can decrypt the data correctly.

Furthermore, the predetermined key assignment method may further perform the determining of 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.

Accordingly, in distributing the same encrypted data to each terminal, the encryption of the data can be performed using a key that is common to the three or more terminals, therefore the amount of data that is distributed can be reduced compared to if each individual key is used in encryption.

Furthermore, the data protection system may include a key storage device that, when it is supposed that each terminal is corresponded with a node on a lowest level in an N-ary (N being a natural number equal to or greater than three) tree structure having a plurality of hierarchies, determines, for each node other than the nodes on the lowest level, a plurality of combination patterns, for each one of N nodes that are reached from the node (parent node), 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, decides an individual decryption key for each determined combination pattern and stores each decided decryption key in correspondence with the node (parent node), and further stores an individual decryption key in correspondence with each node in the lowest level; and a decryption key determining device that executes the predetermined key assignment method, and that determines the decryption key group that is assigned to each terminal, determines decryption keys to be assigned to each terminal, the decryption keys being (a) from among the decryption keys stored in the key storage device in correspondence with each node on a same path, the path being 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, 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 the key storage device in correspondence with the terminal, wherein 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, and the encryption key designation unit, when the tree structure is supposed, prescribes all nodes from which a node on the lowest level that corresponds to an invalid terminal is reached as invalid nodes, and performs encryption key designation processing first with a node on the highest level as a processing target node, and repeatedly performs the encryption key designation processing until all processing target nodes have been processed, wherein the encryption key designation processing is performed on one processing target node at a time that has not been processed, and is processing that (c) when a combination pattern exists that relates to a combination that includes all nodes, other that invalid nodes, one level below the processing target node, designates a decryption key that corresponds to the decryption key stored by the key storage device in correspondence with the combination pattern, (d) 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 the key storage device 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 sets all the nodes one level below, other than invalid nodes, processing target nodes, and (e) if one or more invalid nodes exists one level below the processing target node, and if the one level below is not the lowest level, newly makes all of the one or more invalid nodes processing target nodes.

In this way, information such as a decryption key is corresponded with each node in the tree structure, and the decryption keys assigned to each terminal are determined based on the information and the position of each node in tree structure.

According to the method of designating the encryption key used in encryption of distribution data, the above-described object is relatively easily achieved. In other words, a system is realized that achieves the object of controlling the amount of encrypted data to be distributed, and, when a designated terminal has its decryption keys exposed by a dishonest party, making that designated terminal unable to correctly decrypt the data, while other terminals can correctly decrypt the data.

Furthermore, the determination of the plurality of combination patterns by the key storage device for each node excluding the nodes on the lowest level when the tree structure is supposed, may be performed 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 node (parent node), and the key storage device stores the determined decryption keys in correspondence with the node (parent node).

Accordingly, in a method in which the encryption keys to be used in encryption of distribution data to the terminals is designated using an n-ary tree, the number of encryption keys can be kept relatively low, and as a result it is possible to make the amount of encrypted distribution data relatively small.

Furthermore, the determination of the plurality of combination patterns by the key storage device for each node excluding the nodes on the lowest level when the tree structure is supposed, may be performed 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 node (parent node), and the key storage device stores the determined decryption keys in correspondence with the node (parent node).

Accordingly, in a method in which the encryption keys to be used in encryption of distribution data to the terminals is designated using an n-ary tree, the number of decryption keys assigned to each terminal can be kept relatively low, and as a result it is possible to make the amount of data of the decryption key group that each terminal stores relatively small.

Furthermore, the encryption unit may output in correspondence, for each encryption key designated by the encryption key designation device, encrypted distribution data generated by encrypting using the designated encryption key, and encryption key node identification information for designating a position in the tree structure of the node that is corresponded by the key storage device with the decryption key corresponding to the encryption key, and each terminal may store the decryption key that has been individually assigned according to the predetermined key assignment method, in correspondence with decryption key node identifying information of the node with which the decryption key is corresponded, obtains the encrypted distribution data group and an encryption key node identification information group, and decrypts the encrypted distribution data that corresponds to encryption key node identification information that is stored by the terminal and that matches the decryption key node identification, using the decryption key that corresponds with the decryption key node identification information.

Accordingly, after obtaining an encrypted distribution data group, which is distribution data that has been encrypted using each of one or more encryption keys, each terminal can easily designate which of the decryption keys that it holds is to be used in decryption, by referring to the encryption key node identification information group. The time required to correctly decrypt is reduced compared to a process whereby each of the decryption keys is used successively by trial and error.

Furthermore, the encryption key designation device may include an encryption key storage unit for storing the corresponding encryption key of each decryption key that is stored by the key storage device, the corresponding encryption key and decryption key being different.

Accordingly, even if a decryption key is exposed according to dishonest analyzing or the like of a terminal, it is possible to prevent a situation in which an encryption key that is used for encrypting data so that the data can be correctly decrypted by a plurality of terminals is dishonestly known and misused.

Furthermore, the output by the encryption unit may be recording of the generated encrypted distribution data group on at least one data recording medium, and each terminal may read the encrypted distribution data from one of the at least one data recording mediums, and decrypts the encrypted distribution data.

Accordingly, data is encrypted and then recorded on an optical disk such as a DVD-ROM or another type of recording medium, therefore recording mediums having the same content as the recording medium can be mass produced and distributed to many people either for a charge or free of charge. People who receive one of the recording mediums can set their recording medium in their terminal and use the data recorded on the recording medium through the terminal.

Furthermore, the encryption unit may include: a content storage unit for storing content data that is a digital work; a random number generation unit for generating the distribution data that is a random number; a content encryption unit for encrypting the content data, using the generated distribution data as a key, to generate encrypted content data, the encryption unit generating an encrypted distribution data group by encrypting the generated distribution data successively using each of the encryption keys designated by the encryption key designation device, and recording the encrypted distribution data group and the generated encrypted content on the at least one data recording mediums, and each terminal reading the encrypted content and the encrypted distribution data group from one of the at least one recording mediums, decrypting the encrypted distribution data, and using the resulting distribution data to decrypt the encrypted content data.

Accordingly, the keys necessary for decrypting encrypted digital content such as video and audio are encrypted, and data including the encrypted digital content and the encrypted keys is recorded on the recording medium, therefore recording mediums having the same content as the recording medium can be mass produced and distributed to many people either for a charge or free of charge. People who receive one of the recording mediums can set their recording medium in their terminal and use the data recorded on the recording medium through the terminal.

Furthermore, the data protection system may further include: an encryption key designation information recording device for recording encryption key designation information that designates the encryption key designated by the encryption key designation device; each terminal including: a random number generation unit for generating the distribution data that is a random number; a content storage unit for storing content data that is a digital work; an encryption key selection unit for reading the encryption key designation from the data recording medium, and selecting the encryption key designated by the encryption key designation information, from amongst the encryption key group that corresponds to the decryption key group stored by the terminal, wherein the encryption unit encrypts distribution data by successively using all the encryption keys designated by the encryption keys designated by the encryption designation device to generate an encrypted distribution data group, and outputs the generated encrypted distribution data group.

Accordingly, in a system in which the user of a terminal can record arbitrary digital content such as video and audio on a recording medium such as a DVD-RAM, and distribute the recording medium, a designated terminal that has its decryption keys exposed by analyzing by a dishonest party cannot correctly decrypt the data, whereas other terminals can correctly decrypt the data.

Furthermore, the output according to the encryption unit may be transmission of the generated encrypted distribution data group to each terminal, and each terminal may receive the transmitted encrypted distribution data group and decrypt the received encrypted distribution data group.

Accordingly, since distribution data is encrypted and transmitted to terminals, by receiving the distribution data the terminals can easily use the distribution data.

Furthermore, the decryption key determining device of the present invention is 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, including: 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 a relationship is satisfied such that any of the terminal groups that shares a same terminal as a member with another one or more of the terminal groups does not completely include and is not completely included in the other one or more of the terminal groups, 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, the corresponded decryption key and all 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.

Furthermore, the decryption key determining method of the present invention is 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, including: a terminal group determination step 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 a relationship is satisfied such that any of the terminal groups that shares a same terminal as a member with another one or more of the terminal groups does not completely include and is not completely included in the other one or more of the terminal groups; a decryption key correspondence step for corresponding an individual decryption key with each terminal and each determined terminal group; and a decryption key group assignment step for determining, for each terminal, the corresponded decryption key and all 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.

Furthermore, the decryption terminal system of the present invention is a decryption terminal system including three or more terminals for obtaining encrypted data and decrypting the obtained encrypted data, each terminal including: 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 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 a relationship is satisfied such that any of the terminal groups that shares a same terminal as a member with another one or more of the terminal groups does not completely include and is not completely included in the other one or more of the terminal groups, (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 decided in correspondence with the terminal and all decryption keys decided in correspondence with all terminal groups that include the terminal.

Furthermore, the decryption terminal of the present invention is a decryption terminal that obtains encrypted data and decrypts the obtained encrypted data, including: 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 one of the stored decryption keys, wherein the predetermined key assignment method, (a) when it is supposed that there are three or more terminals including the terminal, determines two or more terminal groups so that the terminal belongs to a plurality of terminal groups, each of which includes two or more terminals as members, and so that the plurality of terminal groups exist so as to satisfy a relationship that a terminal group that includes the terminal as a member does not completely include and is not completely included in another of the two or more terminal groups, (b) decides one or more individual decryption keys in correspondence with the terminal and in correspondence with each determined terminal group, and (c) assigns to the terminal the one or more individual decryption keys decided in correspondence with the terminal and the one or more individual decryption keys decided in correspondence with all the terminal groups that include the terminal.

Accordingly, for example, when encrypted data is recorded on recording mediums and each recording medium distributed to a terminal, not only can the amount of data recorded on the recording medium can be suppressed, but also if a decryption key held by a designated terminal is exposed by a dishonest party, encryption can be performed so that the designated terminal cannot decrypt data correctly, while other terminals can decrypt data correctly.

Furthermore, the encrypted data obtaining unit may obtain the encrypted data by reading the encrypted data from a data recording medium Accordingly, by recording encrypted data on the data recording medium and distributing the data recording medium to the user of each terminal, the user of each terminal can use the data.

Furthermore, the data recording medium may have recorded thereon encryption key designation information for designating at least one encryption key, each terminal may further include: 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 from the stored decryption key group at least one decryption key that corresponds to the at least one encryption key designated by the encryption key designation information; a key data encryption unit for generating an encrypted data group by encrypting the generated key data successively using all of the at least one selected encryption keys, and recording the encrypted key data group on the data recording medium; a content encryption unit for generating encrypted content data by encrypting the stored content data using the generated key data, and recording the encrypted content data on the data recording medium, wherein the encryption data obtaining unit obtains the recorded encrypted key data and the recorded encrypted content data, the decryption unit obtains key data by decrypting the obtained encrypted key data using one of the stored decryption keys, and each terminal further includes: a content decryption unit for decrypting the obtained encrypted content data using the generated key data.

Accordingly, the user of each terminal can encrypt digital content such as audio and video, and record the encrypted digital content on a recording medium.

Furthermore, the encrypted data may have been transmitted from an external transmission device, and the encrypted data obtaining unit may obtain the encrypted data by receiving the encrypted data.

Accordingly, by receiving transmitted data such as digital content, each terminal can use the data easily.

Furthermore, the encryption key designation device of the present invention is 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, including: 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 a relationship is satisfied such that any of the terminal groups that shares a same terminal as a member with another one or more of the terminal groups does not completely_include and is not completely included in the other one or more of the terminal groups, 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 the decryption key decided in correspondence with the terminal and all decryption keys decided in correspondence with all terminal groups that include the terminal; an invalid terminal designation unit for designating one or more terminals as invalid terminals; and an encryption key designation unit, when all the decryption keys corresponded with the terminals by the decryption key group correspondence unit, other than decryption keys assigned to the one or more invalid terminals, are prescribed as valid decryption keys, and supposing that a procedure for selecting an assigned valid decryption key for the most terminals to which a selected valid decryption key is not assigned is repeated until all terminals have been assigned a selected valid decryption key, for designating encryption keys that respectively correspond to all of the valid decryption keys that are selected as a result of the procedure.

Furthermore, the encrypting device of the present invention is an encryption device for encrypting distribution data that is to be distributed to three or more terminals, including: 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 a relationship is satisfied such that any of the terminal groups that shares a same terminal as a member with another one or more of the terminal groups does not completely include and is not completely included in the other one or more of the terminal groups, 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 the decryption key decided in correspondence with the terminal and all decryption keys decided in correspondence with all terminal groups that include the terminal; an invalid terminal designation unit for designating one or more terminals as invalid terminals; an encryption key designation unit, when all the decryption keys corresponded with the terminals by the decryption key group correspondence unit, other than decryption keys assigned to the one or more invalid terminals, are prescribed as valid decryption keys, and supposing that a procedure for selecting an assigned valid decryption key for the most terminals to which a selected valid decryption key is not assigned is repeated until all terminals have been assigned a selected valid decryption key, for designating encryption keys that respectively correspond to all of the valid decryption keys that are selected as a result of the procedure; a encryption unit for encrypting the distribution data successively using all designated encryption keys, to generate an encrypted distribution data group; and an output unit for outputting the generated encrypted distribution data externally.

Furthermore, the encryption key designation method of the present invention is 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, including: a terminal group determination step 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 a relationship is satisfied such that any of the terminal groups that shares a same terminal as a member with another one or more of the terminal groups does not completely_include and is not completely included in the other one or more of the terminal groups; a decryption key group correspondence step for 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, in addition to the decryption key corresponded with the terminal, all decryption keys prescribed in correspondence with all terminal groups that include the terminal; an invalid terminal designation step for designating one or more terminals as invalid terminals; and an encryption key designation step, when all the decryption keys corresponded with the terminals by the decryption key group correspondence step, other than decryption keys assigned to the one or more invalid terminals, are prescribed as valid decryption keys, and supposing that a procedure for selecting an assigned valid decryption key for the most terminals to which a selected valid decryption key is not assigned is repeated until all terminals have been assigned a selected valid decryption key, for designating encryption keys that respectively correspond to all of the valid decryption keys that are selected as a result of the procedure.

Accordingly, for example, when encrypted data is recorded on recording mediums and each recording medium distributed to a terminal, not only can the number of encryption keys used in encryption be kept to a relatively small number, but also if a decryption key held by a designated terminal is exposed by a dishonest party, encryption can be performed so that the designated terminal cannot decrypt data correctly, while other terminals can decrypt data correctly.

In accordance with the present invention, the foregoing objectives are realised as defined in the independent claims. Preferred embodiments are defined in the dependent claims.

Brief Description of the Drawings

<ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Fig. 1</figref> shows an outline of the construction of a data protection system 100 of a first embodiment of the present invention;</li><li><figref idref="f0002">Fig. 2</figref> is function construction drawing of an encryption device 101 and a decryption device 103a;</li><li><figref idref="f0003">Fig. 3</figref> is a function construction drawing of a key setting system 104;</li><li><figref idref="f0004">Fig. 4</figref> shows a 4-ary tree structure;</li><li><figref idref="f0005">Fig. 5</figref> shows an example of a 4-ary tree structure when there are 64 decryption devices;</li><li><figref idref="f0006">Fig. 6</figref> shows an example of invalidation information of the root;</li><li><figref idref="f0007">Fig. 7</figref> shows an example of invalidation information of the root;</li><li><figref idref="f0008">Fig. 8</figref> shows keys assigned in correspondence with the nodes on level 0 and level 1 of a 4-ary structure;</li><li><figref idref="f0009">Fig. 9</figref> shows the structure of key information stored in a key information storage unit 301;</li><li><figref idref="f0010">Fig. 10</figref> is a flowchart showing key assignment processing executed by a decryption key determining unit 305;</li><li><figref idref="f0011">Fig. 11</figref> shows a decryption key group 905 that is determined according to the key assignment process as the decryption keys assigned to a decryption terminal (terminal 1) that corresponds to a leaf on level 3 having a relative number 1, supposing that there are only 64 decryption devices;</li><li><figref idref="f0012">Fig. 12</figref> is a flowchart showing invalidation information updating processing executed by a key information updating unit 304;</li><li><figref idref="f0013">Fig. 13</figref> is a flowchart showing key designation processing executed by an encryption key designation unit 306;</li><li><figref idref="f0014">Fig. 14</figref> shows encryption keys and the like in a state in which there are no invalid terminals, in a case in which there are only 64 decryption devices;</li><li><figref idref="f0015">Fig. 15</figref> shows encryption keys and the like in a state in which terminal 1 is an invalid terminal, in a case in which there are only 64 decryption devices;</li><li><figref idref="f0016">Fig. 16</figref> shows one example of key designation information corresponding to the encryption keys shown in <figref idref="f0015">Fig. 15</figref>;</li><li><figref idref="f0017">Fig. 17</figref> shows an example of four 4-ary tree structures when there are 64 decryption devices in a second data protection system of a second embodiment;</li><li><figref idref="f0018">Fig. 18</figref> shows encryption keys and the like in a state in which terminal 1 is an invalid terminal in the second data protection system;</li><li><figref idref="f0019">Fig. 19</figref> shows decryption keys assigned to each node in a 4-ary tree structure used in a third embodiment;</li><li><figref idref="f0020">Fig. 20</figref> shows a decryption key group 1705 assigned to a decryption terminal (terminal 1) that corresponds to a leaf in a level 3 having a relative number 1, supposing that there are only 64 decryption devices;</li><li><figref idref="f0021">Fig. 21</figref> shows encryption keys and the like in a state in which terminals 1, 2, and 17 are invalid terminals, supposing that there are only 64 decryption devices; and</li><li><figref idref="f0022">Fig. 22</figref> is an outline of the construction of a fourth data protection system in a fourth embodiment of the present invention.</li></ul>

Best Mode for Carrying Out the Invention

First Embodiment

The following describes a data protection system of a first embodiment of the present invention, with use of the drawings. &lt;Overall construction&gt;

<figref idref="f0001">Fig. 1</figref> shows an outline of the construction of a data protection system 100 of the first embodiment of the present invention.

The data protection system 100, as shown in <figref idref="f0001">Fig. 1</figref>, is composed of an encryption device 101, a plurality of decryption devices (terminals) 103a to 103n, and a key setting system 104. The data protection system 100 is for encrypting digital content consisting of digital data showing video, audio and so on, recording the encrypted digital content on one or more optical disks 102, which are DVD-ROMs or the like, and distributing the optical disks 102 to a plurality of terminals.

Here, the key setting system 104 is a system that determines an encryption key to be set in the encryption device 101 and individual decryption keys to be set in each of the decryption devices 103a to 103n.

The encryption device 101 holds the encryption key that has been designated by the key setting system 104, and is for encrypting content and recording the encrypted content on the optical disk 102. Note that it is assumed that one optical disk 102 is replicated to create multiple optical disks 102 having identical content.

Furthermore, the decryption devices 103a to 103n represent a large number of decryption terminals, for example one billion decryption terminals, and each decryption terminal holds a decryption key that has been individually determined by the key setting system 104. Each decryption terminal reads encrypted content from one of the optical disks 102, decrypts the encrypted content, and reproduces the obtained content.

Note that it is assumed that when the data protection system 100 is used for protecting copyright of content the key setting system 104 and the encryption device 101 are used by an organization that performs copyright protection. It is assumed that the decryption devices are used by general users. Furthermore, the key setting system 104 is basically used once for determining the decryption keys for each decryption device, and once for designating the encryption key to be used first. In addition, whenever it is judged that a decryption key held by a particular decryption device has been dishonestly exposed, the key setting system 104 is used for designating a new encryption key to be used in recording content to optical disks in the encryption device 101, with a purpose of preventing the particular decryption device from decrypting encrypted content recorded on an optical disk.

The following describes the encryption device 101, the decryption devices 103a to 103n, and the key setting system 104 in more detail.

<Construction of the encryption device>

<figref idref="f0002">Fig. 2</figref> is a function construction drawing of the encryption device 101 and the decryption device 103a.

As <figref idref="f0002">Fig. 2</figref> shows, the encryption device 101 has a content storage unit 201, a random number generation unit 202, an encryption key group storage unit 203, a key encryption unit 204, a content encryption unit 205, and an output unit 206.

Here, the content storage unit 201 is a storage device such as a hard disk that stores content consisting of digital data showing video, audio, etc.

The random number generation unit 202 has a function of generating a random number to be a key used in encrypting content (hereinafter "content key") . Note that the content key is 64-bit data made up of a random number.

The encryption key group storage unit 203 stores one or a plurality of encryption keys designated by the key setting system 104. Furthermore, the encryption key group storage unit 203 is a storage device such as a memory that stores key designation information that is used by the decryption device to designate the decryption key that corresponds to the stored encryption key. Note that when an encryption key is newly designated by using the key setting system 104, the encryption key that was held by the encryption key group storage unit 203 before the new encryption key was designated is deleted, and only the newly designated encryption key is stored in the encryption key storage unit 203. The storage of this new encryption key and key designation information corresponding to the new encryption key may be performed, for example, by being input by an operator, or by being received from the key setting system 104.

The key encryption unit 204 has a function of encrypting, using each encryption key stored in the encryption key storage unit 203, the content key obtained from the random number generation unit 202, and transmitting the resulting encrypted content keys to the output unit 206.

The content encryption unit 205 has a function of encrypting, using the content key obtained from the random number generation unit 202, content stored in the content storage unit 201, and conveying the resulting encrypted content to the output unit 206.

Furthermore, the output unit 206, which includes hardware that is capable of recording data on an optical disk, has a function of obtaining the key designation information from the encryption key group storage unit 203, and recording the key designation information, the encrypted content transmitted from the content encryption unit 205, and the encrypted content key transmitted from the key encryption unit 204, on the optical disk 102.

According to the recording by the encryption device 101, encrypted content, one or a plurality of encrypted content keys, and key designation information are recorded on the optical disk 102. Note that the number of encrypted content keys recorded on the optical disk 102 is equal to the number of encryption keys designated by the key setting system 104 and stored in the encryption key group storage unit 203.

The hardware of the encryption device 101 includes a CPU and a memory. All or some of the functions of the above-described random number generation unit 202, key encryption unit 204, content encryption unit 205, and output unit 206 are realized according to the CPU executing a control program that is stored in the memory.

<Construction of the decryption device>

The decryption device 103a is a terminal for reproducing optical disks, and, as shown in <figref idref="f0002">Fig. 2</figref>, has an obtaining unit 211, a decryption key group storage unit 212, a decryption key selection unit 213, a key decryption unit 214, a content decryption unit 215, and a reproduction unit 216.

Here, the obtaining unit 211 includes hardware that is capable of reading data from an optical disk. The obtaining unit 211 has a function of reading and conveying encrypted content, the encrypted content key, and the key designation information from the optical disk 102 to the content decryption unit 215, the key decryption unit 214, and the decryption key selection unit 213 respectively.

The decryption key group storage unit 212 is a <u>storage</u> device, such as a non-volatile memory, that stores a plurality of decryption keys that have been determined for the decryption device 103a by the key setting system 104, and so on. The decryption keys are stored, for example, during the decryption device manufacturing process.

The decryption key selection unit 213 has a function of judging, based on the key designation information conveyed from the obtaining unit 211, which of the decryption keys in the decryption key group stored in the decryption key group storage unit 212 can be used, and selecting one decryption key from amongst the keys decryption keys that can be used.

The key decryption unit 214 obtains, through the obtaining unit 211, the encrypted content key that can be decrypted by using the decryption key selected by the decryption key selection unit 213, and generates a content key by decrypting the obtained encrypted content key using the selected decryption key.

The content decryption unit 215 has a function of decrypting, using the content key generated by the key decryption unit 214, the encrypted content conveyed from the obtaining unit 211 to generate content, and conveying the generated content to the reproduction unit 216.

Furthermore, the reproduction unit 216 has a function of reproducing the content transmitted from the content decryption unit 215. Note that if the content treated by the data protection system 100 is, for example, moving image data compressed according to a compression method such as that specified by MPEG (Moving Picture Expert Group), it is necessary for the reproduction unit 216 to be, for example, a so-called MPEG decoder or the like and to include a function for expanding content and outputting a video signal.

The hardware of the decryption device 103a includes a CPU and a memory. All or some of the functions of the above-described obtaining unit 211, decryption key selection unit 213, key decryption unit 214, content decryption unit 215, and reproduction unit 216 are realized according to the CPU executing a control program that is stored in the memory.

Note that the other decryption devices 103b to 103n have an identical construction to the decryption device 103a. However, all or some of the contents stored in the decryption key group storage device 212 are different for each decryption device.

<Construction of the key setting system>

<figref idref="f0003">Fig. 3</figref> is a function construction drawing of the key setting system 104.

As <figref idref="f0003">Fig. 3</figref> shows, the key setting system 104 has a key information storage unit 301, a key information generation unit 302, an invalid terminal designation unit 303, a key information updating unit 304, a decryption key determining unit 305, and an encryption key designation unit 306.

Here, the key information storage unit 301 is a storage device, such as a hard disk, for storing key information that is described later.

The key information generation unit 302 determines a tree-structure so that the decryption devices in the data protection system 100 correspond to nodes on a lowest level in a 4-ary tree, assigns one or a plurality of keys to each node in the tree structure, and generates key information that shows information such as the one or a plurality of keys assigned to each node. Note that the key information is used for designating an encryption key and a decryption key, and includes invalidation information that serves as a basis for judging whether each of the keys assigned to a node can be used as an encryption key. This key information and the 4-ary tree are described in detail later.

The invalid terminal designation unit 303 has a function of receiving a specification of a decryption device whose decryption key has been exposed, from an operator via an input device such as a keyboard or a pointing device, and designating the specified decryption device as a terminal to be invalidated (hereinafter "invalid terminal"). An invalid terminal is a decryption device in regard to which it is necessary to encrypt content in a way that the encrypted content cannot be decrypted correctly by that terminal.

The key information updating unit 304 has a function of updating the invalidation information in the key information stored in the key information storage unit 301, based on the invalid terminal designated by the invalid terminal designation unit 303.

The decryption key determining unit 305 has a function of determining a plurality of decryption keys to be set for each decryption terminal, based on the key information stored in the key information storage unit 301. Note that the decryption keys determined for each decryption device are stored along with information showing the node corresponded in the key information, in the decryption key group storage unit in the decryption device. The decryption keys and the information are stored, for example, during the decryption device manufacturing process. Therefore, the key setting system 104 transmits information showing the determined decryption keys and the correspondence between the decryption keys and the node to, for example, a manufacturing system used in manufacturing the decryption device.

Furthermore, the encryption key designation unit 306 has a function of designating one or a plurality encryption keys to be set in the encryption device, based on the key information stored in the key information storage unit 301, and outputs key designation information that serves as a basis for judging which decryption key is to be used for decryption by showing the correspondence between the designated encryption key and the node. This key designation information is output together with the designated encryption key.

This output denotes, for example, transmission to the encryption device 101, or recording on a portable recording medium. Note that when the encryption key designation unit 306 records the encryption key to a portable recording medium, in actual operation, it is necessary for an operator to have the contents of the recording medium copied to the encryption key group storage unit 203 in the encryption device 101.

<Key information>

The following describes the key information that is generated by the key information generation unit 302 and stored in the key information storage unit 301.

First, the 4-ary tree is described.

<figref idref="f0004">Fig. 4</figref> shows the 4-ary tree.

The 4-ary tree is structured so that each node that composes a node group 406 that is the lowest level (hereinafter "leaf" is also used to describe each of the nodes on the lowest level) is in a one-to-one correspondence with one of the decryption devices (terminals) . The 4-ary tree has a structure in which each node branches out into four nodes. Note that here a structure in which each node branches out into n-nodes branches is called an n-ary tree. Therefore if each node in the tree structure branches out into four nodes, it is called a 4-ary tree. Furthermore, a node that branches out into four branches is called a parent node in relation to the four nodes that branch from the node, while the four nodes that branch from the parent node are called child nodes in relation to the parent node. Furthermore, a node 405 on the highest level is called a root.

When the number of decryption devices in the data protection system 100 is not a power of four, the number of nodes on the lowest level is the smallest power of four that is greater than the number of decryption devices. Here, for simplicity it is assumed that the number of decryption devices and the number of nodes on the lowest level are the same.

The key information generation unit 302 prescribes the highest level in the tree structure in <figref idref="f0004">Fig. 4</figref> as level 0, the level therebelow as level 1, and so on, successively increasing the number given to each level below by 1. The level 403 above the lowest level is prescribed as level D-1, and the lowest level 404 is prescribed as level D. Each node on each level is prescribed a relative number in the level in order starting from 1. Accordingly, the relative number 1 on level D corresponds to the decryption device 103a, the relative number 2 on level D corresponds to the decryption device 103b, and the relative number 4 to the power D on level D corresponds to the last decryption device 103n.

<figref idref="f0005">Fig. 5</figref> shows and example of a 4-ary tree when there are 64 decryption devices.

In the example in <figref idref="f0005">Fig. 5</figref>, since the 4-ary tree is constructed to have 64 leaves, the lowest level is level 3.

The following describes invalidation information that is prescribed in correspondence with each node.

The invalidation information of a node, supposing that node is a parent node, is information combining flags that show whether the child nodes are invalid nodes or not, in order from the child node having the lowest relative number. A flag has a value of 1 if the corresponding child node is an invalid node, and a value of 0 if the corresponding child node is not an invalid node. Accordingly, if the four child nodes are not invalid nodes, the invalidation information of the parent node is "0000", if the four child nodes are invalid nodes, the invalidation information of the parent node is "1111".

Note that it is assumed here that if the decryption device that corresponds to a leaf is an invalid terminal, the invalidation information of the leaf is "1111", and "0000" if the decryption device is not an invalid terminal.

Note that "invalid node" denotes a leaf that corresponds to an invalid terminal, or a node that is reached from a leaf that corresponds to an invalid terminal. Therefore, it can also be said that an invalid node is a node whose corresponding invalidation information has a value other that "0000".

Here, a node being "reached" from another node denotes, supposing there is a chain strung between each child node and its and parent node, nodes that are connected by one or a plurality of chains in either an upwards direction or a downwards direction. Accordingly, in the tree-structure, a node on the lowest level that is reached by traversing one or a plurality of chains consistently from the node on the highest level is a node that is "reached" from the top node. Conversely, the node on the highest level is a node that is "reached" from the node on the lowest level. For example, the root can be reached from any leaf, and any leaf can be reached from the root, but one leaf cannot be reached from another leaf.

The value of the invalidation information of all the nodes is "0000" before a decryption key held by any decryption device is exposed, since there are no invalid terminals.

<figref idref="f0006">Fig. 6</figref> and <figref idref="f0007">Fig. 7</figref> show examples of invalidation information of the root.

The example in <figref idref="f0006">Fig. 6</figref> shows the invalidation information of the root being "0000" when none of the child nodes of the root is invalid.

The example in <figref idref="f0007">Fig. 7</figref>, in which an invalid node is shown by a cross, shows invalidation information of the root being" 1000" when the child node that corresponds to the relative number 1 is an invalid node.

The following describes the keys assigned to each node.

A set consisting of an encryption key and a decryption key that corresponds to the encryption key is assigned individually to each node by the key information generation unit 302. Note that each leaf is assigned one set of keys unique to the corresponding decryption device, and nodes other than leafs are assigned a plurality of sets of keys as described below.

<figref idref="f0008">Fig. 8</figref> shows keys assigned corresponding to the nodes of level 0 and level 1 in a 4-ary tree.

In <figref idref="f0008">Fig. 8</figref>, an encryption key and the corresponding decryption key are expressed collectively as 0-1K0000, 0-1K0001, etc. for convenience.

Note that it is possible in the data protection system 100 to pre-determine employment of either a system in which each encryption key and the corresponding decryption key have different values or a system in which the each encryption key and the corresponding decryption key have the same value. When each encryption key and the corresponding decryption key have different values, for example, a decryption key expressed as 0-1K0000 and an encryption key expressed as 0-1K0000 have different values. When a system in which the encryption key and the decryption key have the same value is employed in the data protection system 100, an encryption key and a decryption key expressed as, for example, 0-1K0000 have the same value.

Hereinafter an expression that a decryption key is assigned to each node, or that an encryption key is assigned to each node is used. However, in reality, when employing a system in which the decryption key and the encryption key have different values, the decryption key and the corresponding encryption key are assigned to each node, while when employing the system in which the decryption key and the corresponding decryption key have the same value, a key that is both the decryption key and the encryption is assigned to each node. Accordingly, the assigned decryption key and so on are set in the key information. Note that the encryption key and the decryption key are, for example, 64-bit data.

As shown in <figref idref="f0008">Fig. 8</figref>, nodes other than leaves are assigned 11 decryption keys.

Here, "invalidation pattern" denotes values amongst the possible values "0000", "1000", etc. that the invalidation information of a node can have in which the number of "ones" is less than (n-1) in an n-ary tree structure. Consequently, there are 11 types of invalidation patterns that have less than three "ones" in the four branches: "0000", "0001", "0010", "0011", "0100", "0101", "0110", "1000", "1001", "1010", and "1100" . The nodes other than the leaves are assigned decryption keys of all the 11 invalidation patterns.

Here, a key having an invalidation pattern X and being for a node on a level A having a relative number B and is expressed as "A-BKX".

Accordingly, "0-1K0000" shows a decryption key etc. corresponding to an invalidation pattern "0000" for a node on level 0 having a relative value 1.

<figref idref="f0009">Fig. 9</figref> shows the structure of the key information stored in the key information storage unit 301.

As shown in <figref idref="f0009">Fig. 9</figref>, key information 500 is information, for each node, in which a node ID 501, an invalidation pattern 502, a key 503, and invalidation information 504 are corresponded with each node.

The node ID 501 shows a level showing the position of the node in the tree structure and the relative number of the node. For example, the node ID of a node on level A having a relative number B is expressed as "A-B".

The invalidation pattern 502, as explained earlier, is a value in which the number of "ones" in the possible values of the invalidation information is less than three.

The key 503 is the decryption key and the encryption key assigned to a node shown by the corresponding node ID.

The invalidation information 504 is invalidation information about the node shown by the corresponding node ID. The initial value is "0000".

Note that in the key information there is no invalidation pattern corresponding to leaves. The key 503 for the leaf is one set of a decryption key and an encryption key.

<Key assignment processing>

The following describes the key assignment processing in the key setting system 104 performed by the decryption key determining unit 305 for determining decryption keys to be set in each of the decryption devices 103a to 103n, in other words for assigning a plurality of keys to each decryption device, after key information has been stored in the key information storage unit 301 by the key information generation unit 302.

<figref idref="f0010">Fig. 10</figref> is a flowchart showing the key assignment processing executed by the decryption key determining unit 305.

The decryption key determining unit 305 first makes the decryption device (terminal) that corresponds to the leaf having the relative number 1 in the 4-ary tree a target of key assignment ("target terminal") (step S11), and focusing on the leaf that corresponds to the target terminal, in other words the corresponding node on the lowest level, designates one decryption key to the node ("focus node") (step S12). Note that specifically "focusing on a node" denotes, for example, storing the address in the storage area of the information of that node in the key information in a variable for internal processing.

Next, the decryption key determining unit 305 designates all the decryption keys corresponding to the invalidation pattern set in the key information stored in the key information storage unit 301 for the node (parent node) that is one level above the focus node that show that the focus node is valid, in other words that the focus node is not an invalid node, and newly prescribes the parent node as the focus node (step S13).

Continuing from step S13, the decryption key determining unit 305 judges whether the current focus node is the root (step S14), and if it is not the root, repeats the processing at step S13 until the current focus node is the root.

At step S14, if the current focus mode is the root, the decryption key determining unit 305 determines all the keys that were designated at steps S12 and S13 to be decryption keys to be set in the assignment target terminal (step S15), and judges whether the assignment target terminal is the last terminal, in other words, whether the assignment target terminal is the decryption terminal corresponding to the leaf having the highest relative number (step 16). If the assignment target terminal is the last terminal, the key assignment processing ends.

Furthermore, when it is judged at step S16 that the assignment target terminal is not the last terminal, the decryption key determining unit 305 newly prescribes the next terminal after the current assignment target terminal, in other words the decryption device that corresponds to the leaf whose relative number is one higher than the leaf corresponding to the current assignment target terminal, as the assignment target terminal (step S17), and performs the processing at step S12.

Such a key assignment processing determines the decryption key group to be set for each decryption terminal. Each decryption device is constructed to hold its decryption key group as a result of being subject to this key assignment processing.

<figref idref="f0011">Fig. 11</figref> shows a decryption key group 905 determined according to the key assignment processing, as the decryption key group assigned to a decryption device (terminal 1) that corresponds to a leaf on level 3 having the relative number 1, supposing that there are only 64 decryption devices.

Note that in <figref idref="f0011">Fig. 11</figref> 3-1K expresses a decryption key assigned only to the leaf 904 on level 3 having the relative number 1.

Supposing there are only 64 decryption devices, as shown in <figref idref="f0011">Fig. 11</figref>, the terminal 1 is assigned a total of 22 decryption keys. The decryption keys assigned to the terminal 1 are 3-1K, 2-1K0000, 2-1K0001, 2-1K0010, 2-1K0011, 2-1K0100, 2-1K0101, 2-1K0110, 1-1K0000, 1-1K0001, 1-1K0010, 1-1K0011, 1-1K0100, 1-1K0101, 1-1K0110, 0-1K0000, 0-1K0001, 0-1K0010, 0-1K0011, 0-1K0100, 0-1K0101, and 0-1K0110. The decryption key 3-1K is the decryption key assigned to the leaf 904 on level 3 having the relative number 1. The other 21 decryption keys correspond to seven invalidation patterns "0000", "0001", "0010", "0011", "0100", "0101", and "0110" that show that the first child node is not invalid. Specifically, the decryption keys 2-1K0000, 2-1K0001, 2-1K0010, 2-1K0011, 2-1K0100, 2-1K0101, 2-1K0110 are from amongst the decryption keys assigned to the node 903 on level 2 having the relative number 1 and that is the parent node of the leaf 904. The decryption keys 1-1K0000, 1-1K0001, 1-1K0010, 1-1K0011, 1-1K0100, 1-1K0101, and 1-1K0110 are from amongst the decryption keys assigned to the node 902 on level 1 having the relative number 1 and that is the parent node of the node 903. The decryption keys 0-1K0000, 0-1K0001, 0-1K0010, 0-1K0011, 0-1K0100, 0-1K0101, and 0-1K0110 are from amongst the decryption keys assigned to the node 901 on level 0 having the relative number 1 on level 0 and that is the parent node of the node 902.

Accordingly, in this case the 22 decryption keys assigned to the terminal 1 are stored in the decryption key group storage unit 212, for example, during the manufacturing process of the terminal 1.

Note that the invalidation patterns that correspond to each node, other than the leaves are information joining a series of "ones" (if a child node is an invalid node) and "zeros" (if a child node is a valid node) in order starting from the child node having the lowest relative number in the level. Assigning decryption keys to invalidation patterns equals assigning decryption keys to a terminal group whose members are all the terminals corresponding to all the leaves that are reached from all the child nodes that are shown as valid nodes in the invalidation patterns belong. Therefore, each terminal is assigned a decryption key that is unique to the terminal and decryption keys that are assigned to all of the terminals, including that terminal, in the group.

<Designation of encryption keys>

The encryption key designation unit 306 in the key setting system 104, when there are no invalid terminals, in other words when none of the decryption keys has been exposed, designates the encryption key 0-1K0000 assigned to the root, in other words the encryption key corresponding to the decryption key 0-1K0000, as the encryption key to be set in the encryption key group storage unit 203 in the encryption device 101.

Accordingly, the encryption device 101 stores in the encryption key storage unit 203 the designated encryption key, and key designation information designating the decryption key 0-1K0000 that is assigned to the root of the tree-structure. The encryption key and the key designation information are, for example, received from the key setting system 104.

When recording content on the optical disk 102, the encryption device 101 encrypts the content key generated by the random number generation unit 202 in the key encryption unit 204, using the encryption key stored in the encryption key group storage unit 203, and the output unit 206 records the encrypted content key and key designation information in correspondence on the optical disk. Furthermore, the content encryption unit 205 encrypts the content stored in the storage unit 201, using the content key, and the output unit 206 records the encrypted content on the optical disk 102.

The following describes invalidation information updating processing executed by the key information updating unit 304 in the key setting system 104.

When the invalid terminal designation unit 303 has designated an invalid terminal, the key information updating unit 304, has the invalidation information "1111" in the key information stored in the key information storage unit 301 of the leaf corresponding to the invalid terminal show that that leaf is an invalid node, and then performs the invalidation information updating processing to update the invalidation information corresponding to the nodes in the key information.

<figref idref="f0012">Fig. 12</figref> is a flowchart showing the invalidation information updating processing executed by the key information updating unit 304.

First, the key information updating unit 304 focuses on the level in the 4-ary tree that is one level above the lowest level (step S21). In other words, if the lowest level is level D, the key information updating unit 304 focuses on level (D-1).

Next, the key information updating unit 304 successively focuses on each node in the level being focused on (the focus level) in order starting from the node having the lowest relative number, and updates the invalidation information of the node that is being focused on (focus node) so that the combination pattern of the invalidation information matches, for the four child nodes of the focus node (step S22). For example, if the child nodes, in order starting from that with the lowest relative number, are "invalid node", "not invalid node", "not invalid node", and "not invalid node", the key information updating unit 304 makes the invalidation information of the focus node "1000".

After step S22, the key information updating unit 304 judges whether the current focus level is the highest level, in other words level 0 (step S23), and if not, focuses on the level one above the focus level (step 24), and repeats the processing at step S22.

In the judgement at step S23, the key information updating unit 304 repeats steps S22 to S24 for each level until the current focus level is the highest level, and ends the invalidation information updating processing when the current focus level is the highest level.

As a result, the invalidation information for all the nodes in the 4-ary tree that are reached from the leaf corresponding to the invalid terminal have a value other than "0000".

The following describes key designation processing performed by the encryption key designation unit 306 to designate the encryption key group to be set in the encryption key group storage unit 203 in the encryption device 101, after the invalid terminal is designated by the invalid terminal designation unit 303 and the invalidation information in the key information is updated by the key information updating unit 304 in the key setting system 104.

<figref idref="f0013">Fig. 13</figref> is a flowchart showing key designation processing executed by the encryption key designation unit 306.

First, the encryption key designation unit 306 focuses on the node on the highest level in the 4-ary tree, in other words, the root (step S31).

Next, the encryption key designation unit 306 refers to the key information stored in the key information storage unit 301 for the node being focused on (the focus node) to judge whether there is an invalidation pattern that matches the invalidation information of the focus node (step S32), and if there is a matching invalidation pattern, designates the encryption key corresponding to the invalidation pattern of the focus node as the encryption key to be set in the encryption device 101 (step S33). Only when it is judged that the level below the focus node is not the lowest level in the tree structure (step S34), and when an invalid node exists amongst the child nodes of the focus node, does the encryption key designation unit 306 prescribe all the invalid nodes as scheduled focus nodes (step S35).

If it is judged at step S32 that there is no invalidation pattern that matches the invalidation information, the encryption key designation unit 306 judges whether the level that the child nodes of the focus node are on is the lowest level in the tree-structure (step S36), and if so, designates the encryption keys assigned to the child nodes of the focus node other than the leaf corresponding to the invalid terminal as encryption keys to be set in the encryption device 101 (step S37).

If it is judged at step S36 that the level that the child nodes of the focus node are on is not the lowest level, the encryption key designation unit 306 sets all the child nodes of the target node as scheduled focus nodes (step S38).

After steps S35, S37, and S38, or in step 34 after it has been judged that the level one below the focus node is the lowest level, the encryption key designation unit 306 judges whether there are any scheduled focus nodes that have not been focused on yet (step S39), and if there are any, newly focuses on one of the schedule focus nodes (step S40), and returns to the judgement processing at step S32.

Furthermore, at step S39, when it is judged that there are no scheduled focus nodes that have not been focused on yet, the encryption key designation unit 306 ends the key designation processing.

As a result of the key designation processing, all the encryption keys designated by step S33 or step S37 are output from the encryption key designation unit 306 together with the key designation information, and stored in the encryption key storage unit 203 of the encryption device 101.

<figref idref="f0014">Fig. 14</figref> shows encryption keys and so on in a state in which there are no invalid terminals, supposing that there are only 64 decryption devices. In this case, the encryption key that is stored in the encryption key group storage unit 203 of the encryption device and that is used in encrypting the content key in recording content to the optical disk 102 is the encryption key 0-1K000, in other words the encryption key that corresponds to the decryption key expressed by 0-1K0000.

<figref idref="f0015">Fig. 15</figref> shows the encryption keys and so on in a state in which the terminal 1 is an invalid terminal, supposing there are only 64 decryption devices.

When the terminal 1 is the only invalid terminal, as a result of the above-described invalidation information updating processing in the key information stored in the key information storage unit 301, the invalidation information of each of a node 1103 on level 2 having a relative number 1, the invalidation information a node 1102 on level 1 having a relative number 1, and the invalidation information of the a node 1101 on level 0 having a relative number 1 becomes "1000".

The following describes, supposing the above-described situation, based on the example in <figref idref="f0015">Fig. 15</figref>, specific content of the key designation processing (see <figref idref="f0013">Fig. 13</figref>).

First, the encryption key designation unit 306 focuses on the node in the highest level, in other words the root 1101 (step S31). Then, the encryption key designation unit 306 refers to the key information stored in the key information storage unit 301, and since the invalidation information "1000" of the node 1101 matches one of the above-described eleven invalidation patterns (step S32), designates the encryption key 0-1K1000 corresponding to the matching invalidation pattern as the encryption key to be set in the encryption device 101 (step S33), and since the level one below the level of the focus node is not the lowest level (step S34), sets the node 1102, which is an invalid node amongst the child nodes of the focus node, as a scheduled focus node (step S35).

After step S35, since the node 1102 exists as a scheduled focus node that has not been focused on yet (step S39), the encryption key designation unit 306 newly prescribes the node 1102 as the focus node (step S40), and returns to the judgement processing at step S32.

Next, the encryption key designation unit 306 refers to the key information stored in the key information storage unit 301 for the focus node, and since the invalidation information "1000" for the node 1102 matches one of the above-described eleven invalidation patterns (step S32), designates the encryption key 1-1K1000 corresponding to the matching invalidation pattern as the encryption key to be set in the encryption device 101 (step S33). Since the level one below the focus node is level 2 and is not the lowest level (step S34), the encryption key designation unit 306 sets the node 1103, which is an invalid node among the child nodes of the focus node, as a scheduled focus node (step S35) .

After step S35, since the node 1103 exists as a scheduled focus node that has not been focused on yet (step S39), the encryption key designation unit 306 newly sets the node 1103 as the focus node (step S40), and returns to the judgement processing at step S32.

Next, the encryption key designation unit 306 refers to the key information stored in the key information storage unit 301 for the focus node, and since the key information "1000" for the node 1103 matches one of the above-described eleven invalidation patterns (step S32), designates the encryption key 2-1K1000 corresponding to the matching invalidation pattern as the encryption key to be set in the encryption device 101 (step S33) . Since the level 3 one below the level of the focus node is the lowest level (step S34), the encryption key designation unit 306 skips step S35, and since there are no longer any scheduled focus nodes that have not been focused on (step S39), ends the key designation processing.

The result of the key designation processing is that the encryption key group that is the encryption key group stored in the encryption key group storage unit 203 and that is used to encrypt the content key in recording content on the optical disk 102 is encryption keys 0-1K1000, 1-1K1000, and 2-1K1000.

Note that the encryption key designation unit 306 generates key designation information corresponding to each of the encryption keys designated in the above-described key designation processing, from the node ID, invalidation pattern, and so on in the key information 500 (see <figref idref="f0009">Fig. 9</figref>), and outputs the generated key designation information. This key designation information is stored in the encryption key group storage unit 203 of the encryption device 101, and is recorded on the optical disk 102 with content and so on by the encryption device 101.

<figref idref="f0016">Fig. 16</figref> shows an example of key designation information that corresponds to the encryption keys shown in <figref idref="f0015">Fig. 15</figref>.

In the example in <figref idref="f0016">Fig. 16</figref> the key designation information has a structure that combines a character string that is the node ID in the key information 500, the letter K, and the character string of the invalidation pattern.

Note that the output unit 206 in the encryption device 101 records the key designation information shown in <figref idref="f0016">Fig. 16</figref> on an optical disk in a manner that the encrypted content generated by encrypting using the encryption key 0-1K1000, the encrypted content generated by encrypting using the encryption key 1-1K1000, and the encrypted content generated by encrypting using the encryption key 2-1K1000 can be distinguished in the stated order.

<Decryption of encrypted content>

The following describes in detail the procedure for one decryption device 103n for decrypting and reproducing content from the optical disk 102. Here, encrypted content keys generated by encrypting using the encryption keys shown in <figref idref="f0015">Fig. 15</figref>, and the key designation information shown in <figref idref="f0016">Fig. 16</figref> are recorded on the optical disk. Note that the decryption device 103n has the same structure as the encryption device 103a, and only the content of the decryption key group storage unit 212 differs. Here, the various units of the decryption device 103n are described with use of the reference numbers in <figref idref="f0002">Fig. 2</figref>.

The decryption key selection unit 213 in the decryption device 103n reads the key designation information from the optical disk 102 via the obtaining unit 211, by referring to information showing the correspondence between the decryption keys held in the decryption key group storage unit 212, for example the node ID and invalidation pattern corresponding to the decryption keys in the key designation information, and the key designation information, and selects the encryption key designated in the key designation information and the decryption key that corresponds to the invalidation pattern for the node, in other words, selects the decryption key corresponding to the encryption key. The decryption key selection unit 213 extracts the selected decryption key from the decryption key group storage unit 212, and gives the extracted decryption key to the key decryption unit 214. On receiving the decryption key, the key decryption unit 214 decrypts the encrypted content key, which has been obtained through the obtaining unit 211, using the decryption key. According to this procedure, for example, if the decryption key 0-1K1000 is included in the decryption key group storage unit 212 in the decryption device 103n, the decryption device 103n uses the decryption key 0-1K000 to decrypt the encrypted content key that has been encrypted using the encryption key 0-1K1000 on the optical disk 102 on which the key designation information shown in <figref idref="f0016">Fig. 16</figref> is recorded, to obtain the content key.

After obtaining the content key, the content decryption unit 215 in the decryption device 103n decrypts the encrypted content that has been obtained via the obtaining unit 211, using the content key, to obtain content, and reproduces the obtained content in the reproduction unit 216.

Note that when the terminal 1 is the decryption device 103a, the decryption device 103a holds only the 22 keys shown in <figref idref="f0011">Fig. 11</figref>, and does not hold the decryption keys 0-1K1000, 1-1K1000, and 2-1K1000, therefore the decryption device 103a cannot correctly decrypt the encryption content keys recorded on the optical disk 102 generated by encrypting using the encryption keys shown in <figref idref="f0015">Fig. 15</figref>. Consequently, the decryption device 103a cannot correctly decrypt the encrypted content recorded on the optical disk 102, and cannot reproduce the content.

<Remarks>

When the number of decryption devices in the data protection system 100 is approximately one billion (≒4¹⁵), it is necessary to have a 4-ary tree consisting of levels 0 to 15.

In this case, supposing one decryption device is an invalid terminal, in the key designation processing by the encryption key designation unit 306 an encryption key corresponding to one invalidation pattern for each of the fifteen nodes on the path from the root to the leaf that corresponds to the invalid terminal (excluding that leaf) is designated. As a result, 15 encryption keys are used for encrypting the content key in the encryption device 101. At this time, the encrypted content and the 15 encrypted content keys, and the key designation information are recorded on the optical disk 102.

Furthermore, if for example approximately sixteen thousand (≒4⁷) of the one billion decryption devices are invalid terminals, approximately 131,072 (4^7∗(15-7)) encryption keys are used to encrypt the content key in the encryption device 101. In this case, approximately 131,072 encrypted content keys, and key designation information are recorded on the optical disk 102.

Supposing that each encrypted content key is 64 bits, in other words 8 bytes, the 131,072 encrypted content keys total approximately 1 MB. Therefore, the total amount of data of the encrypted content keys is sufficiently small in relation to the capacity of an ordinary optical disk.

The following discusses the overall data amount of encrypted content keys in cases when encryption is performed in a manner other than that of the present embodiment. <ol id="ol0001" compact="compact" ol-style=""><li>(1) If the encryption content key is 8 bytes, and supposing that there are approximately 1 billion decryption devices of which approximately 16,000 are invalid terminals, and supposing that each of the decryption devices has one decryption key that is different to all other decryption devices, and a method is used in which the content key is encrypted using each encryption key that corresponds to each decryption key held by each decryption device other than the invalid terminals, the total number of encryption keys to be recorded on the optical disk is approximately 999,984,000. This means that the total amount of encrypted content would be approximately 7600 MB. This large amount of data is not practical.</li><li>(2) Supposing the same as above, if only one decryption key is assigned to each node in the 4-ary tree that corresponds each decryption device and leaf, and each decryption device has the decryption key assigned to each node reached in an upwards direction from the leaf that corresponds to the decryption device, and that a method is employed in which content is encrypted using each encryption key assigned to all valid child nodes that are child nodes of all the nodes (in other words invalid nodes) that are reached in an upwards direction from leaves that correspond to invalid terminals. Here, the lowest level in the tree structure is level 15, the total number of encrypted content keys to be recorded on the optical disk is approximately 393,216 (4^7∗(15-7)^∗3), and the total amount of encrypted content is approximately 3 MB. This amount of data is considerably larger than that in the data protection system 100 of the present embodiment.</li><li>(3) Supposing the same as above, if only one decryption key is assigned to each node in a binary tree in which each decryption device is corresponded with a leaf, and each decryption device has the decryption key assigned to each node reached in an upwards direction from the leaf that corresponds to the decryption device, and that a method is employed in which content is encrypted using each encryption key assigned to all valid child nodes that are child nodes of all the nodes (in other words invalid nodes) that are reached in an upwards direction from leaves that correspond to invalid terminals. Here, the lowest level in the tree-structure is level 30, the total number of encrypted content of keys to the recorded on the optical disk is 262,144 (2^14∗(30-14)), the total amount of data of the encrypted content is 2 MB. This amount of data is considerably larger than that in the data protection system 100 of the present embodiment.</li></ol>

Second embodiment

The following describes a data protection system (hereinafter "second data protection system") of the second embodiment of the present invention, with use of the drawings.

The difference between the data protection system 100 and the second data protection system is that the second data protection system uses a plurality of tree structures for determining decryption keys and encryption keys.

The second data protection system basically has the same constituent features as the data protection system 100 in the first embodiment (see <figref idref="f0001 f0002 f0003">Figs. 1 to 3</figref>) . Therefore, the reference numerals used in <figref idref="f0001 f0002 f0003">Figs. 1 to 3</figref> are used in describing the constituent features of the second data protection system. Here, the description focuses on the features of the second data protection system that are different to the data protection system 100, and omits the similarities.

The specific operation content of a key information generation unit 302, a key information updating unit 304, a decryption key determining unit 305 and an encryption key designation unit 306 of the second data protection system differ to the respective corresponding units in the data protection system 100, however the basic processing content of each unit (procedures and so on shown in <figref idref="f0010">Figs. 10</figref>, <figref idref="f0012">12</figref>, and <figref idref="f0013">13</figref>) is almost the same. In the key information storage unit 301 in the second data protection system the eleven invalidation patterns decryption keys and encryption keys shown in <figref idref="f0009">Fig. 9</figref> are stored in correspondence with each node other than those on the lowest level, and one set of a decryption key and an encryption key is stored correspondence with each node on the lowest level.

In the key setting system 104 in the second data protection system, the key information generation unit 302 makes four 4-ary trees, such as those shown in <figref idref="f0017">Fig. 17</figref>. Each of the leaves in each of the 4-ary trees is made to correspond to one of the decryption terminals 103a to 103n. Consequently, there are four roots 1301 to 1304, and each decryption device corresponds to a leaf in one of the tree structures.

<figref idref="f0017">Fig. 17</figref> shows an example of four 4-ary trees when there are 64 decryption devices in the second data protection system of the second embodiment.

In this case, since the four 4-ary trees are structured so that there are 64 leaves, the lowest level in each tree is level 2.

For example, a decryption key group that is held by the terminal 1 shown in <figref idref="f0017">Fig. 17</figref> and that as been assigned according to key assigning processing (see <figref idref="f0010">Fig. 10</figref>) consists of fifteen decryption keys. Specifically, the fifteen decryption keys are 2-1K, 1-1K0000, 1-1K0001, 1-1K0010, 1-1K0011, 1-1K0100, 1-1K0101, 1-1K0110, 0-1K0000, 0-1K0001, 0-1K0010, 0-1K0011, 0-1K0100, 0-1K0101 and 0-1K0110. The decryption key 2-1K is the decryption key assigned to the leaf on level 2 having the relative number 1. The other fourteen decryption keys correspond to seven invalidation patterns "0000", "0001", "0010", "0011", "0100", "0101", and "0110" that show that the first child node is not invalid. Specifically, the seven decryption keys 1-1K0000, 1-1K0001, 1-1K0010, 1-1K0011, 1-1K0100, 1-1K0101, and 1-1K0110 are from amongst the decryption keys assigned to the node on level 1 having the relative number 1. The decryption keys 0-1K0000, 0-1K0001, 0-1K0010, 0-1K0011, 0-1K0100, 0-1K0101 and 0-1K0110 are from amongst the decryption keys assigned to the node on level 1 having the relative number 1.

Furthermore, for example, the fifteen decryption keys assigned to the terminal 17 shown in <figref idref="f0017">Fig. 17</figref> are 2-17K, 1-5K0000, 1-5K0001, 1-5K0010, 1-5K0011, 1-5K0100, 1-5K0101, 1-5K0110, 0-2K0000, 0-2K0001, 0-2K0010, 0-2K0011, 0-2K0100, 0-2K0101 and 0-2K0110. The decryption key 2-17K is the decryption key assigned to the leaf on level 2 having the relative number 17. The other fourteen decryption keys correspond to seven invalidation patterns "0000", "0001", "0010", "0011", "0100", "0101", and "0110" that show that the first child node is not invalid. Specifically, the seven decryption keys 1-5K0000, 1-5K0001, 1-5K0010, 1-5K0011, 1-5K0100, 1-5K0101, and 1-5K0110 are from amongst the decryption keys assigned to the node on level 1 having the relative number 5. The seven decryption keys 0-2K0000, 0-2K0001, 0-2K0010, 0-2K0011, 0-2K0100, 0-2K0101 and 0-2K0110 are from amongst the decryption keys assigned to the node on level 0 having the relative number 2.

Furthermore, as shown in <figref idref="f0017">Fig. 17</figref>, the encryption keys that are designated (see <figref idref="f0013">Fig. 13</figref>) by the encryption key designation unit 306 in the second data protection system when there are no invalid terminals, and that are set in the encryption device 101 and used in encrypting the content key when recording are four encryption keys 0-1K0000, 0-2K0000, 0-3K0000, and 0-4K0000.

<figref idref="f0018">Fig. 18</figref> shows the encryption keys and so on when the terminal 1 is an invalid terminal, in the second data protection system.

When the terminal 1 is the only invalid terminal, as a result of the invalidation information updating processing (see <figref idref="f0012">Fig. 12</figref>), the key information stored in the key information storage unit 301 is "1000" for the invalidation information of the node 1405 on level 1 having the relative number 1 and "1000" for the invalidation information of the node 1401 on level 0 having the relative number 1, and the encryption keys designated by key designation processing by the encryption key designation unit 306 as the encryption keys to be set in the encryption device 101 are five encryption keys 0-1K1000, 1-1K1000, 0-2K0000, 0-3K0000, and 0-4K0000.

Note that the operations of the encryption device 101 and the decryption devices 103a to 103n in the second data protection system are the same as those in the data protection system 100 in the first embodiment.

Third Embodiment

The following describes a data protection system (hereinafter "third data protection system") of a third embodiment of the present invention, with use of the drawings.

The third data protection system is characterized in that it uses invalidation patterns whose contents differ to the invalidation patterns shown in the first and second embodiments. In other respects, the third data protection system is basically the same as the data protection system 100.

The third data protection system has basically the same constituent features as the data protection system 100 in the first embodiment (see <figref idref="f0001 f0002 f0003">Figs. 1 to 3</figref>) . Therefore, the reference numerals used in <figref idref="f0001 f0002 f0003">Figs. 1 to 3</figref> are used in describing the constituent features of the third data protection system. Here, the description focuses on the features of the third data protection system that are different to the data protection system 100, and omits the similarities.

The key information storage unit 301 in the third data protection system stores an invalidation pattern and five sets of decryption keys and encryption keys in correspondence with each node in the levels other than the lowest level, and one set of a decryption key and an encryption key in correspondence with each node in the lowest level.

In the first and second embodiments "invalidation pattern" denotes a value, amongst the possible values "0000", "1000" etc. of the invalidation information of a node, in which the number of "ones" is less than (n-1) when the tree structure is an n-ary tree. However, in the third embodiment "invalidation pattern" denotes the values from amongst the possible invalidation information values in which the number of "ones" is less than two.

Consequently, there are five types of invalidation patterns: "0000", "0001", "0010", "0100", and "1000". Five groups of an encryption key and a decryption key for each of the invalidation patterns are corresponded with each node other than the leaves by the key information generation unit 302. Key information that consists of one group of an encryption key and decryption key in correspondence with each leaf is generated in correspondence with each leaf, and stored in the key information storage unit 301.

<figref idref="f0019">Fig. 19</figref> shows decryption information that is assigned to each node in the 4-ary tree used in the third embodiment.

As <figref idref="f0019">Fig. 19</figref> shows, for example, five decryption keys 0-1K0000, 0-1K0001, 0-1K0010, 0-1K0100, and 0-1K1000 are assigned to the root, and five decryption keys 1-1K0000, 1-1K0001, 1-1K0010, 1-1K0100, and 1-1K1000 are assigned to the node on level 1 having a relative number 1 on level 1.

The following describes operations of the third data protection system an example in which there are only 64 decryption devices.

The <figref idref="f0020">Fig. 20</figref> shows a decryption key group 1705 that is assigned to a decryption terminal (terminal 1) corresponding to a leaf on level 3 having a relative number 1, supposing that there are only 64 decryption devices.

The decryption key group 1705 shown in <figref idref="f0020">Fig. 20</figref> that is held in the terminal 1 and assigned according to the key assignment processing of the decryption key determining unit 305 (see <figref idref="f0010">Fig. 10</figref>) in the third data protection system, consists of thirteen decryption keys. Specifically, the thirteen decryption keys are 3-1K, 2-1K0000, 2-1K0001, 2-1K0010, 2-1K0100, 1-1K0000, 1-1K0001, 1-1K0010, 1-1K0100, 0-1K0000, 0-1K0001, 0-1K0010 and 0-1K0100. The decryption key 3-1K is the decryption key assigned to the leaf 1704 on level 3 having the relative number 1. The other twelve decryption keys correspond to four invalidation patterns "0000", "0001", "0010", and "0100" that show that the first child node in not invalid. Specifically, the decryption keys 2-1K0000, 2-1K0001, 2-1K0010 and 2-1K0100 are from amongst the decryption keys assigned to the node 1703 on level 2 having a relative number 1 and being the parent node of the leaf 1704. The decryption keys 1-1K0000, 1-1K0001, 1-1K0010 and 1-1K0100 are from amongst the decryption keys assigned to the node 1702 on level 1 having a relative number 1 and that is the parent node of the node 1703. The decryption keys 0-1K0000, 0-1K0001, 0-1K0010 and 0-1K0100 are from amongst the decryption keys assigned to the node 1701 on level 0 having a relative number 1 and that is the parent node of the node 1702. Consequently, the number of decryption keys held by each terminal in the third data protection system is less that that in the data protection system 100 in the first embodiment.

Note that the decryption key group assigned to each terminal by the decryption key determining unit 305 is stored in the decryption key group storage unit 212 of each terminal in the manufacturing process or the like.

The following describes the encryption key that is necessary in recording content and the like to the optical disk 102, when the third data protection system is actually put into operation.

When there are only 64 decryption devices and there are no invalid terminals, the encryption key that is designated according to the key designation processing of the encryption key designation unit 306 in the third data protection system, stored in the encryption key group storage 203 of the encryption device 101, and used in encrypting the content key in recording content to the optical disk 102 is the encryption key 0-1K0000, in other words one encryption key that corresponds to the decryption key 0-1K0000.

<figref idref="f0021">Fig. 21</figref> shows encryption keys and so on when terminals 1, 2, and 17 are invalid terminals, supposing there are only 64 decryption devices.

The key information in the key information storage unit 301 is updated according to the invalidation information updating processing (see <figref idref="f0012">Fig. 12</figref>) of the key information updating unit 304 in the third data protection system. Note that the invalidation information updating processing is identical to the content performed by the key information updating unit 304 of the data protection system 100 of the first embodiment. As a result, in the key information stored in the key information storage unit 301, the invalidation information of the leaves on level 3 having relative numbers 1, 2, and 17 is "1111", the invalidation information of the node 1806 on level 2 having a relative number 1 is " 1100" , the invalidation information of the node 1807 on level 2 having a relative number 5 is "1000", the invalidation information of the node 1802 on level 1 having a relative number 1 is "1000" , the invalidation information of the node 1803 on level 1 having a relative number 2 is "1000", the invalidation information of the node 1801 on level 0 having a relative number 1 is "1100", and the invalidation information of other nodes is "0000". Note that nodes whose corresponding invalidation information is "0000" are valid nodes, and other nodes are invalid nodes.

After the invalidation information updating processing, the encryption key designation unit 306 designates the encryption key according to the key designation processing (see <figref idref="f0013">Fig. 13</figref>).

The following describes a specific example of the processing content of the key designation processing based on the example showed in <figref idref="f0021">Fig. 21</figref>, with use of <figref idref="f0013">Fig. 13</figref>.

In this example the lowest level is level 3.

First, the encryption key designation unit 306 focuses on a node on the highest level, in other words the root 1801 (step S31). Next, the encryption key designation unit 306 refers to the key information stored in the key information storage unit 301 for the focus node to judge whether the invalidation information "1100" of the node 1801 matches any of the aforementioned five types of invalidation patterns (step S32). Since the invalidation information of the node 1801 does not match any of the invalidation patterns, the encryption key designation unit 306 then judges whether the level one below the node 1801 that is being focused on is the lowest level (step S36), and since the level one level below the node 1801 is not the lowest level, prescribes all the child nodes of the node 1801 as scheduled focus nodes (step S38).

According to step S38, nodes 1802 to 1805 become scheduled focus nodes.

Next, the encryption key designation unit 306 judges whether there are any scheduled focus nodes that have not yet been focused on (step S39), and since there are, focuses on the node 1802 which is one of those (step S40). Then the encryption key designation unit 306 returns to the judgement processing at step S32, and refers to the key information to judge whether the invalidation information "1000" of the node 1802 that is being focused on matches any of the aforementioned five types of invalidation patterns (step S32). Since the invalidation information "1000" does match one of the invalidation patterns, the encryption key designation unit 306 designates the encryption key 1-1K1000 that corresponds to the invalidation pattern "1000" as the encryption key to be set in the encryption device 101 (step S33). Then, since level 2, which is one level below level 1 that is being focused on, is not the lowest level (step S34), the encryption key designation unit 306 prescribes the node 1806, which is an invalid node from amongst the child nodes of the node 1802, as a scheduled focus node (step S35).

After step S35, the encryption key designation unit 306 judges whether there are any scheduled focus nodes that have not yet been focused on (step S39), and since there are, focuses on the node 1806 which is one of those (step S40). Next, the encryption key designation unit 306 returns to the judgement processing at step S32.

Next, the encryption key designation unit 306 refers to the key information to judge whether the invalidation information "1100" of the node 1806 matches any of the aforementioned five types of invalidation patterns (step S32), and since it does not, then judges whether the level one below the node 1806 that is being focused on is the lowest level (step S36). Since level 3, which is the level that is one level below the node 1806, is the lowest level, the encryption key designation unit 306 designates encryption keys 3-3K and 3-4K that correspond to the leaves 1808 and 1809 respectively, which are the valid nodes among the child nodes of the node 1806, as encryption keys to be set in the encryption device 101 (step S37). Then, the encryption key designation unit 306 judges whether there are any scheduled focus nodes that have not yet been focused on yet (step S39), and since there are, focuses on the node 1803 which is one of those, (step S40), and returns to the judgement processing at step S32.

Next, the encryption key designation unit 306 refers to the key information to judge whether the invalidation information "1000" of the node 1803 matches any of the aforementioned five invalidation patterns (step S32), and since it matches one of the invalidation patterns, designates the encryption key 1-2K1000 that corresponds to the invalidation pattern "1000" as the encryption key to be set in the encryption device 101 (step S33) . Then, since level 2, which is the level one below the node 1803 that is being focused on, is not the lowest level (step S34), the encryption key designation unit 306 prescribes the node 1807, which is an invalid node among the child nodes of the node 1803, as a scheduled focus node (step S35).

Next, the encryption key designation unit 306 judges whether there are any scheduled focus nodes that have not been focused on yet (step S39), and since there are, focuses on the node 1807 that is one of those (step S40). Then the encryption key designation unit 306 returns to the judgement processing at step S32, and refers to the invalidation information "1000" of the node 1807 that is being focused on to judges whether the invalidation information "1000" matches any of the aforementioned five invalidation patterns (step S32). Since the invalidation information " 1000" of the node 1807 matches one of the invalidation patterns, the encryption key designation unit 306 designates the encryption key 2-5K1000 that corresponds to the invalidation pattern "1000" as an encryption key to set in the encryption device 101 (step S33). Since level 3, which is the level one level below the node 1807 that is being focused on, is the lowest level (step S34), the encryption key designation unit 306 skips the processing at step S35, and judges whether there are any scheduled focus nodes that have not yet been focused on (step S39), and since there are, focuses on the node 1804 which is one of those (step S32), and returns to the judgement processing at step S32.

Next, the encryption key designation unit 306 refers to the key information to judge whether the invalidation information "0000" of the node 1804 matches any of the aforementioned five types of invalidation patterns (step S32), and designates the encryption key 1-3K0000 that corresponds to the invalidation pattern "0000" as an encryption key to be used in the encryption device 101 (step S33). Then, since level 2, which is the level below the node 1804 that is being focused on, is not the lowest level (since S34), the encryption key designation unit 306 tries to prescribe an invalid node among the child nodes of the node 1804 as a scheduled focus node (step S35). However, since all the child nodes of the node 1804 are valid nodes, no new scheduled focus nodes are prescribed.

Next, the encryption key designation unit 306 judges whether there are any scheduled focus nodes that have not yet been focused on (step S39), focuses on the node 1805 that is one of those (step S40), and returns to the judgement processing at step S32. Then, the encryption key designation unit 306 refers to the key information to judge whether the invalidation information "0000" of the node 1805 matches any of the aforementioned five invalidation patterns (step S32), and since it matches one of the invalidation patterns, designates the encryption key 1-4K0000 that corresponds to the invalidation pattern "0000"of the node 1805 as the encryption key to be set in the encryption device 101 (step S33). Since level 2, which is one level below the node 1805 that is being focused on, is not the lowest level (step S34), the encryption key designation unit 306 tries to prescribe an invalid node among the child nodes of the node 1805 as a scheduled focus node (step S35). However, since all the child nodes of the node 1805 are valid nodes, no new scheduled focus nodes are prescribed.

Next, the encryption key designation unit 306 judges whether there are any scheduled focus nodes that have not yet been on (step S39), and since there are no longer any, ends the key designation processing.

As a result of such key designation processing, seven encryption keys 1-1K1000, 1-2K1000, 1-3K0000, 1-4K0000, 2-5K1000, 3-3K, and 3-4K are designated to be set in the encryption device 101.

Note that the seven encryption keys are stored later in the encryption key group storage unit 203 in the encryption device 101, and used in encryption of the content key by the key encryption unit 204. Furthermore, each encrypted content key that is generated by encryption using each encryption key is recorded by the output unit 206 on the optical disk 102 together with the key designation information that is used to designate the decryption key that corresponds to each encryption key, and encrypted content.

As a result of the assignment by the decryption key determining unit 305 of decryption keys to each terminal, none of the decryption keys that corresponds to the seven encryption keys is held by any of the terminals 1, 2, and 7. Furthermore, the other terminals are assigned one or more of the decryption keys corresponding to the seven encryption keys.

Consequently, after content has been recorded on the optical disk 102 according to encryption processing using the seven encryption keys, decryption processing of the content cannot be performed normally using the decryption keys exposed from the terminals 1, 2, and 7. Furthermore, other terminals are able to perform decryption processing of the content normally.

Fourth Embodiment

The following describes a data protection system of a fourth embodiment (hereinafter "fourth data protection system"), with use of the drawings.

In the data protection system 100 shown in the first embodiment, the optical disk 102 on which the encryption device 101 records encrypted content, and which is distributed to the decryption deices 103a to 103n, is a DVD-ROM or the like. However, the fourth data protection system has both a prerecroded recording medium such as a DVD-ROM and a recordable medium such as a DVD-RAM of which the optical disk 102 is only the recordable medium.

In other words, in contrast to the optical disk 102 that is recordable, in the fourth data protection system certain information is recorded by the system side. The user has the terminal encrypt arbitrary content and then record the encrypted content to the optical disk 102. Then the user is able to distribute the optical disk 102. In addition, a user of the same terminal or another terminal can use their terminal to decrypt and use the content recorded on the optical disk 102. Note that here the description focuses on the features of the fourth data protection system that are different to the data protection system 100, and omits the similarities.

<figref idref="f0022">Fig. 22</figref> is an outline of the construction of the fourth data protection system of the fourth embodiment of the present invention.

The fourth data protection system, as shown in <figref idref="f0022">Fig. 22</figref>, is composed of a key designation information recording device 1501, a plurality of user data encryption devices (terminals) 1502a to 1502n, a plurality of decryption devices (terminals) 103a to 103n, a key setting system 104. It is assumed that, for example, the key setting system 104 and the key designation information recording device 1501 are used by an organization that manages copyright protection, and the terminals are used by general users.

Note that the decryption terminals 103a to 103n are the same as those shown in the first embodiment. Furthermore, all or some of the user data encryption devices 1502a to 1502n may be provided in the same terminal as all or some of the decryption devices 103a to 103n.

Furthermore, the key setting system 104 in the fourth data protection system is basically the same as that in the first embodiment, but there are some additional functions. Specifically, in the key setting system 104 in the fourth data protection system, supposing that a 4-ary tree structure in which each terminal is corresponded in advance with a leaf in the fourth data protection system, decryption keys are assigned to each terminal according to the key assignment processing shown in <figref idref="f0010">Fig. 10</figref>. It is supposed here that if an assignment target terminal is a decryption terminal that a decryption key group is assigned to the terminal, and if the assignment target terminal is a user data encryption device that an encryption key group that corresponds to the decryption key group is assigned to the terminal. Note that the key setting system 104 generates and then outputs information that shows the correspondence between the keys assigned to a terminal and the leaf in the tree structure.

For convenience, in this fourth embodiment it is assumed that each of the user data encryption devices 1502a to 1502n is provided in the same terminal as the same respective decryption device 103a to 103n. Furthermore, corresponding encryption keys and decryption keys are assumed to have the same value. Consequently, each terminal holds a key group consisting of encryption keys and decryption keys that has been assigned by the key setting system 104 in advance, and information showing the correspondence between the key group and the node in the 4-ary tree.

Furthermore, the key setting system 104 in the fourth data protection system has a further function of outputting the key designation information showing one or more encryption keys designated as a result of the invalidation information updating processing (see <figref idref="f0012">Fig. 12</figref>) and the key designation processing (see <figref idref="f0013">Fig. 13</figref>) when in operation, to the key designation information recording device 1501. For example, when there are no invalid terminals, the key designation information is "0-1K0000" only.

The key designation information recording device 1501, which includes hardware capable of recording data to an optical disk, is a device which has a function of recording key designation information input from the key setting system 104 to the optical disk 102.

Furthermore, each of the user data encryption devices 1502a to 1502n has equivalent functions to those of the encryption devices shown in the first embodiment (see <figref idref="f0002">Fig. 2</figref>). However, the user can freely store digital contents in the content storage unit 201. In addition, the content of the encryption key group storage unit 203 is the encryption key group obtained from the above-described key setting system 104 and held by the terminal, and information showing the correspondence between each encryption key and the 4-ary tree. The encryption key used in the encryption of the content key obtained by the key encryption unit 204 from the random number generation unit 202 is selected as a result of an encryption key selection processing described later. Furthermore, the output unit 206 does not record the key designation information to the optical disk 102, but instead records encrypted content and the encrypted content key to the optical disk 102.

Furthermore, each of the user data encryption devices 1502a to 1502n have a further function of reading the key designation information that has been recorded by the key designation information recording device 1501 from the optical disk 102, and performing encryption key selection processing to select the encryption key for use in encrypting the content key. This encryption key selection processing is processing for verifying the information showing the correspondence between each encryption key in the encryption key group storage unit 203 and the nodes and the key designation information, and if the same node is shown in both pieces of information, selecting the encryption key that corresponds to that node, and sending the selected encryption key to the key encryption unit 204. The encryption key selection processing is similar to the processing for selecting a decryption in the decryption key selection unit 213 of the decryption device 103a shown in the first embodiment.

Specifically, when encrypting content with the content key and recording the encrypted content to the optical disk 102, each of the user data encryption devices 1502a to 1502n has a function of encrypting the content key using the encryption key following the key designation information recorded in advance on the optical disk 102, and recording the encrypted content key on the optical disk 102.

Consequently, according to the fourth data protection system, the numerous terminals whose decryption key and so on have not been exposed cannot decrypt with a decryption key that has been exposed from another terminal. The content can be encrypted and recorded on the optical disk 102 so that it can be correctly decrypted in the numerous terminals that have decryption keys that have not been exposed.

<Supplementary Remarks>

The data protection system of the present invention has been described based on the first to fourth embodiments, but the present invention is not limited to these embodiments. Specifically: <ol id="ol0002" compact="compact" ol-style=""><li>(1) The content shown in the first to fourth embodiments is video, audio and the like but is not limited to being so. The content may be a computer program or other data, or a combination of a computer program or other data and video or the like.</li><li>(2) The decryption devices shown in the first to fourth embodiments have a reproduction unit 216 for reproducing content, but they may instead have a function of outputting decrypted content to an external device.</li><li>(3) In the first to third embodiments encrypted content is recorded on the optical disks 102 and distributed to the decryption devices. However, besides distribution on a recording medium, it is also possible to transmit content via a wireless or wired transmission path. When an embodiment in which content is transmitted is employed, it is necessary for the output unit 206 in the encryption device 101 to have hardware that has a communication function, and to transmit encrypted content, the encrypted content key, and the key designation information to each decryption device (terminal) . Furthermore, it is necessary for the obtaining unit 211 in the decryption device 103a and other decryption devices to have hardware that has a communication function, and to receive and obtain the encrypted content, the encrypted content key and the key designation information. Note that the transmission method may be, for example, one in which the encryption device 101 records to a recording medium in a server that is connected to, for example, the Internet, and the decryption device 103a and so on receive the content of the recording medium via the Internet or the like. Furthermore, the recording medium used when an embodiment in which content is transmitted is employed is not limited to being an optical disk, but may be an IC card, a flexible disk, magnetic tape, a ROM, or the like. </li><li>(4) The method of determining the invalidation pattern corresponding to each node in the key information shown in the first embodiment is simply one example. For example, it is possible to have nodes other than the root not have an invalidation pattern "0000", but instead have invalidation patterns "0111", "1101", "1011", and "1110". Here, the contents of the key assignment processing (see <figref idref="f0010">Fig. 10</figref>) and the key designation processing (see <figref idref="f0013">Fig. 13</figref>) are varied to a degree to fit these patterns. Furthermore, in the first to fourth embodiments the invalidation patterns are prescribed supposing a 4-ary tree structure. However, it is sufficient for the tree structure to have at least one part that has a structure of at least three branches, in other words one parent node in at least one level having at least three child nodes. The parent node may have three branches or five branches, or parent nodes having differing numbers of branches, for example three or four, may exist in one level. Furthermore, the number of "ones" in the invalidation patterns of each node in the 4-ary tree is limited to being less than three in the first embodiment, and less than two in the third embodiment, however, for example in a 5-ary tree structure the number of "ones" in the invalidation pattern of each node may be limited to less than two, less than three, or less than four. </li><li>(5) It is desirable for the units used in decryption in the decryption devices 103a to 103n shown in the first to the fourth embodiments and the units used in encryption in the user data encryption devices 1502a to 1502n in the fourth embodiment to be constructed according to so-called tamper-resistant techniques so that the system, data and so on used in decryption and encryption are protected.</li><li>(6) The key setting system in the embodiments outputs the decryption keys determined when determining the decryption keys to be assigned to each terminal and information showing the node in the tree structure to which the decryption key corresponds, the result of which is that the decryption devices 103a to 103n hold a key group and information showing the node that corresponds to each decryption device. However, the decryption devices do not necessarily have to hold the information showing the node that corresponds to the decryption devices. Even if the decryption devices do not hold the information showing the node that corresponds to the decryption devices, the decryption devices can decrypt content by successively trying to decrypt the encrypted content key recorded on the optical disk, using each decryption key that that decryption device holds. Note that in this case that a rule may be provided such as the head eight bits of the content key being "0", or a general digital signature may be used, so that the authenticity of a decrypted content key can be verified. Here, the decryption device uses the decrypted content key to decrypt content only when the decrypted content key is authentic.</li><li>(7) In the first embodiment the content key, the decryption keys and the encryption keys are 64 bits, but the data size of the keys is not limited to being 64 bits, and may be another number of bits. Note that in <figref idref="f0016">Fig. 16</figref> the character strings formed from a character string that is the node ID in the key information 500, the letter K, and the character string of the invalidation pattern, but the key designation information is not limited to this format.</li><li>(8) In the first to fourth embodiments the content key used in encryption of content is encrypted using encryption keys expressed as 0-1K0000 etc., but besides the content key, various data that must be kept secret may also be encrypted using these encryption keys.</li><li>(9) The invalidation information updating processing, the key assignment processing and the key designation processing (the procedures shown in <figref idref="f0010">Figs. 10</figref>, <figref idref="f0012">12</figref>, and <figref idref="f0013">13</figref>) in the key setting system 104 shown in the first to fourth embodiments may be distributed as a computer program for execution by a computer or a device that has a program execution function, either recorded on a recording medium or via various types of communication paths. Such a recording medium may be an IC card, an optical disk, a flexible disk, a ROM, or the like. The distributed computer program is used by being installed in a computer or the like that then performs invalidation information updating processing, key assignment processing and key designation processing such as that shown in the first to fourth embodiments by executing the computer program.</li></ol>

Industrial Use

The data protection system of the present embodiment can be used to protect copyright of digital works and so on in a system composed of a device for distributing a digital work of audio video and the like through a recording medium and a plurality of usage devices for reproduction and so on of distributed digital works.