Patent Publication Number: US-2010131760-A1

Title: Content using system and content using method

Description:
TECHNICAL FIELD 
     The present invention relates to a content using system and a content using method, and more particularly relates to a content using system and a content using method which use an encrypted digital content. By the way, this application is based upon and claims the benefit of priority from Japanese Patent Application No. 2007-104186, and the disclosure of Japanese Patent Application No. 2007-104186 is incorporated herein in its entirely by reference. 
     BACKGROUND ART 
     In recent years, a service and a system are spread in which an STB (Set Top Box), a computer apparatus or a mobile communication terminal obtains, reproduces and uses digital content (hereafter, referred to as content), such as video, music and the like which are delivered by a content sever. Content delivered in such a system is usually encrypted such that the content can be reproduced to be viewed or listened by only a terminal having a license to view or listen to it. DRM (Digital Rights Management) is a technique that allows only a person having such license to view or listen to it. 
     According to the DRM, content which is encrypted (hereafter, referred to as encrypted content) is provided, and a decryption key for decrypting the encrypted content is provided to a terminal having a license. The license is a right to reproduce or copy the content. Information for limiting a number of reproduction or copy and a usable period is provided to the terminal to which the license is assigned. As products in which the DRM is used, for example, there are [Windows Media Technology (registered mark)] and [Media Commerce Suite]. 
     Also, for example, as fcr a service (for example, a service for providing audition of music) in which a number of access is limited, [Isamu Teranishi, Jun Furukawa, and Kazue Sako, “k-times anonymous Authentication,” ASIACRYPT 2004, LNCS 3329, pp. 308-322. Springer-Verlag, 2004] describes a technique in which the number of access is set for each user and the user is authenticated while ensuring anonymity. 
     On the other hand, Japanese Laid Open Patent Application (JP-P 2004-280851A) describes a content providing method that provides content as a consideration for an electronic value transmitted from a mobile terminal. Also, Japanese Laid Open Patent Application (JP-P 2004-103013A) describes a conventional technique with regard to a buying, presenting and using method of an electronic merchandise coupon by using a mobile communication terminal or the like. Moreover, Japanese Laid Open Patent Application (JP-P 2005-327113A) describes a conventional technique in which content protected in copyright is provided in response to a request from a user terminal. 
     Also, Japanese Laid Open Patent Application (JP-P 2000-306005A) describes a system which uses a mobile terminal to exhibit a redemption certification or reception certification issued by a server to a fixed terminal and to obtain, from the fixed terminal, an electronic ticket that can be exchanged with an electronic value (content). Here, the fixed terminal transmits the exhibited redemption certification or reception certification to the server such that the server judges whether or not it is usable. If it is usable, the fixed terminal obtains the electronic ticket from the server and transfers the electronic ticket to the mobile terminal. Also, the redemption certification or the reception certification includes information with regard to an expiration date and the content, and is signed by the server or a provider of the content. For this reason, only when the redemption certification or reception certification is verified to be usable, a user can obtain the electronic ticket that can be exchanged with the content. However, in the obtainment of the electronic ticket, there is a risk that information with regard to the user (mobile terminal) is leaked to the fixed terminal. 
     DISCLOSURE OF INVENTION 
     The present invention proposes a method to ensure, for an audio-visual terminal that reproduces content, anonymity of a user while enabling reproduction of the content. Also, the present invention proposes a method to unitarily manage information with regard to a user using content. Moreover, the present invention proposes a method to trace an issuer of an electronic ticket for providing a right to use content. 
     A content using system according to the present invention includes a communication terminal, an audio-visual terminal and a license server. The license server transmits license information including a using condition of content to the communication terminal. The communication terminal issues an electronic ticket including the license information to which a group sign is provided. After verifying the group sign of the electronic ticket obtained from the communication terminal, the audio-visual terminal transmits the electronic ticket to the license server. After verifying the group sign of the electronic ticket transmitted from the audio-visual terminal, the license server judges based on the using condition whether or not the electronic ticket is usable. When the electronic ticket is judged to be usable, the license server assigns a license of the content to the audio-visual terminal. The audio-visual terminal decrypts the content which is encrypted by using a decryption key obtained based on the license and reproduces the content. 
     In this way, the group sign is provided to the electronic ticket according to the present invention. Thus, it is possible to certify a validity of the electronic ticket while concealing information with regard to the communication terminal from the audio-visual terminal. Also, the audio-visual terminal can make the license server, which assigns the license, to verify whether or not the electronic ticket is usable. 
     The license server is preferred to include a group database which manages a group and the communication terminal such that they are correlated with each other. The communication terminal registered in the group database obtains a member key and member certification information specific to the communication terminal. The member certification information is generated by using a group public key. The communication terminal generates the group sign by using the member key and the member certification information. Also, the audio-visual terminal verifies the group sign by using the group public key. In the content using system according to the present invention, the group is unitarily managed by the license server and only the terminal group-registered in the license server can generate the electronic ticket. 
     The license server is preferred to further include a subscriber information database which stores information with regard to a user of the communication terminal and a terminal ID of the communication terminal such that they are correlated with each other. In this case, the license server stores the terminal ID and the group in the group database such that they are correlated with each other. Consequently, the communication terminal that can provide the group sign and the information with regard to the user of the communication terminal are linked to each other. Thus, the information with regard to the user which uses the electronic ticket can be obtained. 
     The electronic ticket is preferred to include: the license information; ticket information including information specifying content which can be used based on the electronic ticket; and the group sign provided to the license information and the ticket information. In this case, the electronic ticket serves as an electronic certificate for using the content specified by the communication terminal. 
     The license server is preferred to further include a license database which registers a license ID assigned to the license information. When the license database does not register a license ID included in the electronic ticket transmitted from the audio-visual terminal, the license server does not assign the license to the audio-visual terminal. Thus, this can prevent an illegally-generated electronic ticket from being used. 
     The license information includes: content information including information with regard to one or a plurality of contents included in a product; the using condition; and a digital sign which is provided to the content information and the using condition based on a license server secret key possessed by only the license server. The using condition is preferred to include a reproducing condition of the content for the audio-visual terminal; and an issuing condition of the electronic ticket for the communication terminal. Here, the issuing condition is preferred to include information indicating a period in which or a number of times at which the electronic ticket can be issued based on the license information. Also, the reproducing condition is preferred to include information indicating a number of times at which the audio-visual terminal can reproduce content based on the electronic ticket. 
     Also, the content information is preferred to include reproducible periods of one or a plurality of contents included in the product. In this case, the license server refers to the reproducible periods included in the electronic ticket transmitted from the audio-visual terminal to judge whether or not the electronic ticket is usable. 
     According to the present invention, for an audio-visual terminal that reproduces content, it is possible to ensure anonymity of a user while enabling reproduction of the content. Also, it is possible to unitarily manage information with regard to a user using content. Moreover, it is possible to trace an issuer of an electronic ticket for providing a right to use content. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a configuration in an exemplary embodiment of a content using system according to the present invention; 
         FIG. 2  is a block diagram showing a schema of operation of the content using system according to the present invention; 
         FIG. 3  shows functions of a communication terminal program according to the present invention; 
         FIG. 4  shows functions of an audio-visual terminal program according to the present invention; 
         FIG. 5  shows a configuration of a storage device of a license server according to the present invention; 
         FIG. 6  shows functions of a license server program according to the present invention; 
         FIG. 7  shows one example of a configuration of a license database according to the present invention; 
         FIG. 8  shows one example of a configuration of a subscriber information database according to the present invention; 
         FIG. 9  is a sequence diagram showing an operation of a group registration process in the content using system according to the present invention; 
         FIG. 10  shows kinds of keys possessed by respective elements of the content using system according to the present invention; 
         FIG. 11  is a sequence diagram showing an operation from an obtainment of license information to an issue of a ticket in the content using system according to the present invention; 
         FIG. 12  shows a configuration of content information according to the present invention; 
         FIG. 13  shows a configuration of license information according to the present invention; 
         FIG. 14  shows a configuration of a ticket according to the present invention; 
         FIG. 15  is a sequence diagram showing an operation of a ticket verification process in the content using system according to the present invention; and 
         FIG. 16  is a sequence diagram showing one example of an operation from a delivery of content to a reproduction of the content in the content using system according to the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Exemplary embodiments of a content using system according to the present invention will be described below with reference to the attached drawings. The same or similar reference symbols in the drawings indicate the same, similar or equivalent elements. 
     An exemplary embodiment of a content using system according to the present invention will be described below with reference to  FIGS. 1 to 16 . The present exemplary embodiment is described by exemplifying a system in which video content provided from a content server is reproduced by an STB (Set to Box). 
     (Configuration) 
     A configuration of a first exemplary embodiment of the content using system according to the present invention will be described with reference to  FIGS. 1 to 8 .  FIG. 1  shows the configuration of the content using system according to the present invention. 
     With reference to  FIG. 1 , the content using system includes a communication terminal  10 , an audio-visual terminal  20 , a license server  30  and a content server  40 . The communication terminal  10 , the audio-visual terminal  20 , the license server  30  and the content server  40  are connected to a carrier network  100 . However, actually, the communication terminal  10 , the audio-visual terminal  20 , the license server  30  and the content server  40  are not limited to physical equipments but may be virtual machine (VM) environments installed in computers. 
     The communication terminal  10  is connected through the carrier network  100  to the license server  30  and connected through a short-range wireless communication line  200  to the audio-visual terminal  20 . The audio-visual terminal  20  is connected through a communication line  900  to the license server  30  and connected through a communication line  400  to the content server  40 . The license server  30  is connected through a communication line  300  to the content server  40 . The respective elements of the content using system according to the present invention are not required to be always connected, and they are preferred to be connected as necessary. 
     The carrier network  100  is a communication network, such as a mobile telephone communication network or a fixed telephone communication network, which is managed by a communication common carrier or an enterprise (hereafter, referred to as a carrier). 
     The license server  30  is preferred to be a computer apparatus managed by the carrier. 
     The communication terminal  10  is capable of communicating with another communication terminal  10  (not shown) by using the carrier network  100 . 
     The audio-visual terminal  20  is a video viewing apparatus exemplified as an STB. In the present exemplary embodiment, the audio-visual terminal  20  is described by using the STB as an example. However, the audio-visual terminal  20  is not limited to the STB but may be a music player, a monitor, a PDA, a mobile telephone with a displaying function, a computer apparatus, storage or the like. 
     The content server  40  is a computer apparatus that delivers or broadcasts content as products. 
     The content in the present exemplary embodiment is video data. However, the content is not limited to the video data but may be music data, image data or text data. 
     Although not shown, the content using system according to the present invention is preferred to include a plurality of communication terminals  10 , audio-visual terminals  20  and content servers  40 . 
     The schema of the content using system according to the present invention will be described below with reference to  FIG. 2 . 
     The content server  40  registers to the license server  30 , information (content information  500 ) concerning to products (including one or a plurality of contents) which are provided by the content server  40 . 
     The license server  30  transmits a right to use a product purchased by a user of the communication terminal  10 , as license information  600  to the communication terminal  10 . 
     The communication terminal  10  authorizes the license information by providing a group sign thereto to generate an electronic ticket (hereafter, referred to as a ticket  700 ) and transmits the ticket  700  to the audio-visual terminal  20 . 
     The audio-visual terminal  20  verifies the group sign of the ticket  700  to check that there is no modification. After that, in order to verify whether or not the ticket  700  is usable, the audio-visual terminal  20  transmits the ticket  700  to the license server  30 . 
     The license server  30  verifies whether or not the ticket  700  is usable based on a using condition and license ID which are included in the ticket  700 , and transmits the verification result to the audio-visual terminal  20 . At this time, if the ticket  700  is usable, the license server  30  assigns a license of the content specified by the ticket  700  to the audio-visual terminal  20  which has requested the verification. 
     The audio-visual terminal  20  to which the license is assigned, based on the using condition included in the ticket, obtains an encrypted content  800  from the content server  40  and reproduces the obtained encrypted content  800 . 
     The configuration of the content using system according to the exemplary embodiment will be described below in detail. 
     The communication terminal  10  is a communication terminal (for example, a mobile communication terminal or PDA) which is capable of being connected to the carrier network  100  by using an I/O interface  14 . 
     As shown in  FIG. 1 , the communication terminal  10  includes a CPU  11 , a RAM  12 , a storage device  13 , the I/O interface  14  and an I/O interface  15 . The CPU  11 , the RAM  12 , the storage device  13 , the I/O interface  14  and the I/O interface  15  are connected to each other through a bus. The CPU  11  controls the I/O interface  14  and the I/O interface  15  to control transmission and reception of various data. 
     The I/O interface  14  is connected through a wireless line or wired line to the carrier network  100  and controls transmission and reception of data between the communication terminal  10  and the carrier network  100 . The I/O interface  14  is preferred to be capable of being connected through the carrier network  100  to a global network such as the Internet or the like. In this case, the communication terminal  10  communicates with the license server  30  through the global network. 
     Also, the I/O interface  15  is capable of being connected through the short-range wireless communication line  200  to the audio-visual terminal  20 . The I/O interface  15  controls transmission and reception of data to and from the audio-visual terminal  20  connected through the short-range wireless communication line  200  to the I/O interface  15 . 
     The communication terminal  10  transmits the ticket  700  through the short-range wireless communication line  200  to the audio-visual terminal  20 . Here, a communication is preferable that uses a communication protocol such as HTTPS (Hypertext Transfer Protocol over Transport Layer Security), which enables a confidential communication. 
     The short-range wireless communication line  200  is preferred to be an infrared communication line, Bluetooth (registered trademark), NFC (Near Field Communication) or a communication line based on visible light communication, for example. By the way, the short-range wireless communication line  200  is required only to enable transmission and reception of data between the communication terminal  10  and the audio-visual terminal  20 . Thus, the short-range wireless communication line  200  is not limited to a wireless line but may be a wired line. Also, data transmitted or received through the short-range wireless communication line  200  may be transmitted or received through the carrier network  100 . 
     Also, the CPU  11  executes a communication terminal program  17  stored in the storage device  13  to implement respective functions shown in  FIG. 3 . 
     With reference to  FIG. 3 , the communication terminal program  17  includes a license request unit  171 , a sign verification unit  172  and a ticket generation unit  173 . In detail, the CPU  11  executes the communication terminal program  17  temporarily stored in the RAM  12  to implement the respective functions of the license request unit  171 , the sign verification unit  172  and the ticket generation unit  173 . That is, a configuration example of the communication terminal program  17  shown in  FIG. 3  indicates a configuration example of the CPU  11  when the communication terminal program  17  is executed. 
     The license request unit  171  requests the license server  30  for group subscription and issue of the license information  600  of a desired content. 
     In the content using system according to the present invention, the license information  600  is not issued to the communication terminal  10  that is not group-registered in the license server  30 . Thus, the communication terminal  10  is required to be group-registered in the license server  30  prior to the reception of the issue of the license information  600 . 
     The sign verification unit  172  uses a license server public key  93  to verify a license server sign  602  provided to the license information  600 . Also, the sign verification unit  172  uses a content server public key  96  to verify a content server sign  602  provided to the content information  500 . 
     The ticket generation unit  173  authorizes the license information  600  issued by the license server  30  to generate the ticket  700 . With reference to  FIG. 13 , the ticket generation unit  173  specifies content as a target of reproduction and a method to obtain the content as ticket information  701  for the license  500  and provides a group sign  702  to the license  500  by using a member key  94  generated at the group-registration to generate the ticket  700 . 
     The ticket information  701  includes, for example, a name, an identifier, a resolution, a method to obtain (method to download) from the content server  40  and the like of content reproduced by the audio-visual terminal  20 . 
     As mentioned above, by executing the communication terminal program  17 , the communication terminal  10  obtains the license information  600  from the license server  30  and generates the ticket  700  for reproducing the purchased content. 
     Here, the detail of the group sign  702  provided when the ticket  700  is generated is described. For example, the present invention uses a group sign method described in [G. Ateniese, J. Camenisch, M. Joye and G. Tsudik, “A Practical and Provably Secure Coalition-Resistant Group Signature Scheme”, In advances in Cryptology CRYPTO 2000, LNCS 1880, p 255-270, Springer-Verlag, 2000.]. 
     Since being group-registered in the license server  40 , the communication terminal  10  obtains the member key  94  and member certification information  95  which are specific to the communication terminal  10 . In detail, the communication terminal  10  generates a terminal public key, a terminal secret key and sign partial information from random number and a group public key  91  and transmits the terminal public key and the sign partial information to the license server  30 . 
     The license server  30  generates a digital sign by using a group secret key for sign,  90 A and the sign partial information received from the communication terminal  10  and outputs the digital sign as the member certification information  95 . 
     Since the communication terminal  10  holds the member certification information  95  signed by the license server  30 , the registration in the group is insured. 
     On the other hand, the license server  30  transmits partial information for generating the member key  94  specific to the terminal to the communication terminal  10 . 
     The communication terminal  10  generates the member key  94  specific to the communication terminal  10  by using random number generated at the group-registration, the partial information from the license server  30  and the terminal secret key corresponding to the terminal public key. 
     As mentioned above, the communication terminal  10  group-registered in the license server  4 U can obtain the member key  94  specific to the terminal and the member certification information  95 . 
     The communication terminal  10  provides the group sign to an electronic certificate by using the member key  94  and the member certification information  95 . In detail, the ticket generation unit  173  of the communication terminal  10  generates conversion data from the member key  94 , random number generated by a random number generator (not shown) and the member certification information  95 . The ticket generation unit  173  generates the group sign  702  by using the conversion data and the messages (the content information  500 , the license information  600  and the ticket information  701 ). By the way, the group sign according to the present invention is preferred to be a group sign which has a high processing speed and uses a bilinear property of Weil pairing or Tate pairing on an elliptic curve. 
     The group sign can be verified by using the group public key  91  which is common in the group. Thus, the audio-visual terminal  20  which verifies the sign cannot specify the communication terminal  10  which has provided the group sign. 
     However, the license server  30  can extract the terminal public key of the communication terminal  10  which has provided the sign from the group sign by using a group secret key for encryption,  90 B. Meanwhile, at the group-registration, the license server  30  records a terminal ID and the terminal public key of the communication terminal  10 , the digital sign provided to the member certification information  95  and the like such that they are correlated with each other. 
     For this reason, by referring to the terminal ID corresponding to the terminal public key extracted from the group sign, it is possible to specify the communication terminal  10  which has generated the ticket  700 . Also, as described later, the terminal ID is correlated with subscriber information of the user having the communication terminal  10 . Thus, the license server  30  can specify the user who has issued the ticket  700 . 
     The audio-visual terminal  20  is a computer apparatus (for example, STB, a mobile communication terminal, a computer, storage or the like) capable of being connected to the content server  40  through an I/O interface  25 . 
     As shown in  FIG. 1 , the audio-visual terminal  20  includes a CPU  21 , a RAM  22 , a storage device  23 , an I/O interface  24 , the I/O interface  25  and an I/O interface  26 . The CPU  21 , the RAM  22 , the storage device  23 , the I/O interface  24 , the I/O interface  25  and the I/O interface  26  are connected to each other through a bus. The CPU  21  controls the I/O interface  24 , the I/O interface  25 , and the I/O interface  26  to control transmission and reception of various data. 
     The I/O interface  24  is capable of being connected through, the short-range wireless communication line  200  to the communication terminal  10 . The audio-visual terminal  20  obtains the ticket  700  from the communication terminal  10  through the short-range wireless communication line  200 . Here, a communication is preferable that uses a communication protocol (for example, HTTPS) which enables a confidential communication. 
     Also, the I/O interface  25  is connected through a wireless line or a wired line to the communication line  400 , and controls transmission and reception of data between the audio-visual terminal  20  and the content server  40 . Here, the communication line  400  is preferred to be a global network such as the Internet or a broadcast line. However, the communication line  400  may be a local network, a communication line using a private line, or a communication line through the carrier network  100 . 
     Moreover, the I/O interface  26  is capable of being connected through a communication line  900  to the license server  30 . The audio-visual terminal  20  transmits the ticket  700  through the communication line  90  to the license server  30 . The communication line  900  is preferred to be a global network such as the Internet. However, the communication line  900  may be a local network, a communication line using a private line, or a communication line through the carrier network  100 . 
     Also, the CPU  21  executes an audio-visual terminal program  27  stored in the storage device  23  to implement various functions shown in  FIG. 4 . 
     With reference to  FIG. 4 , the audio-visual terminal program  27  includes a sign verification unit  271 , a verification request unit  272  and a content reproduction unit  273 . In detail, the CPU  21  executes the audio-visual terminal program  27  temporarily stored in the RAM  22  to implement the respective functions of the sign verification unit  271 , the verification request unit  272  and the content reproduction unit  273 . That is, a configuration example of the audio-visual terminal program  27  shown in  FIG. 4  indicates a configuration example of the CPU  21  when the audio-visual terminal program  27  is executed. 
     The sign verification unit  271  verifies the group sign  702 , the license server sign  602  and a content server sign  502  which are provided to the ticket  700  obtained from the communication terminal  10  by using the group public key  91 , license server public key  93  and content server public key  96  which are obtained in advance to judge a validity of the ticket  700  and the presence or absence of modifications in the ticket  700 . In order to examine whether or not the obtained ticket  700  is usable, the verification request unit  272  transmits the ticket  700  to the license server  30 . 
     Also, the verification request unit  272  obtains the verification result from the license server  30 . When the ticket  700  is judged to be usable, the verification request unit  272  obtains by using some method a decryption key for decrypting the encrypted content  800  which is obtained from the content server  40 . By the way, examples of a method to obtain the decryption key will be described later. 
     The content reproduction unit  273  reproduces the encrypted content  800  obtained from the content server  40  by using a method based on the ticket information  701  included in the ticket  700  to output the encrypted content  800  to a not-shown output device (for example, a monitor (display device)). In detail, the content reproduction unit  273  refers to the ticket information  701  included in the ticket  700  to determine the matter of the reproduction target content, an obtaining method (download method) thereof and the like. The content reproduction unit  273  obtains the reproduction target content (encrypted content  800 ) from the content server  40  by using the determined obtaining method. The content reproduction unit  273  decrypts the encrypted content  800  by using the decryption key obtained by the verification request unit  272  to reproduce the encrypted content  800 . 
     The license server  30  is a computer apparatus capable of being connected to the carrier network  100  through an I/O interface  34 . 
     As shown in  FIG. 1 , the license server  30  includes a CPU  31 , a RAM  32 , a storage device  33 , the I/O interface  34 , an I/O interface  35  and an I/O interface  36 . The CPU  31 , the RAM  32 , the storage device  33 , the I/O interface  34 , the I/O interface  35  and the I/O interface  36  are connected to each other through a bus. The CPU  31  controls the I/O interface  34 , the I/O interface  35 , and the I/O interface  36  to control transmission and reception of various data. 
     The I/O interface  34  is preferred to be capable of being connected through the carrier network  100  to a global network such as the Internet or the like. In this case, the license server  30  communicates with the communication terminal  10  through the global network. 
     Also, the I/O interface  35  is connected through the communication line  300  to the content server  40 . In detail, the I/O interface  35  is connected through a communication line to the carrier network  100  and controls transmission and reception of data between the license server  30  and the carrier network  100 . Consequently, the I/O interface  35  controls transmission and reception of data to and from the content server  40 . Here, the communication line  300  is preferred to be a local network or a communication line which uses a private line. However, the communication line  400  may be a global network such as the Internet or a communication line through carrier network  100 . 
     Moreover, the I/O interface  36  is connected through the communication line  900  to the audio-visual terminal  20 . The I/O interface  36  controls transmission and reception of data to and from the audio-visual terminal  20 . 
     With reference to  FIG. 5 , the storage device  33  includes a license server program  37 , a license database (license D/B)  38 , a group database (group D/B)  39 , a content information database (content information D/B)  50  and a subscriber information database (subscriber information D/B)  51 . 
     The CPU  31  executes the license server program  37  stored in the storage device  33  to implement various functions shown in  FIG. 6 . 
     With reference to  FIG. 6 , the license server program  37  includes a content management unit  371 , a license information generation unit  372 , a group management unit  373 , a sign verification unit  374  and a using condition verification unit  375 . In detail, the CPU  31  executes the license server program  37  temporarily stored in the RAM  32  to implement the various functions of the content management unit  371 , the license information generation unit  372 , the group management unit  373 , the sign verification unit  374  and the using condition verification unit  375 . That is, a configuration example of the license server program  37  shown in  FIG. 3  indicates a configuration example of the CPU  31  when the license server program  37  is executed. 
     The content management unit  371  records the content information  500 , which is transmitted from the content server  40 , in the content information D/B  50  and manages the content information  500 . 
     With reference to  FIG. 12 , the content information  500  includes certificate information  501  for specifying a product provided by the content server  40  and the content server sign  502  provided to the certificate information  501 . Here, the product is one or a plurality of pieces of video content data provided by the content server  40 . 
     The certificate information  501  includes product information  511  for specifying respective names and respective resolutions of all of the contents included in the product, respective reproducible periods  512  of all of the contents included in the product, and a product ID  513  specifically provided to the product. 
     The product information  511  includes, for example, names, identifiers (content IDs), resolutions and obtaining methods (download methods or broadcast channels) of all of the contents included in the product. The download method is, for example, a VoD (Video on Demand) download, a P2P (Peer to Peer) download, or the like. 
     The reproducible period  512  indicates an expiration date until which the content included in the product can be reproduced by the audio-visual terminal  20  to which the license is assigned. The reproducible period  512  is preferred to include reproducible periods  12  respectively corresponding to all of the contents included in the product. However, the reproducible period  512  may be a reproducible period with respect to the product. 
     When the user purchases the product, the content management unit  371  records the identifier of the user (the user ID) and the product ID  513  of the product in the content information D/B  50  such that they are correlated with each other. 
     The license information generation unit  372  generates and issues the license information  600  serving as the electronic certificate for assigning the license, in response to the request from the communication terminal  10 . 
     The license information generation unit  372  generates the license information  600  by setting for the content information  500  corresponding to the product requested by the communication terminal  10 , an using condition  601  based on which the communication terminal  10  issues the license, and by providing a license server sign  602  to the content information  500  by using a license server secret key  92 . 
     That is, as shown in  FIG. 13 , the license information  600  includes the content information  500 , the using condition  601  and the license server sign  602 . 
     With reference to  FIG. 13 , the using condition  601  includes: ticketing condition information  611  in which a condition for validating the license information  600  to generate the ticket  700  is set; reproducing condition information  612  in which a condition for reproducing licensed content is set; and a license ID  613  specifically assigned to the license information  600 . 
     The ticketing condition information  611  includes, for example, information defining an expiration date (ticketing expiration date) until which the communication terminal  10  can generate the ticket  700  and the number of generation of the ticket  700  (the number of ticketing). The reproducing condition information  612  includes information with regard to: the number of reproduction of the content allowed for the audio-visual terminal  20  (the maximum reproduction number); and limitation to the copy of the content. Also, the license server secret key  92  is possessed by only the license server  30 . 
     When generating the license information  600 , the license information generation unit  372  registers the license ID, which is assigned to the license information  600 , in the license D/B  38 . The license ID is preferred to be set at random such that the license ID is different for each piece of license information  600 . 
     As shown in  FIG. 7 , the license information generation unit  372  records a license ID  383 , a terminal ID of the communication terminal  10  to which the license is assigned, a product ID of a product purchased by the communication terminal  10 , a content ID of the content included in the product, and usable/unusable information for controlling usable/unusable of the product (content) in the license D/B  38  such that they are correlated with each other. 
     Consequently, the communication terminal  10  to which the license is assigned and the product as the target of the license can be managed such that they are correlated with each other. Also, the usable/unusable information is preferred to be provided for respective content ID. Thus, the license server  30  can control the usable/unusable of respective content. Of course, the usable/unusable information may be provided for respective product ID. 
     Here, the terminal ID is an identifier assigned to the communication terminal  10  that uses the carrier network  100 . The terminal ID is preferred to be a serial number which is specific to the terminal and recorded on, for example, a UIM (User Identity Module) card or a SIM (Subscriber Identity Module) card. In this case, since the terminal ID is information that can be known by only the carrier which manages the license server  30 , it is difficult that a third party specifies the user using the content. 
     The group management unit  373  registers a plurality of communication terminals  10  in the group D/B  39  such that the communication terminals  10  are grouped for predetermined categories. For example, the grouping is based on companies which provide contents used by the communication terminals  10  or based on categories corresponding to regions in which the communication terminals  10  are used by users thereof or corresponding to personal information of the users. 
     The group management unit  373  generates group secret keys  90  (group secret key for sign,  90 A and group secret key for encryption,  90 B) and group public keys  91  (group public key for sign,  91 A and group public key for encryption,  91 B) for respective group. 
     Also, the group management unit  373 , in response to a group registration request, transmits the group public keys to the communication terminal  10  and generates partial information for generating the member key  94  and the member certification information  95  specific to the communication terminal  10 . 
     The group D/B  39  records the terminal ID of the communication terminal  10 , the group secret key  90 , the group public key  91 , the member certification information  95  and the terminal public key such that they are correlated with each other. The communication terminal  10  registered in the license D/B  38  can validate the license information issued by the license server  40  and use the validated license information as the electronic certificate (ticket  700 ). 
     The sign verification unit  374  verifies the content server sign  502 , the license server sign  602  and the group sign  702  which are provided to the ticket  700  by using the content server public key  96 , the license server public key  93  and the group public key  91  which are obtained in advance. 
     The using condition verification unit  375  judges whether or not the ticket  700  is usable by referring to the using condition included in the ticket  700 . Here, whether usable or not is judged based on a delivery expiration date of the content as a target of reproduction or an expiration date of the ticket  700 , for example. 
     Also, the using condition verification unit  375  verifies whether or not the ticket  700  is generated based on the license information  600  issued by the license server  30  by referring to the license ID  613  included in the ticket  700 . At this time, when the license ID  613  is not recorded in the license D/B  38  or when the usable/unusable information corresponding to the license ID  613  is set to be unusable or already-used, the using condition verification unit  375  inhibits the use of the ticket  700 . 
     Also, the using condition verification unit  375  sets usable/unusable information corresponding to content. IDs included in the already-used tickets  700 , to be already-used. At this time, when setting all the contents included in the product to be already used, the using condition verification unit  375  deletes the license IDs corresponding to those tickets  700  from the license D/B  38 . At this time, the using condition verification unit  375  may record the change histories of the license IDs in a not-shown archive database. 
     When the license server  30  is requested to verify the unusable ticket  700 , there is a possibility that the ticket  700  is illegally generated or used by mistake. The license server  30  can specify the user who illegally uses or uses by mistake the ticket  700  by using the subscriber information D/B  51  in which subscriber information  102  is recorded for respective communication terminals  10 . 
     With reference to  FIG. 8 , the subscriber information D/B  51  includes a terminal ID  101  and the subscriber information  102 . 
     Here, the subscriber information D/B  51  records the subscriber information  102  being correlated with the terminal ID  101 . Here, the terminal ID  101  is the terminal ID registered in the SIM card and an identifier specific to the communication terminal  10 , which is provided by the carrier. The subscriber information  102  is personal information which is notified to the carrier when the user makes a contract with the carrier with respect to the use of communication. For example, the subscriber information  102  includes a name, a birth date, a sex, an occupation, an address, contact information (telephone number or e-mail address) and the like of the user. 
     The content server  40  is a computer apparatus capable of being connected through an I/O interface  44  and an I/O interface  45  to the communication line  300  and the communication line  400 , respectively. 
     As shown in  FIG. 1 , the content server  40  includes a CPU  41 , a RAM  42 , a storage device  33 , the I/O interface  44  and the I/O interface  45 . The CPU  41 , the RAM  42 , the storage device  33 , the I/O interface  44  and the I/O interface  45  are connected to each other through a bus. The CPU  41  controls the I/O interface  44  and the I/O interface  45  to control transmission and reception of various data. 
     The I/O interface  44  controls transmission and reception of data to and from the license server  30  through the communication line  300 . The I/O interface  45  controls transmission and reception of data to and from the audio-visual terminal  20  through the communication line  400 . 
     Although not shown, the storage device  43  includes a content database that records video content data as products. Also, the storage device  43  includes a content server program for generating the content information  500  and the encrypted content  800 . 
     The CPU  41  executes the content server program temporarily stored in the RAM  42  to generate the content information  500 . 
     At this time, the content server  40  provides a content server sign  502 A to certificate information  501 A by using a content server secret key to certify the certificate information. 
     Also, the content server  40  transmits the content server public key  96  to the communication terminal  10 , the audio-visual terminal  20  and the license server  30  by using some method. 
     The product provided by the content server  40  includes one or a plurality of contents. The information with regard to the product is, notified as the content information  500  to the license server  30 . 
     Also, the CPU  41  executes the content server program to encrypt the content by using the secret key to generate the encrypted content  800 , and delivers or broadcasts the encrypted content  800  to the audio-visual terminal  20  to which the license is assigned. Here, the content server  40  transmits the decryption key for decrypting the encrypted content  800  to the audio-visual terminal  20  to which the license is assigned, by using some method. 
     Based on the above-mentioned configuration, the content using system according to the present invention can realize the following points. 
     (1) The license server sign  602  certifies the matter and issuer of the license information  600  for providing a right to use the product.
 
(2) The group sign enables the sign verification of the ticket  700  while ensuring the anonymity of the communication terminal  10  from the audio-visual terminal  20 .
 
(3) Since the communication terminal  10  (terminal ID) as an entity to which an issue is performed is correlated with the license ID of the issued license information  600  in the license server  30 , traceability of the ticket  700  can be provided.
 
(4) The license server  30  can unitarily execute management, selling, verification of illegal use, and prevention of illegal use of products provided by a plurality of content servers  40 .
 
(5) By providing the using condition to the ticket  700 , a matter, a reproduction method, and a number of reproduction of content can be set arbitrarily in accordance with a matter of purchased product.
 
     (Operation) 
     The operations in the exemplary embodiment of the content using system according to the present invention will be described below in detail with reference to the drawings. The communication terminal  10  is required to be group-registered before obtaining the using right (license information  600 ) of the product from the license server  30 . 
     The operation in the group registration of the communication terminal  10  according to the present invention will be described below with reference to  FIG. 9 . 
     Here, prior to the group registration process of the communication terminal  10 , the license server  30  generates the group secret keys  90  (group secret key for sign,  90 A and group secret key for encryption,  90 B) and the group public keys  91  (group public key for sign,  91 A and group public key for encryption,  91 B) for respective group. 
     (1) Step S 1   
     The communication terminal  10  issues, to the license server  330 , a registration request to the group. 
     (2) Step S 2   
     In response to the group registration request, the license server  30  registers the communication terminal  10  to a group which is set in advance. 
     Although not shown, when the group registration request is issued from the communication terminal  10 , the group management unit  373  of the license server  30  determines the group to which the communication terminal  10  is registered based on information (for example, information with regard to an enterprise to which the communication terminal  10  is desired to be registered) included in the registration request. Then, a group management unit  471  transmits the group public keys  91  of the determined group to the communication terminal  10 . 
     The communication terminal  10  generates a terminal public key, a terminal secret key and sign partial information by using random number and the group public key  91 , and transmits the terminal public key and the sign partial information to the license server  30 . 
     The group management unit  373  generates the member certification information  95  specific to the communication terminal  10  by using sign information and the group secret key for sign,  90 A. At this time, the group management unit  373  correlates to the group, the terminal ID transmitted by the communication terminal  10  together with the group registration request. In detail, the terminal ID of the communication terminal  10 , the member certification information  95 , the group (the group secret key  90  and the group public key  91 ), and the terminal public key which is transmitted by the communication terminal  10  at the time of the group registration are stored in the group D/B  39  such that they are linked to each other. 
     By the way, at the step S 2 , the group management unit  373  can register the communication terminal  10  to either the group (group secret key  90 , group public key  94 ) which is newly set in response to the group registration request from the communication terminal  10  or an existing group. Also, the terminal ID transmitted from the communication terminal  10  is identical to the terminal ID recorded in the subscriber information D/B  51 . 
     (3) Step S 3   
     After the completion of the group registration, the license server  30  transmits to the communication terminal  10  the group public key  91 , the license server public key  93 , the member certification information  95 , and the partial information for generating the member key  94 . Here, the license server public key  93  may be distributed to the communication terminal  10  in advance. 
     (4) Step S 4   
     The communication terminal  10  generates the member key  94  specific to the communication terminal  10  by using the member certification information  95 , the partial information and the random number generated at the step S 1 . Here, the license server public key  93  may be distributed to the communication terminal  10  in advance. 
     On the other hand, the audio-visual terminal  20  obtains the group public key  91  and the license server public key  93  from the communication terminal  10 , before receiving the ticket  700 . The ticket  700  will be described bellow. Also, the communication terminal  10  and the audio-visual terminal  20  obtain the content server public key  96  by using some method. The content server public key  96  may be directly obtained from the content server  40  or may be obtained through the license server  30 . By the way, the audio-visual terminal  20  may obtain the license server public key  93  through the content server  40 . 
     As mentioned above, the communication terminal  10  is registered in the group managed by the license server  30 . Therefore, the communication terminal  10  can validate the license information  600  issued by the license server  30  to generate the ticket  700 . Also, since obtaining the group public key  91  and the license server public key  93 , the communication terminal  10  and the audio-visual terminal  20  can verify the signs of the license information  600  and the ticket  700 , respectively. 
       FIG. 10  shows states of holding keys for the respective elements in the content using system according to the present invention. Here, the states of holding keys are shown at the time when the communication terminal  10  is group-registered, generates the ticket  700  and uses the ticket  700 . 
     With reference to  FIG. 13 , the storage device  13  includes the group public key  91 , the member key  94 , the license server public key  93  and the member certification information  95 . The storage device  23  includes the group public key  91 , the license server public key  93  and the content server public key  96 . The storage device  33  includes the group secret keys  90 , the group public keys  91 , the license server secret key  92  and the license server public key  93 . By the way, in the storage device  33 , the group secret keys  90  include the group secret key for sign,  90 A and the group secret key for encryption,  90 B. The group public keys  91  include the group public key for sign,  91 A and the group public key for encryption,  91 B. 
     Based on the group registration, the communication terminal  10  obtains the group public key  91 , the member key  94 , the license server public key  93  and the member certification information  95  and stores them in the storage device  13 . Also, the communication terminal  10  stores and holds the terminal ID, which is assigned to the communication terminal  10 , in the storage device  13  or the not-shown SIM card. The audio-visual terminal  20  obtains the group public key  91 , the license server public key  93  and the content server public key  96  and stores them, in the storage device  23 . The license server  30  stores the group secret keys  90 , the group public keys  91 , the license server secret key  92  and the license server public key  93  in the storage device  33 . 
     Although not shown, the license server  30  holds the content public key  96  for verifying the content server sign  502  of the content information  500 . Moreover, the license server  30  records the terminal public key of the group-registered communication terminal  10  and the terminal ID specific to the terminal in the group D/B  39  such that they are correlated with each other. Thus, the content server  40  can specify the communication terminal  10  by using the terminal ID. 
     Next, with reference to  FIG. 11 , the operation in the exemplary embodiment will be described below in detail from the purchase of the product (content) to the issue and reception of the ticket for using the content. 
     (1) Step S 11   
     The user operates the communication terminal  10  to select the product including the content that is desired to be viewed. At this time, the user, while viewing a menu displayed on a not-shown display screen of the communication terminal  10 , operates keys to select the desired product. 
     (2) Step S 12   
     The communication terminal  10  issues a license request to the license server  30  and transmits want list information specifying the product desired to be purchased and the member certification information  95  to the license server  30 . 
     (3) Step S 13   
     The license server  30  receives the license request and generates the license information  600  corresponding to the product (product ID) specified by the want list information. In detail, at first, the license information generation unit  372  of the license server  30  refers to the member certification information  95  to check whether or not the communication terminal  10  is group-registered. 
     When the communication terminal  10  is group-registered, the license information generation unit  372  extracts the content information  600  corresponding to the specified product from the content information D/B  50 . Then, the license information generation unit  372  provides by using license server secret key  92  the license server sign  602  to information including the content information  500  to which the using condition  601  is added, and thus generates the license information  600 . 
     (4) Step S 14   
     The license server  30  issues the generated license information  600  through the carrier network  100  to the communication terminal  10  as a requester. 
     (5) Step S 15   
     At this time, the license server  30  registers in the license D/B  38  the license ID  613  assigned to the issued license information  600 . 
     With reference to  FIG. 7 , the license ID registered in the license D/B  38  is an identifier specifically assigned to the license information  600 . Usually, one product ID is assigned to one product. The product includes one or a plurality of contents, and specific content IDs are provided to respective contents in accordance with matters of the contents. 
     Also, the contents in the product are respectively correlated to pieces of usable/unusable information for respectively controlling whether or not the contents are usable. For example, when communication terminal  10  (of which terminal ID is “1111”) purchases a product (of which product ID is “Product A”) including contents A, B and C, a license ID of “123456” assigned to the license information  600 , the terminal ID of “1111” and the product ID of “Product A” are correlated with each other. The product A includes the plurality of contents A, B and C which are respectively correlated with pieces of usable/unusable information. In this case, all of the contents in the product are correlated with the pieces of usable/unusable information. By the way, the number of the contents included in the product may be one (for example, a product B). 
     Also, a piece of usable/unusable information may be provided per a product in place of being provided per a content. For example, as in the case of a product corresponding to a license ID of “123123”, the product is correlated with a piece of usable/unusable information. Moreover, the number of products that can be purchased by the communication terminal  10  may be plural. The usable/unusable information includes a flag setting content or product, which corresponds to the license information  600 , to be usable or unusable, or includes a flag indicating that the content or product is already-used. The sign verification unit  374  or the using condition verification unit  375  sets the flag of the usable/unusable information based on the verification result of the ticket  700 . 
     Also, the carrier may set the flag of the usable/unusable information without depending on the using condition provided to the ticket  700 . For example, when the license assigned to the communication terminal  10  is deprived, the usable/unusable information corresponding to the license information  600  as the target of the deprivation is set to be unusable. 
     By the way, a method may be used which controls the product or contents included in the product not to be used by deleting the license ID or the content ID. 
     (6) Step S 16   
     When receiving the license information  600 , the communication terminal  10  verifies the license server sign  602  by using the license server public key  93 . Consequently, it is confirmed that the license information  600  is validly issued by the license server  30 . 
     (7) Step S 17   
     If the sign passes the verification, the communication terminal  10  generates the ticket  700  that functions as an electronic certificate for using the content, based on the license information  600 . 
     Although not shown, the content information  500  and the using condition  601  that are included in the license information  600  are displayed on the display screen of the communication terminal  10 . The user checks the using condition of the purchased product and the matters of the contents on the display screen and operates a not-shown input device to select the content desired to be used, the using condition and the like. 
     The ticket generation unit  173  of the communication terminal  10  provides the group sign  701  to the information (message) by using the member key  94  and the member certification information  95  and the like to generate the ticket  700  as shown in  FIG. 14 . The information (message) includes the license information  600  to which the information (ticket information  701 ) with regard to the content as a target of use, which is selected by the user, is added. 
     At this time, the ticket generation unit  173  may provide the group sign  702  by using random number, the group public key  91 , a common variable or the like. The group sign  702  ensures that the ticket  700  is issued by a member of the group registered in the license server  30 . 
     The ticket information  701  set in the ticket  700  includes: information specifying the content that can be used by using the ticket  700  (for example, a content name, an address of an entity which delivers the content, and a resolution); and information specifying the reproducing condition of the content in the audio-visual terminal  20  (for example, reproducible period, number of reproduction and the like) and the download method of the content and the like. Also, the ticket information may include the address of the license server  30  which verities the ticket  700 . 
     (8) Step S 18   
     The communication terminal  10  transmits the generated ticket  700  to the audio-visual terminal  20  in response to the operation of the user. When the ticket  700  is issued, the communication terminal  10  is connected to the audio-visual terminal  20  through the short-range wireless line  200 . Because of transmission through the short-range wireless line  200 , the user can easily issue the ticket  700  at small communication cost. 
     By the way, the ticket  700  may be issued through the carrier network  100 . When the ticket  700  is issued through the carrier network  100 , the ticket can be transmitted to the audio-visual terminal  20  far from the communication terminal  10 . In this case, the ticket  700  is preferred to be transmitted through a communication that uses a communication protocol capable of a confidential communication. 
     Also, the number of generation of the ticket  700  and the timing of generation of the ticket  700  by the communication terminal  10  are limited in accordance with the number of ticketing and a period in which ticketing is possible which are included in the license information  600 . 
     (9) Step S 19   
     When receiving the ticket  700 , the audio-visual terminal  20  verifies the content server sign  502  and the license server sign  602  by using the content server public key  96  and the license server public key  93 , and verifies the group sign  702  by using the group public key  91 . In this way, the audio-visual terminal  20  judges whether or not the ticket  700  is valid electronic money. 
     At this time, since the group sign  702  is provided to the ticket  700 , the audio-visual terminal  20  can verify the ticket  700  without knowing the information with regard to the issuer (communication terminal  10 ) of the ticket  700 . For example, when the audio-visual terminal  20  is installed in a public facility used by the public, since the information with regard to the issuer of the ticket  700  is not disclosed to the audio-visual terminal  20 , the leakage of the personal information is prevented. 
     Next, the verifying process of the ticket  700  will be described with reference to  FIG. 15 . 
     When obtaining the ticket  700 , the audio-visual terminal  20  can verify whether or not the ticket  700  is usable at an arbitrary timing. Prior to the verification, the audio-visual terminal  20  is connected through the communication line  900  to the license server  30 . Here, the audio-visual terminal  20  specifies the license server  30  by using the address of the license server included in the ticket information  701 . 
     (1) Step S 21   
     The audio-visual terminal  20  transmits the ticket  700  together with a verification request to the license server  30 . At this time, the audio-visual terminal  20  transmits own terminal ID together with the verification request to the license server  30 . 
     (2) Step S 22   
     In response to the verification request, the sign verification unit  374  of the license server  30  verifies the content server sign  502  and the license server sign  602  by using the content server public key  96  and the license server public key  93 , and verities the group sign  702  by using the latest group public key  91 . 
     (3) Step S 23   
     When the ticket  700  is a valid electronic certificate based on the license information  600 , the using condition verification unit  375  verifies whether or not the ticket  700  is usable. 
     At the step S 23 , the verification of usable or unusable based on the license ID and the verification of usable or unusable based on the using condition are carried out. The using condition verification unit  375  checks whether or not a certificate ID equal to a certificate ID  513  included in the ticket  700  is registered in the license D/B  38 . At this time, when the certificate ID equal to the license ID  613  of the ticket  700  is registered in the license D/B  38 , the using condition verification unit  375  judges that it passes. When the certificate ID equal to the license ID  613  of the ticket  700  is not registered in the license D/B  38 , the using condition verification unit  375  judges that it does not pass. 
     Also, the using condition verification unit  375 , by referring to the ticket information  701  included in the ticket  700 , specifies the use target content and judges whether or not the content is usable. 
     The using condition verification unit  375 , by referring to the license D/B  38 , judges whether or not the content is usable based on the license ID  613  of the ticket  700  and the usable/unusable information corresponding to the content ID of the use target content. At this time, when the unusable flag or the already-used flag is set in the usable/unusable information, the using condition verification unit  375  judges that the content is unusable. 
     Moreover, the using condition verification unit  375 , by referring to the reproducible period  512  in the certificate information  501 , judges whether or not the content is usable based on whether or not a current time is within the reproducible period  512 . 
     (4) Step S 24   
     After the completion of the verifying process, the license server  30  updates the usable/unusable information corresponding to the license ID of the verified ticket  700  (for respective content ID). In detail, when the ticket  700  is judged to be usable in the verification process, the license server  30  sets the already-used flag in the usable/unusable information of the content ID (product ID) corresponding to the ticket  700 . Also, the license server  30  may correlate the terminal ID of the audio-visual terminal  20  to the license ID and the content ID which have become usable. 
     Consequently, the license server  30  can manage the audio-visual terminal  20  to which the license of the content is assigned. In this case, the license server  30  obtains the terminal ID of the audio-visual terminal  20  at the step S 21 . When the ticket is judged to be unusable in the verification process, the license server  30  sets the unusable flag in the usable/unusable information of the content ID (product ID) corresponding to the ticket  700 . 
     As mentioned above, the license server  30  can determine to assign or not to assign the license to the audio-visual terminal based on the verification with respect to whether or not the ticket  700  is usable. 
     Also, since the license D/B  38  records the terminal ID of the communication terminal  10  and license ID such that they are correlated with each other, the license server  30  can specify the communication the communication terminal  10  which has issued the unusable ticket  700 . 
     Moreover, even when the license ID  613  of the ticket  700  is not registered in the license D/B  38 , the license server  30  can specify the communication terminal  10  as a issuer of the ticket  700 . In detail, the license server  30  extracts from the ticket  700  the terminal secret key of the communication terminal  10  which has provided the group sign  702 A by using the group secret key for encryption,  90 B. The license server  30  manages the terminal public key and the information (the terminal ID  101  and the subscriber information  102 ) specifying the communication terminal  10  such that they are correlated with each other. Thus, the carrier which possesses the license server  30  can know the terminal that illegally issues the ticket  700  or transmits the ticket  700  by mistake. 
     (5) Step S 25   
     After verifying whether the ticket  700  is usable or not, the license server  30  notifies the audio-visual terminal  20  of the verification result in which the ticket  700  is determined to be usable or unusable. 
     The verifying process from the step S 21  to the step S 25  enables the audio-visual terminal  20  to which the license is assigned by the license server  30  to reproduce the content set in the ticket information  701  of the ticket  700 . At this time, the audio-visual terminal  20  determines the matter, the resolution, and the download method of the content to be reproduced based on the ticket information  701  set in the ticket  700 . 
     Also, the audio-visual terminal  20  sets the reproducible period of the content and the maximum number of reproduction of the content based on the reproducing condition information  612 . The obtaining and reproducing methods of the content are preferred to be methods based on a DRM technique. 
     The audio-visual terminal  20  to which the license is assigned obtains by using some method the decryption key with which the encrypted content  800  corresponding to the ticket  700  can be decrypted. Then, the audio-visual terminal  20  decrypts the encrypted content  800  delivered by the content server  40  by using the decryption key to reproduce the content. 
     The methods of obtaining the decryption key from the content server  40  and reproducing the content will be described below with reference to  FIG. 16 . 
     (1) Step S 31   
     The audio-visual terminal  20  to which the license is assigned issues a content delivery request, which specifies the content ID included in the ticket information  701 , to the content server  40 . At this time, the audio-visual terminal  20  transmits information (terminal ID or address) for specifying the audio-visual terminal  20  to the content server  40 . 
     (2) Step S 32   
     The content server  40  issues to the license server  30  a license check request which specifies the terminal ID of the audio-visual terminal  20  and the content ID. 
     (3) Step S 33   
     The license server  30  judges whether or not the specified audio-visual terminal  20  has the license of the specified content by referring to the license D/B  38 . 
     When the content ID specified by the content server  40  is correlated with the terminal ID of the audio-visual terminal  20  and the content ID, the license server  30  judges that the license is assigned to the audio-visual terminal  20 . On the other hand, when the terminal ID of the audio-visual terminal  20  is not correlated, the license server  30  judges that the license is not assigned to the audio-visual terminal  20 . 
     Preferably, the information registered in the license D/B  38  is deleted when the expiration date set for respective license information  600  lapses and its change history is recorded in the not-shown archive database. The expiration date is preferred to be equal to the end of the reproducible period set in the reproducing condition information  612 . Consequently, when a status of assignment of the license is checked after the elapse of the expiration date, it is judged that the license is not assigned. 
     (4) Step S 34   
     The license server  30  notifies the content server  40  of the status of assignment of the license, which is checked at the step S 33 . However, when the license server  40  notifies the content server  40  of the audio-visual terminal  20  to which the license is assigned, the content usable based on the license, the expiration date of the license and the like after whether or not the ticket  700  is usable is verified at the step  23 , the steps S 32  to S 34  are omitted. 
     (5) Step S 35   
     The content server  40  makes sure that the license is assigned to the audio-visual terminal  20  based on the notified status of assignment of the license and transmits the encrypted content  800  generated by encrypting the required content and the decryption key to the audio-visual terminal  20 . When the license is not assigned to the audio-visual terminal  20 , the content server  40  does not transmit at least the decryption key. 
     (6) Step S 36   
     The audio-visual terminal  20  decrypts the obtained encrypted content  800  by using the decryption key to reproduce the content. At this time, the content can be reproduced for the number of reproduction and during the reproducible period which are set by the reproducing condition information  612 . 
     When the DRM technique is used as the reproduction protocol for the content, the content server  40  uses the terminal information (for example, the terminal ID) specific to the audio-visual terminal  20 , which is transmitted by the audio-visual terminal  20  at the step S 31  or by the license server  30  at the step S 34 , to generate the decryption key. When the terminal information possessed by the audio-visual terminal  20  agrees with terminal information in the decryption key, the audio-visual terminal  20  can decrypt the encrypted content  800  by using the decryption key to reproduce the content. 
     The method by which the audio-visual terminal  20 , to which the license is assigned, obtains the decryption key is not limited to the above-described method. The audio-visual terminal  20  may obtain the decryption key by issuing a request for the decryption key to the license server  30  which obtains the decryption key from the content server  40  in advance. 
     Also, the audio-visual terminal  20  may record the content (encrypted content  800 ) by downloading it or may record the content (encrypted content  800 ) that is stream delivered or broadcasted. Also, the content may be delivered in response to a request from the audio-visual terminal  20  or may be delivered at a predetermined timing or periodically. 
     As mentioned above, according to the content using system according to the present invention, the control of sales and using rights of the products (contents) provided by a plurality of companies can be unitarily managed by one carrier. The product (content) provided by the content server  40  is a pay product, a free product or a product which can be changed with points issued by a company. In a case of the pay product, a fee of the product is preferred to be dealt with similarly to a communication fee when the carrier network  100  is used. That is, the product fee is preferred to be collected together with the communication fee by the carrier. 
     Consequently, the payments of the fees can be made unitary, and subscribers (clients) of the carrier can be converted to users of the content. Also, points corresponding to the product fee are preferred to be added to points issued by the carrier. In this case, since a variety of the points collected by the users are not wide, an eagerness to buy the content is enhanced. 
     The communication terminal  10  to which the using right of the content is assigned can issue the ticket  700  to which various reproducing conditions of the content are provided. At this time, the group sign enables the certification of the matter of the ticket  700  while concealing the information of the issuer of the ticket  700  from the audio-visual terminal  20 . Also, only the terminal group-registered in the license server  30  can provide the group sign. Thus, only the license server  30  can trace the communication terminal  10  which issues the ticket  700 . 
     The license server  30  can generate an issuing history of the ticket  700  from the matter which is recorded in the license D/B  38  at the verification of whether or not the ticket  700  is usable. Also, the license server  30  can know who views which content based on the issuing history and the subscriber information D/B  50 . That is, the license server  3  can obtain information with respect to a market research of the content. The carrier can obtain a new income by selling the information to the provider of the content. 
     Also, the license server  30  can cancel the license assigned to the communication terminal  10  by controlling the license D/B  38  (by deleting the license ID or by changing the usable/unusable information). For example, the license server  30  can cancel the license in response to a request for cancelling the license from the communication terminal  10 . Also, the license server  30  can extend the term of the license, add a license and change the license by providing new license information  600  in response to a request from the communication terminal  10 . 
     Moreover, in a case that the communication terminal  10  set an issued time of the ticket in the ticket information  701 , the license server  30  can know the issued time of the ticket at the verification of the ticket  700 . Also, the content information  500  may include image data or program for displaying information (for example, a sample image or sample video) with respect to the content. The content information  500  may records a URL of a cite (for example, a download page of the content) providing information with respect to the content or a hash value of a matter of that. 
     As mentioned above, the exemplary embodiment of the present invention has been described in detail. However, the concrete configuration is not limited to the above-mentioned exemplary embodiment. The configuration with modifications within the range of the scope of the present invention is included in the present invention. 
     The exemplary embodiment is described with regard to the case in which the ticket  700  includes the reproducing condition information  612  such as the number of reproduction of the content, the reproducible period of the content and the like. However, the ticket  700  may includes the reproducing condition information  612  as well as a condition (a number of copying and a period in which copying is allowed) for the audio-visual terminal  20  to copy the content, or the like. 
     Also, the management server secret key  92  may be equal to the group secret key for sign,  90 A, and the management server public key  93  may be equal to the group secret key for encryption,  90 B. In this case, a labor for managing the keys and a labor for verifying the electronic certificate can be partially omitted. 
     Moreover, the communication terminal  10  may copy the ticket  700  and transmit copies of the ticket  700  to a plurality of audio-visual terminals  20 . In this case, among the plurality of audio-visual terminals  20 , only the audio-visual terminal  20  performing the first verification of the ticket  700  in the license server  30  can use the ticket  700 . 
     For example, when the user having the communication terminal  10  views the content on one of the audio-visual terminals  20  respectively placed in a plurality of rooms in a house of the user, the transmission of the ticket  700  to every audio-visual terminal  20  enables that the user does not required to view the content on a specific audio-visual terminal  20  but the user can view the content on an audio-visual terminal  20  in a desired room. In this case, the tickets  700  stored in the audio-visual terminals  20  other than the audio-visual terminal  20  which has used the content are not authenticated by the license server  30 , and thus cannot be used. 
     As mentioned above, the present invention has been described by referring to the exemplary embodiment. However, the present invention is not limited to the above-mentioned exemplary embodiment. The configuration and details of the present invention can be modified within a scope of the present invention by performing various modifications which can be understood by those skilled in the art.