Patent Publication Number: US-2009232312-A1

Title: Encrypted content reproduction device, encrypted content reproduction method, program, and recording medium for storing the program

Description:
TECHNICAL FIELD 
     The invention relates to an encrypted content playback equipment that decrypts an encrypted content, an encrypted content playback method, a program, and a recording medium for storing the program. 
     BACKGROUND ART 
     Conventionally, when storing digital copyrighted goods in a recording medium such as a semiconductor memory, a magnetic disk, or a magneto-optical disk, the digital copyrighted goods are generally encrypted, so that copyrights of the digital copyrighted goods are protected. The encrypted digital copyrighted goods (hereinafter called “encrypted content”) are usually stored in an area (hereinafter called “user&#39;s area”) not requiring authentication of the recording medium. 
     Key information used for encrypting the digital copyrighted goods is often stored in the recording medium so that it may not be easily seen by the user. For example, it is proposed that an area (hereinafter called “protected area”) requiring mutual authentication is provided in the recording medium, and the key information is stored in the protected area, so that the encrypted content is protected (see patent document 1). 
     An encrypted content playback equipment for playing back the encrypted content obtains the key information and the encrypted content from the inserted recording medium, and decrypts the encrypted content by key information to play back the content. 
     It may be also considered to obtain the encrypted content from other recording medium than the recoding medium (hereinafter called “key medium”) which stores the key information, or from a server on network. 
     When obtaining the encrypted content from the key medium, or when obtaining the encrypted content from the network, key information is essential for decrypting the encrypted content and playing back the digital copyrighted goods. 
     Patent document 1: JP-A-2003-233795 
     DISCLOSURE OF INVENTION 
     Problems to be Solved by the Invention 
     The prior art, however, has the following problems. The conventional encrypted content playback equipment can use key information only while the key medium is inserted. Accordingly, if a plurality of key information and a plurality of encrypted contents are stored in one key medium, while one encrypted content playback equipment is playing back any of the encrypted contents, other encrypted content playback equipment cannot play back other encrypted content recorded in the same key medium. If the encrypted content stored in the user&#39;s area is copied in other encrypted content playback equipment, the specification does not allow the key information stored in the protected area to be copied, and a plurality of key information stored in one key medium cannot be used simultaneously. 
     In a network for home, when an encrypted content is stored in a server device, and when desired to enjoy the encrypted content by accessing one server device from a plurality of rooms, if there is only one key medium in which key information is recorded, the key medium must be physically moved to individual rooms. That is, the encrypted content in the server cannot be used simultaneously in a plurality of rooms. Or the key medium must be always inserted in the server device. 
     The invention is devised in view of the problems discussed above, and it is intended to present an encrypted content playback equipment capable of decrypting and playing back an encrypted content by key information without inserting key medium having the key information, an encrypted content playback method, a program, and a recording medium for storing the program. 
     Means for Solving the Problems 
     The encrypted content playback equipment of the invention has an insertion section, in which a key medium that stores key information for decrypting an encrypted content is inserted, and which obtains the key information; a storage section that stores provisionally the key information obtained from the key medium as a provisional key; a provisional-key usage limitation processing section that controls permission of use of the provisional key; and a decryption section that decrypts the encrypted content by using the provisional key. 
     According to the invention, by copying the key information of the key medium to the storage section of an internal memory as provisional key, the encrypted content can be decrypted and played back by using the provisional key without inserting the key medium. Besides, since the use of the provisional key is limited by the provisional-key usage limitation processing section, the copyright of the encrypted content can be protected. 
     The provisional-key usage limitation processing section may control permission of use of the provisional key based on a condition specified by provisional-key usage limitation information. 
     The provisional-key usage limitation processing section may have a register that stores temporarily the provisional-key usage limitation information. The provisional-key usage limitation information may be stored in the storage section. 
     When the provisional-key usage limitation information is stored in the key medium together with the key information, the provisional-key usage limitation processing section may obtain the provisional-key usage limitation information from the key medium. 
     When the provisional-key usage limitation processing section judges based on provisional-key usage limitation information that the provisional key cannot be used, the provisional-key usage limitation processing section may delete the provisional key from the storage section. 
     The provisional-key usage limitation information may contain a limit number for limiting the number of times of playback of the content. The provisional-key usage limitation information may contain the time limit for limiting playback time of the content. The provisional-key usage limitation information may contain a limit distance for limiting the distance capable of playing back the content. 
     When the encrypted content is stored in the key medium, the encrypted content playback equipment may obtain the encrypted content from the key medium. 
     When the encrypted content is stored in a server connected with the encrypted content playback equipment through a network, the encrypted content playback equipment may obtain the encrypted content from the server. 
     In case where the key medium stores the encrypted key information in a user&#39;s area being read or written regardless of result of mutual authentication, and stores a decrypting key for decrypting the encrypted key information in a protected area being read or written only when mutual authentication is successful, when the key medium is inserted in the insertion section, the storage section stores the encrypted key information as a provisional key and also stores the decrypting key, and the decryption section first decrypts the encrypted provisional key by using the decrypting key, and then decrypts the encrypted content by using the decrypted provisional key. 
     If the provisional-key usage limitation information is contained in the encrypted content, the provisional-key usage limitation processing section may obtain the provisional-key usage limitation information from the encrypted content being decrypted by the decryption section, and may continue decrypting of the encrypted content when the provisional key is judged to be usable, or may stop decrypting of the encrypted content when the provisional key is judged to be not usable, based on provisional-key usage limitation information. 
     An encrypted content playback method of the invention provides the steps of: obtaining the key information from a key medium having key information for decrypting an encrypted content and storing the key information as a provisional key; controlling permission for use of the provisional key; and decrypting the encrypted content by using the provisional key. 
     According to the invention, by copying the key information of key medium as the provisional key, the encrypted content can be decrypted and played back by using the provisional key even when the key medium is not inserted. Since the use of the provisional key is limited, the copyright of the encrypted content can be protected. 
     At the step of controlling permission for use of the provisional key, the permission for use of the provisional key may be controlled based on the condition specified by the provisional-key usage limitation information. The provisional-key usage limitation information may be stored in the key medium together with the key information. 
     The encrypted content playback method of the invention may further have a step of deleting the provisional key when it is judged based on the provisional-key usage limitation information that the provisional key cannot be used. 
     In the encrypted content playback method of the invention, the provisional-key usage limitation information may contain a limit number for limiting the number of times of playback of the content. The provisional-key usage limitation information may contain the time limit for limiting playback time of the content. The provisional-key usage limitation information may contain a limit distance for limiting a distance capable of playing back the content. 
     In case where the key information is encrypted and stored in a user&#39;s area being read or written regardless of result of mutual authentication by the key medium, and a decrypting key for decrypting the encrypted key information is stored in a protected area being read or written only when mutual authentication by the key medium is successful, at the step of storing the provisional key, the encrypted key information may be stored as a provisional key and the decrypting key may be stored, and at the step of decrypting the encrypted content, the encrypted provisional key may be first decrypted by using the decrypting key, and then the encrypted content may be decrypted by using the decrypted provisional key. 
     When the provisional-key usage limitation information is contained in the encrypted content, at the step of controlling permission for use of provisional key, the provisional-key usage limitation information may be obtained from within the encrypted content being decrypted at the step of decrypting the encrypted content, and decrypting of encrypted content may be continued when the provisional key is judged to be usable, or decrypting of the encrypted content may be stopped when the provisional key is judged to be not usable, based on the provisional-key usage limitation information. 
     At least part of the encrypted content playback method of the invention may be realized as a program capable to be executed by a computer resource. Such program may be recorded in a recording medium capable to be read by a computer. 
     EFFECTS OF THE INVENTION 
     According to the encrypted content playback equipment, the encrypted content playback method, the program and the recording medium of the invention, the encrypted content can be decrypted by the key information and played back even when the key medium is not inserted. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing configurations of an encrypted content playback equipment and a key medium in an embodiment 1 of the invention. 
         FIG. 2  is a flowchart showing a method of obtaining key information from the key medium in encrypted content playback equipment in the embodiment 1 of the invention. 
         FIG. 3  is a flowchart of outline of the encrypted content playback method in the embodiment 1 to an embodiment 6 of the invention. 
         FIG. 4  is a flowchart showing a method of playing back the encrypted content by using a provisional key in the embodiment 1 of the invention. 
         FIG. 5  is a block diagram showing configurations of an encrypted content playback equipment and a key medium in an embodiment 2 of the invention. 
         FIG. 6  is a flowchart showing a method of obtaining the encrypted content from the key medium in the encrypted content playback equipment in the embodiment 2 of the invention. 
         FIG. 7  is a flowchart showing a method of playing back encrypted content by a provisional key in the embodiment 2 of the invention. 
         FIG. 8  is a diagram of outline of obtaining key information by a plurality of encrypted content playback equipments in the embodiment 2 of the invention. 
         FIG. 9  is a block diagram showing configurations of an encrypted content playback equipment and a key medium in an embodiment 3 of the invention. 
         FIG. 10  is a flowchart showing a method of playing back the encrypted content by using a provisional key in the embodiment 3 of the invention. 
         FIG. 11  is a diagram of example of provisional-key usage limitation information of the invention. 
         FIG. 12  is an outline diagram for obtaining the encrypted content from a server in a plurality of encrypted content playback equipment in an embodiment 4 of the invention. 
         FIG. 13  is a flowchart showing an encrypted content playback method in the embodiment 4 of the invention. 
         FIG. 14  is a block diagram of configurations of an encrypted content playback equipment and a key medium in an embodiment 5 of the invention. 
         FIG. 15  is a flowchart of method of obtaining a decrypting key from the key medium in the encrypted content playback equipment in the embodiment 5 of the invention. 
         FIG. 16  is a block diagram of configurations of an encrypted content playback equipment and a key medium in an embodiment 6 of the invention. 
         FIG. 17  is a flowchart showing a method of an playing back encrypted content by using a provisional key in embodiment 6 of the invention. 
     
    
    
     DESCRIPTION OF THE REFERENCE NUMERALS 
     
         
           1 ,  1   b  Key information 
           1   a ,  1   ab  Provisional key 
           2 ,  2   b ,  2   c  Provisional-key usage limitation information 
           3 ,  3   c  Encrypted content 
           100 ,  500 ,  900 ,  1500 ,  1700  Encrypted content playback equipment 
           101  Provisional-key usage limitation processing section 
           102  Provisional-key storage memory 
           103  Decryption processing section 
           104  Key medium insertion section 
           105  Playback processing section 
           106  Register 
           120 ,  520 ,  920 ,  1520 ,  1720  Key medium 
           121  Data storage section 
           122  Protected area 
           123  User&#39;s area 
           124  Host interface 
           506  Encrypted content storage memory 
           507  Time measuring section 
           907  Distance measuring section 
           925  Position transmitter 
           1260  Server 
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Embodiments for carrying out the invention are described below with reference to the accompanying drawings. 
     Embodiment 1 
     With reference to  FIG. 1  to  FIG. 4 , an encrypted content playback equipment and an encrypted content playback method in an embodiment 1 of the invention will be described.  FIG. 1  shows principal configurations of the encrypted content playback equipment and a key medium in the embodiment. An encrypted content playback equipment  100  is, for example, a secure digital (SD) audio player, and a key medium  120  is, for example, an SD memory card (the same applies to other embodiments). 
     The key medium  120  is a recording medium in which key information is recorded, and includes a data storage section  121  such as a flash memory having a recording area, and a host interface  124  for exchanging data with the encrypted content playback equipment  100  being a host device. The data storage section  121  includes a protected area  122  allowed to read and write only when successful in mutual authentication with the host device, and a user&#39;s area  123  allowed to read and write without the mutual authentication. 
     The protected area  122  stores key information  1  for decrypting an encrypted content  3 , that is, an encrypted digital copyrighted goods, and provisional-key usage limitation information  2  for limiting the use of a provisional key  1   a . The provisional key  1   a  is key information temporarily stored in the encrypted content playback equipment  100 , and is obtained by copying the key information  1 . 
     The encrypted content playback equipment  100  has a key medium insertion section  104  into which the key medium  120  is inserted, a provisional-key storage memory  102 , that is, a storage section, for storing the key information  1  obtained from the key medium  120  as provisional key  1   a , and a provisional-key usage limitation processing section  101  that limits the use of the provisional key  1   a  according to a the provisional-key usage limitation information  2  obtained from the key medium  120 . The provisional-key usage limitation processing section  101  has a register  106 , and stores the provisional-key usage limitation information  2  obtained from the key medium  120  in the register  106 . 
     The register  106  is means for temporarily storing data necessary for processing as required. For example, after storing the provisional-key usage limitation information  2  concerning certain key information in the register  106 , when the encrypted content playback equipment  100  executes other process than decrypting of an encrypted content  3  relating to the key information, the data used in other process is written in the register  106 , and the stored provisional-key usage limitation information  2  is erased from the register  106 . By contrast, the provisional-key storage memory  102  continues to store the once stored provisional key  1   a  until erased by the provisional-key usage limitation processing section  101 , and the provisional key  1   a  is not erased if the encrypted content playback equipment  100  executes other process than decrypting of the encrypted content  3 . 
     The encrypted content playback equipment  100  further includes a decryption processing section  103  that decrypts the encrypted content  3  by using key information  1  of the key medium  120  or the provisional key  1   a  stored in the provisional-key storage memory  102 , and a playback processing section  105  that plays back the decrypted content. In the embodiment, the encrypted content  3  is preliminarily set in the encrypted content playback equipment  100 . For example, a medium, such as CD, storing the encrypted content  3  is, for example, inserted in the encrypted content playback equipment  100 . 
       FIG. 2  shows a method of obtaining the key information  1  and the provisional-key usage limitation information  2  from the key medium  120  by the encrypted content playback equipment  100 . When the key medium  120  is inserted in the key medium insertion section  104  of the encrypted content playback equipment  100 , the encrypted content playback equipment  100  performs mutual authentication with the key medium  120  (S 201 ). When mutual authentication is successful (S 202 ), the encrypted content playback equipment  100  obtains the key information  1  from the protected area  122  by way of the host interface  124 , and stores the key information  1  as provisional key  1   a  in the provisional-key storage memory  102  (S 203 ). 
     Next, the encrypted content playback equipment  100  obtains the provisional-key usage limitation information  2  from the protected area  122  by way of the host interface  124 , and stores the provisional-key usage limitation information  2  in the register  106  (S 204 ). 
     In the embodiment, when the key medium  120  is inserted in the encrypted content playback equipment  100 , processing shown in  FIG. 2  is executed automatically. However, after inserting the key medium  120 , by manipulating the switch or the like provided in the encrypted content playback equipment  100 , the key information  1  and provisional-key usage limitation information  2  may be obtained. 
     In the embodiment, the encrypted content playback equipment  100  first stores the key information  1  (S 203 ), and then stores the provisional-key usage limitation information  2  (S 204 ), but the same effect is obtained if the provisional-key usage limitation information  2  is first stored and then the key information  1  is stored. 
       FIG. 3  shows an outline of encrypted content playback method. When playing back the encrypted content  3 , first, the encrypted content playback equipment  100  judges whether the key medium  120  is inserted or not in the key medium insertion section  104  (S 301 ). If the key medium  120  is inserted, the decryption processing section  103  decrypts the encrypted content  3  by using the key information  1  of the key medium  120  (S 302 ). If the key medium  120  is not inserted, the decryption processing section  103  decrypts the encrypted content  3  by using the provisional key  1   a  stored in the provisional-key storage memory  102  (S 303 ). The playback processing section  105  plays back the decrypted content (S 304 ). If the key medium  120  is inserted, the encrypted content  3  may be decrypted by using the provisional key  1   a.    
       FIG. 4  specifically shows a method of decrypting and playing back the encrypted content by using the provisional key at step  303  and step  304  in  FIG. 3 . In the embodiment, the provisional-key usage limitation information  2  is number limit information showing the number of times for limiting the number of playback of a content. The provisional-key usage limitation processing section  101  has a number control function for monitoring the number of playback of the encrypted content  3 , and counts the number of playback when the content is actually played back while the key medium  120  is not inserted in the key medium insertion section  104 . 
     In  FIG. 4 , the provisional-key usage limitation processing section  101  judges whether or not to use the provisional key  1   a  based on the provisional-key usage limitation information  2 . Specifically, the provisional-key usage limitation processing section  101  judges whether or not the number of actual playback is not over the number of limit indicated by the provisional-key usage limitation information  2  (S 401 ). 
     If the playback number exceeds the limit number at step  401 , the provisional-key usage limitation processing section  101  judges that the provisional key  1   a  is not usable, and the provisional key  1   a  of provisional-key storage memory  102  is invalidated (S 403 ). With respect to invalidate, for example, if the provisional key  1   a  has a flag showing validity or invalidity, invalidity may be set in the flag, or the provisional key  1   a  may be deleted from the provisional-key storage memory  102 . 
     If the playback number is not over the limit number, the provisional-key usage limitation processing section  101  judges that the provisional key is usable. Hence, the provisional-key usage limitation processing section  101  processes nothing particular about the provisional key  1   a.    
     The decryption processing section  103  decrypts the encrypted content  3  by using the provisional key  1   a  stored in the provisional-key storage memory  102  (S 402 ). The playback processing section  105  plays back the content (S 404 ). The provisional-key usage limitation processing section  101  counts up the number of times of actual playback of content. 
     According to the embodiment, for example, if the limit number of provisional-key usage limitation information  2  is once, after the key medium  2  is drawn out from the encrypted content playback equipment  100 , the encrypted content can be decrypted and played back only once by using the obtained provisional key  1   a . But after playing back once, the provisional key  1   a  is invalidated and cannot be used, and the content cannot be played back second time and after. 
     Thus, according to the embodiment, without inserting the key medium  120 , the encrypted content playback equipment  100  can play back the encrypted content  3  by using the provisional key  1   a.    
     Also according to the embodiment, since the use of provisional key  1   a  is limited to a finite number of times by using the provisional-key usage limitation information  2 , limitless use of provisional key  1   a  is prevented. Further, by invalidating the provisional key  1   a  according to the provisional-key usage limitation information  2 , the copyright of the content can be protected. 
     When the encrypted content playback equipment  100  executes other process than playback, the provisional-key usage limitation information  2  stored in the register  106  is erased. That is, when the provisional-key usage limitation information  2  is stored in the register  106  as in the embodiment, the provisional key  1   a  can be used only right after the key medium  120  is drawn out, and limitless use of provisional key  1   a  is prevented. 
     The key information  1  stored in the protected area  122  of the key medium  120  may be or may not be encrypted. When the key information  1  of the key medium  120  is encrypted, the encrypted content playback equipment  100  may store the encrypted key information  1  directly as provisional key  1   a , or if the key information  1  is not leaded outside, it may be decrypted and stored. 
     As in the embodiment, when both key information  1  and provisional-key usage limitation information  2  are stored in the protected area  122  of the key medium  120 , correspondence between key information  1  and provisional-key usage limitation information  2  can be achieved easily. However, as far as the correspondence between key information  1  and provisional-key usage limitation information  2  is established, the provisional-key usage limitation information  2  may be obtained aside from the key information  1 . For example, the provisional-key usage limitation information  2  may be obtained by using a network or other medium connected by wire or without wire to the encrypted content playback equipment  100 . 
     In  FIG. 1 , the key information  1  and encrypted content  3  are shown by one piece each, but the combination of the key information  1  and the encrypted content  3  may be either one set or plural sets. In the embodiment, the key information  1  and provisional-key usage limitation information  2  are in pairs, but when the encrypted content  3  and key information  1  are in pairs, the key information  1  and provisional-key usage limitation information  2  may not be in pairs. For example, one provisional-key usage limitation information  2  may limit the use of a plurality of key information  1 . Or the key information  1  may be common to a plurality of encrypted contents  3 , and, use of common key information  1  may be limited by one or more provisional-key usage limitation information  2 . 
     Embodiment 2 
     With reference to  FIG. 5  to  FIG. 8 , an encrypted content playback equipment and an encrypted content playback method according to an embodiment 2 of the invention are described.  FIG. 5  shows principal configurations of an encrypted content playback equipment and a key medium in the embodiment. 
     The encrypted content playback equipment  500  of the embodiment 2 differs from the encrypted content playback equipment  100  of the embodiment 1 only in that a time measuring section  507  and an encrypted content storage memory  506  are provided. The time measuring section  507  measures the time from the moment that a key medium  520  is drawn out from the key medium insertion section  104 , and outputs the measured time. The encrypted content storage memory  506  stores the encrypted content  3 . 
     What the key medium  520  of the embodiment 2 differs from the key medium  120  of the embodiment 1 lies in that a plurality of encrypted contents  3  are stored in the user&#39;s area  123  of the key medium  520 , and that the same number of key information  1  and provisional-key usage limitation information  2  as the number of encrypted contents  3  are stored in the protected area  122 . In the embodiment, the provisional-key usage limitation information  2  is the time limit information showing the time limit indicating the upper limit of time capable of playing back the contents. Other configuration of the embodiment 2 is the same as that in the embodiment 1. 
       FIG. 6  shows a method of obtaining key information  1 , provisional-key usage limitation information  2 , and an encrypted content  3  from the key medium  520  by encrypted content playback equipment  500  of the embodiment. When the key medium  520  is inserted in the key medium insertion section  104  of the encrypted content playback equipment  500 , the encrypted content playback equipment  500  performs mutual authentication with the key medium  520  (S 601 ). When the mutual authentication is successful (S 602 ), the encrypted content playback equipment  500  obtains the key information  1  and the provisional-key usage limitation information  2  from the protected area  122  by way of the host interface  124 , and stores them in the provisional-key storage memory  102  (S 603 ). 
     Next, the encrypted content playback equipment  500  obtains the encrypted content  3  from the user&#39;s area  123  by way of the host interface  124 , and stores it in the encrypted content storage memory  506 . 
     Referring to  FIG. 7 , the content playback method by the encrypted content playback equipment  500  is explained. The provisional-key usage limitation processing section  101  judges whether or not to use the provisional key  1   a  based on the provisional-key usage limitation information  2 . Specifically, the provisional-key usage limitation processing section  101  judges whether or not the time measured by the time measuring section  507  is less than the time limit of provisional-key usage limitation information  2  (S 701 ). 
     If the measured time is less than the time limit, the provisional-key usage limitation processing section  101  judges that the provisional key  1   a  is usable. The decryption processing section  103  decrypts the encrypted content  3  by using the provisional key  1   a  stored in the provisional-key storage memory  102  (S 702 ). The playback processing section  105  plays back the content (S 704 ). 
     At step S 701 , if the measured time is not less than the time limit, the provisional-key usage limitation processing section  101  judges that the provisional key is not usable, and invalidates the provisional key in the provisional-key storage memory  102  (S 703 ). 
     The time measuring section  507  is always outputting the measured time. That is, the provisional-key usage limitation processing section  101  returns to step  701  during playback of the content at step  704 , and compares the measured time with the time limit always or at specified time intervals. When the measured time exceeds the time limit, step  703  is executed, and playback of the content is stopped. 
     According to the embodiment, for example, when the time limit of provisional-key usage limitation information  2  is 1 hour, within 1 hour after the key medium  520  is drawn out, the encrypted content playback equipment  500  can decrypt and play back the encrypted content  3  by using the provisional key. When exceeding 1 hour after removal of key medium  520 , the provisional key is invalidated, and the encrypted content  3  cannot be played back. 
     In the embodiment, the data of provisional-key usage limitation information  2  can be set for each encrypted content. That is, the time limit can be changed for each encrypted content. 
       FIG. 8  is a diagram of state of obtaining key information  1 , provisional-key usage limitation information  2 , and the encrypted content  3  from the same key medium  520  by an audio player  830  and a television  840 . The audio player  830  and the television  840  have the configuration of the encrypted content playback equipment  500  shown in  FIG. 5 , and obtain the key information  1 , provisional-key usage limitation information  2  and the encrypted content  3  from the key media  520  individually according to the flowchart of  FIG. 6 . Then, without inserting key medium  520 , the audio player  830  and television  840  can play back the encrypted content  3  simultaneously according to the flowchart of  FIG. 7 . 
     Thus, according to the embodiment, whether the key medium  520  is inserted or not, the encrypted content playback equipment  500  can play back the encrypted content  3  by using the provisional key  1   a , so that the encrypted content  3  can be played back simultaneously by different equipments. 
     According to the embodiment, since the use of provisional key is limited in finite time by using the provisional-key usage limitation information  2 , limitless use of provisional key is prevented. Further, by invalidating the provisional key according to the provisional-key usage limitation information  2 , the copyright of the content can be protected. 
     In the embodiment, since the encrypted content  3  and key information  1  are stored in the same medium, the encrypted content  3  can be played back if only one medium  520  is available. Correspondence of encrypted content  3  and key information  1  can be established easily. 
     In addition, in  FIG. 6 , the encrypted content  3  may be obtained (S 604 ) prior to obtaining of key information  1  and provisional-key usage limitation information  2  (S 603 ). 
     In the embodiment, the time measuring section  507  starts measuring the time after the key medium  520  is drawn out, and outputs the measured time. But the measured time is not limited to this example. The time measuring section  507  may start measuring the time when playback of the content is started and output the total time up to the present time, or output the total of actual playback time as the measured time. The provisional-key usage limitation processing section  101  may compare the measured time with the time limit of provisional-key usage limitation information  2 , and may judge whether the provisional key is usable or not. 
     If a plurality of encrypted contents  3  are stored in the key medium  520 , the encrypted content playback equipment  500  may obtain all encrypted contents  3  and all key information  1  and provisional-key usage limitation information  2 , or may also obtain selected encrypted contents  3  only, and corresponding key information  1  and provisional-key usage limitation information  2  only. 
     If a plurality of encrypted contents  3  are stored in the encrypted content storage memory  506 , the encrypted content playback equipment  500  may process all encrypted contents  3  as specified in  FIG. 7 , decrypt and play back the encrypted contents  3  sequentially. Or only the selected encrypted contents  3  may be played back according to  FIG. 7 . 
     Embodiment 3 
     With reference to  FIG. 9  to  FIG. 11 , an encrypted content playback equipment and an encrypted content playback method in an embodiment 3 of the invention are described.  FIG. 9  shows principal configurations of an encrypted content playback equipment and a key medium in the embodiment. 
     What the embodiment 3 differs from the embodiment 2 lies in the configuration in which a key medium  920  has a position transmitter  925  for transmitting position information of the key medium, and an encrypted content playback equipment  900  has a distance measuring section  907  that receives the position information of the key medium, measures the distance of the encrypted content playback equipment  900  and the key medium  920 , and outputs the measured distance. The position transmitter  925  is, for example, a global positioning system, and outputs degrees of latitude and longitude of the key medium  920 . In the embodiment, the provisional-key usage limitation information  2  is the distance limit information showing the limit distance as upper limit of distance allowing the provisional key  1   a  to be used. Other configuration of the embodiment 3 is the same as that of the embodiment 2. 
     The encrypted content playback equipment  900  of the embodiment obtains the key information  1 , the provisional-key usage limitation information  2 , and the encrypted content  3  from the key medium  920  according the flowchart in  FIG. 6 . 
     Playback method of the encrypted content when the key medium  920  is not inserted is explained by referring to  FIG. 10 . First, the provisional-key usage limitation processing section  101  judges whether the provisional key is usable or not based on the provisional-key usage limitation information  2 . Specifically, the provisional-key usage limitation processing section  101  judges whether the distance between the key medium  920  and the encrypted content playback equipment  900  measured by the distance measuring section  907  is not over the limit distance shown by the provisional-key usage limitation information  2  (S 1001 ). 
     If the measured distance is not over the limit distance, the provisional-key usage limitation processing section  101  judges that the provisional key is usable. The decryption processing section  103  decrypts the encrypted content  3  by using the provisional key  1   a  stored in the provisional-key storage memory  102  (S 1002 ). The playback processing section  105  plays back the content (S 1004 ). 
     If the measured distance is over the limit distance, the provisional-key usage limitation processing section  101  judges that the provisional key is not usable, and the provisional key in the provisional-key storage memory  102  is invalidated (S 1003 ). 
     The distance measuring section  907  is measuring the distance of key medium  920  and the encrypted content playback equipment  900  always or at specified time intervals. That is, the provisional-key usage limitation processing section  101  returns to step  1001  during content playback at step  1004 , and compares the measured distance and the limit distance, and when the measured distance exceeds the limit distance, the step  1003  is executed, and playback of the content is stopped at the same time. 
     Thus, according to the embodiment, without inserting key medium  920 , the encrypted content playback equipment  900  can play back the encrypted content  3  by using the provisional key  1   a . For example, the same encrypted content can be enjoyed among family members and friends staying within a limit distance range simultaneously by using plural equipments. 
     In the embodiment, since the use of provisional key  1   a  is limited in a finite distance by using provisional-key usage limitation information  2 , limitless use of provisional key  1   a  is prevented. For example, after enjoying the encrypted content, when the equipments are gotten away from one another, the encrypted content cannot be used in the equipment in which the memory card being the key medium is not inserted. Further, by invalidating the provisional key  1   a  according to the provisional-key usage limitation information  2 , the copyright of the content can be protected. 
     Instead of the position transmitter  925  and the distance measuring section  907  of the embodiment, the key medium  920  and the encrypted content playback equipment  900  have a wireless communication section, such as infrared or Bluetooth means, which performs wireless communication, respectively. For example, the provisional-key usage limitation information  2  includes information for limiting the use of the provisional key depending on the distance between the key medium  920  and the encrypted content playback equipment  900 . In this case, the provisional-key usage limitation processing section  101  permits use of the provisional key only when communication is established by the wireless communication section, and invalidates the use of the provisional key when the communication is interrupted. The provisional-key usage limitation information  2  may include the information showing the lower limit of intensity of radio wave applicable to wireless communication, and the provisional-key usage limitation processing section  101  may permit use of the provisional key only when the communication wave is stronger than the intensity shown by the provisional-key usage limitation information  2 , and may invalidate use of the provisional key when the communication wave is weaker than the intensity shown by the provisional-key usage limitation information  2 . 
     In the embodiment 1 through the embodiment 3, the provisional-key usage limitation information  2  respectively includes the limit number, the time limit, and the limit distance, but these conditions may be combined in plural sets as shown in  FIG. 11 . For example, if the condition includes time limit information  1101  permitting use of the provisional key  1   a  for three hours after start of content playback, number limit information  1102  permitting playback of the content for three times by the provisional key  1   a , and distance limit information  1103  permitting use of provisional key  1   a  only when the key medium is present within a distance of 50 meters, the provisional-key usage limitation processing section  101  judges as follows. The provisional-key usage limitation processing section  101  judges that the provisional key  1   a  is usable for 3 hours after start of the encrypted content, but invalidates the provisional key  1   a  when the distance of key medium  920  and the encrypted content playback equipment  900  is longer than 50 meters, so that the encrypted content cannot be played back. Even if the time after start of playback is within three hours and the key medium exists within 50 meters, after the same content is viewed three times, the provisional-key usage limitation processing section  101  invalidates the provisional key  1   a  on the moment, so that the encrypted content  3  cannot be played back again. 
     In the embodiment 1 through the embodiment 3, the provisional-key usage limitation information  2  includes the number limit information for showing the number of times of limit, time limit information for showing the time limit, or distance limit information for showing the limit distance, but these conditions are not particularly specified as far as the use of the key can be limited. 
     Embodiment 4 
     With reference to  FIG. 12  and  FIG. 13 , an encrypted content playback equipment and an encrypted content playback method according to an embodiment 4 of the invention are described. In the embodiment, the encrypted contents  3  are stored in an integrated fashion in a server  1260 , and the encrypted content playback equipment obtains the encrypted contents  3  from the server  1260 , and plays back the encrypted contents  3 . 
     In  FIG. 12 , an audio player  1230 , a television  1240 , and a personal computer  1250  include the configuration of encrypted content playback equipment according to the embodiment 1.  FIG. 12  shows an outline configuration in which the audio player  1230 , the television  1240 , and the personal computer  1250  obtains encrypted content  3  from the server  1260  and obtains key information  1  from key medium  120 . In the embodiment, the audio player  1230 , the television  1240 , and the personal computer  1250  are connected to the server  1260  by wired or wireless network. 
     The key medium  120  is inserted into the audio player  1230 , the television  1240 , and the personal computer  1250  of the embodiment, respectively, and they obtain the key information  1  and provisional-key usage limitation information  2  from the key medium  120  according to the flowchart in  FIG. 2 . 
     The method of playing back the encrypted content by audio player  1230  is explained by referring to  FIG. 13 . The audio player  1230  judges whether the key medium  120  is inserted or not (S 1301 ). When the key medium  120  is inserted in the audio player  1230 , the audio player  1230  obtains the encrypted content  3  from the server  1260  (S 1302 ). The decryption processing section  103  of the audio player  1230  decrypts the encrypted content  3  by the key information  1  of the key medium  120  (S 1303 ), and plays back the content (S 1304 ). 
     If the key medium  120  is not inserted, the provisional-key usage limitation processing section  101  judges whether the provisional key in the provisional-key storage memory  102  is usable or not (S 1305 ). If the provisional key is usable, the audio player  1230  obtains the encrypted content  3  from the server  1260  (S 1306 ). The decryption processing section  103  decrypts the encrypted content  3  by the provisional key (S 1307 ), and the playback processing section  105  plays back the decrypted content (S 1304 ). If the key medium  120  is inserted, it may be also judged if the provisional key is usable or not. 
     The provisional-key usage limitation processing section  101  invalidates the provisional key if the provisional key is judged to be not usable (S 1308 ). 
     As in the case of the audio player  1230 , the television  1240  and the personal computer  1250  obtain the encrypted content  3  from the server  1260  and play back it according to the flowchart in  FIG. 13 . 
     According to the embodiment, in each one of the audio player  1230 , the television  1240 , and the personal computer  1250 , when key information  1  is preliminarily obtained from one key medium  120 . If the key medium  120  is not inserted at the time of playback, the encrypted content  3  can be obtained from the common server  1260 , and played back simultaneously. 
     The audio player  1230 , the television  1240 , and the personal computer  1250  shown in  FIG. 12  include the configuration of encrypted content playback equipment in the embodiment 1, but may also include the configuration of encrypted content playback equipment in the embodiment 2 or the embodiment 3. In this case, the encrypted content  3  received from the server  1260  can be stored in the encrypted content storage memory  506 . Accordingly, the timing for obtaining the encrypted content  3  from the server  1260  is not limited to the step  1302  or the step  1306 , but may be anytime before decrypting of the encrypted content  3 . For example, before the key information  1  is obtained from the key medium  120 , the encrypted content  3  may be received from the server  1260 . 
     In addition, the encrypted content playback equipment is not limited to the audio player, the television or the personal computer. Any other equipment may be used as far as having the constituent elements in the encrypted content playback equipment in the embodiment 1 to the embodiment 3. 
     Embodiment 5 
     With reference to  FIG. 14  and  FIG. 15 , an encrypted content playback equipment and an encrypted content playback method in an embodiment 5 of the invention are described. Different from the embodiment 1 to the embodiment 4, in a key medium  1420  in embodiment 5 shown in  FIG. 14 , key information  1   b  and provisional-key usage limitation information  2   b  are encrypted and stored in user&#39;s area  123 . In protected area  122 , a decrypting key  4 , which is a key for decrypting the encrypted key information  1   b  and provisional-key usage limitation information  2   b , is stored. 
     With reference to  FIG. 15 , a method of obtaining decrypting key  4  and others from the key medium  1420  by the encrypted content playback equipment  1400  in the embodiment 5, will be described. The encrypted content playback equipment  1400  performs mutual authentication with the key medium  1420  (S 1501 ) when the key medium  1420  is inserted in the key medium insertion section  104 . When successful in mutual authentication (S 1502 ), the decrypting key  4  stored in the protected area  122  is read out, and is stored in the provisional-key storage memory  102  (S 1503 ). 
     Next, the key information  1   b  and provisional-key usage limitation information  2   b  stored in the user&#39;s area  123  are read out in encrypted state, and stored in provisional-key storage memory  102  (S 1504 ). The key information  1   b  stored in provisional-key storage memory  102  is called provisional key lab. 
     Using the decrypting key  4 , the decryption processing section  103  decrypts the provisional key lab and the provisional-key usage limitation information  2   b  (S 1505 ). 
     Step  1505  is not particularly specified in sequence as far as it is before decrypting of the encrypted content  3 . For example, step  1505  may be executed after the key medium  1520  is removed from the key medium insertion section  104  after step  1504 . The decrypted provisional key lab and decrypted provisional-key usage limitation information  2   b  may be stored in the provisional-key storage memory  102  instead of the provisional key lab and of provision-key usage limitation information  2   b  before decrypting. 
     The encrypted content  3  and the key information  1   b  are in a relation of a pairs, and when the encrypted content  3  increases, the key information  1   b  also increases. On the other hand, the protected area  122  of the SD memory card being the key medium  1420  is generally smaller in capacity as compared with the user&#39;s area  123 , and much key information cannot be stored. According to the embodiment, however, since more key information can be stored in the key medium  1420  by making use of the user&#39;s area, more encrypted contents  3  can be played back by the provisional key lab. 
     In the embodiment, the decrypting key  4  is stored in the protected area  122  of the key medium  1420  in which key information  1   b  and provisional-key usage limitation information  2   b  are stored, but the decrypting key  4  is not needed in the key medium  1420  not having the protected area  122 . For example, encrypted key information  1   b  and encrypted provisional-key usage limitation information  2   b  may be stored in the key medium  1  not having protected area  122 , and the decrypting key  4  may be stored in other medium having the protected area. Nothing is particularly specified as far as the key information  1   b  and provisional-key usage limitation information  2   b  can be decrypted by the encrypted content playback equipment  1400 . 
     Embodiment 6 
     With reference to  FIG. 16  and  FIG. 17 , an encrypted content playback equipment and an encrypted content playback method in an embodiment 6 of the invention are described. In the embodiment, provisional-key usage limitation information  2   c  is contained in an encrypted content  3   c . The encrypted content  3   c  is stored in the user&#39;s area  123  of the key medium  1620 , and key information  1  is stored in the protected area  122 . The encrypted content playback equipment  1600  in the embodiment reads out the key information  1  from the protected area  122 , and stores it in the provisional-key storage memory  102  as provisional key  1   a , and reads out the encrypted content  3   c  from the user&#39;s area  123 , and stores it in the encrypted content storage memory  3 . 
     Method of playing back the encrypted content  3   c  obtained as described above is explained by referring to  FIG. 17 .  FIG. 17  shows a method of playing back the encrypted content  3   c  when the key medium  1620  is not inserted in the key medium insertion section  104 . First, the decryption processing section  103  starts decrypting the encrypted content  3   c  by using the provisional key  1   a  stored in the provisional-key storage memory  102  (S 1701 ). 
     The provisional-key usage limitation processing section  101  obtains the provisional-key usage limitation information  2   c  from the decrypted portion of the encrypted content  3   c  (S 1702 ). The provisional-key usage limitation processing section  101  judges whether the provisional key is continuously usable or not based on the provisional-key usage limitation information  2   c  (S 1703 ). For example, if the provisional-key usage limitation information  2   c  contains a limit distance for limiting the distance between the key medium  1720  and the encrypted content playback equipment  1600 , same as in embodiment 4, use of provisional key is judged based on the distance between the key medium  1620  and the encrypted content playback equipment  1600 . 
     When the provisional-key usage limitation processing section  101  judges that the provisional key is usable, the decryption processing section  103  continues to decrypt the encrypted content  3   c  (S 1704 ). The playback processing section  105  plays back the decrypted content (S 1705 ). 
     When the provisional-key usage limitation processing section  101  judges that the provisional key is not usable, the provisional-key usage limitation processing section  101  invalidates the provisional key (S 1706 ). 
     In addition, the encrypted content  3   c  containing the provisional-key usage limitation information  2   c  may be separately obtained from a server connected through a network, instead of obtaining from the key medium  1620  having key information  1 . 
     In the embodiment, too, same as in embodiments 1 to 5, the same effects of playing back the encrypted content  3  without inserting the key medium  1620  are obtained. 
     The encrypted content playback method explained in the foregoing embodiments can be executed, at least in part, as a program that can be executed by a computer resource. Such program can be recorded in a recording medium that can be read by a computer. 
     INDUSTRIAL APPLICABILITY 
     The invention is useful for an encrypted content playback equipment and an encrypted content playback method capable of playing back an encrypted content finitely without inserting a medium in which key information is recorded.