Patent Publication Number: US-2007110226-A1

Title: Transmission/reception system, transmission/reception method, receiver device, reception method, and program

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
      The present invention contains subject matter related to Japanese Patent Application JP 2005-235639 filed in the Japanese Patent Office on Aug. 16, 2005, the entire contents of which being incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a transmission/reception system, transmission/reception method, receiver device, reception method, and program, and more particularly to a transmission/reception system, transmission/reception method, receiver device, reception method, and program for enabling a receiving end to easily handle a digital material transmitted from a transmitting end for reproduction purposes no matter whether it is encrypted.  
      2. Description of the Related Art  
      In recent years, a transmission/reception system for digital materials (content) has been in widespread use (refer, for instance, to Japanese Patent Application JP 2003-143548). Further, there is a project for playing digital materials in movie theaters, and the transmission/reception system for use in such a project is being studied and developed.  
      The elements (not shown) constituting such a transmission/reception system would be, for instance, a first server, a second server, a router, and a projector. The first server encrypts and transmits the stream data for a movie. The second server transmits the stream data for movie trailers and commercials without encrypting it. The router selects and transmits the data (output signal) transmitted from the first or second server. The projector receives the data transmitted from the router and plays the movie, trailers, or commercials related to the received data.  
     SUMMARY OF THE INVENTION  
      In the transmission/reception system including the above-mentioned elements, that is, the transmission/reception system including the first server, second server, router, and projector, the projector may receive both encrypted stream data and unencrypted stream data. However, an appropriate method for enabling the projector to handle such stream data in a discriminating manner has not been worked out.  
      The present invention addresses the circumstances described above, and enables the receiving end to easily handle digital materials delivered from the transmitting end for digital material reproduction purposes no matter whether they are encrypted or not.  
      According to one embodiment of the present invention, a transmission/reception system includes: a first sender device configured to encrypt stream data by a predetermined encryption method and transmit the encrypted stream data; a second sender device configured to transmit stream data without encryption; and a receiver device configured to receive stream data that is transmitted from the first sender device or the second sender device. The receiver device includes a receiver configured to receive the stream data that is transmitted from the first sender device or the second sender device, and a generator configured to determine whether the stream data received by the receiver is encrypted, and generate encryption information that indicates the obtained determination result.  
      According to another embodiment of the present invention, a transmission/reception method for a transmission/reception system includes a first sender device configured to encrypt stream data by a predetermined encryption method and transmit the encrypted stream data, a second sender device configured to transmit unencrypted stream data, and a receiver device configured to receive stream data that is transmitted from the first sender device or the second sender device. The receiver device determines whether the received stream data is encrypted, and generates encryption information that indicates the obtained determination result.  
      The transmission/reception system and transmission/reception method according to an embodiment of the present invention is applied to a transmission/reception system. The sender/receiver system includes a first sender device configured to encrypt stream data by a predetermined encryption method and transmit the encrypted stream data, a second sender device configured to transmit unencrypted stream data, and a receiver device configured to receive stream data that is transmitted from the first sender device or the second sender device. The receiver device within the transmission/reception system determines whether the received stream data is encrypted, and generates encryption information that indicates the obtained determination result.  
      According to another embodiment of the present invention, a receiver device receives stream data that is encrypted by a first encryption method or receives unencrypted stream data when transmitted to the device. The receiver device includes: a receiver configured to receive the encrypted stream data or the unencrypted stream data; and a generator configured to determine whether the stream data received by the receiver is encrypted, and generate encryption information that indicates the obtained determination result.  
      The receiver device may further include a decryptor and a decryption controller. The decryptor is configured to perform a first decryption process on encrypted stream data by a first decryption method. The decryption controller is configured to permit the decryptor to perform the first decryption process on the stream data when the encryption information generated by the generator indicates that the stream data is encrypted. The decryption controller is configured to prohibit the decryptor from performing the first decryption process on the stream data when the encryption information generated by the generator indicates that the stream data is not encrypted.  
      The first encryption method and the first decryption method may be an encryption method and a decryption method that are based on a common key. The common key used for encrypting the stream data may be transmitted to the receiver device. The receiver may further receive the common key. The generator may further supply the common key, which is received by the receiver, to the decryptor when the stream data is determined to be encrypted. The decryptor may perform the first decryption process on the stream data by using the common key that is supplied from the generator.  
      The generator may further determine before the first decryption process of the decryptor whether the common key is normally prepared, and generate common key preparation information that indicates the obtained determination result. The receiver device may further include an indicator that identifies the state of the receiver device in accordance with the encryption information and the common key preparation information, which are both generated by the generator, and presents the obtained identification result.  
      The indicator may identify and present a first state, a second state, and a third state. The first state is where unencrypted stream data is received when the encryption information indicates that the stream data is not encrypted. The second state is where encrypted stream data is received and the first decryption process is normally performed on the stream data when the encryption information indicates that the stream data is encrypted and the common key preparation information indicates that the common key is normally prepared. The third state is where encrypted stream data is received but the first decryption process is abnormally performed on the stream data when the encryption information indicates that the stream data is encrypted and the common key preparation information indicates that the common key is not normally prepared.  
      The indicator may include a lamp that can illuminate in a first color or in a second color. The indicator associates the first state, the second state, and the third state with a first lamp state where the lamp is prohibited from illuminating in the first color or in the second color, a second lamp state where the lamp is allowed to illuminate in the first color only, or a third lamp state where the lamp is allowed to illuminate in the second color only. The indicator exercises control to place the lamp in the first lamp state, in the second lamp state, or in the third lamp state as appropriate in order to present the first state, the second state, or the third state.  
      The common key may be transmitted to the receiver device after being further encrypted by a second encryption method. The generator may perform a second decryption process on the encrypted common key by a second decryption method, which is related to the second encryption method, when the encrypted common key is received by the receiver. The generator may supply the common key to the decryptor and generate the common key preparation information indicating that the common key is normally prepared when the second decryption process is successfully performed before the first decryption process of the decryptor. The generator may generate in the other situations the common key preparation information indicating that the common key is not normally prepared.  
      The second encryption method and the second decryption method may be a public key cryptosystem based on a public key and a decryption method that is associated with the public key cryptosystem and based on a private key.  
      The common key encrypted with the public key may be included in meta data of the stream data that is encrypted with the common key. The meta data may be superposed over the stream data, and the stream data over which the meta data is superposed may be transmitted to the receiver device. The generator may generate the encryption information indicating that the stream data is encrypted, extract the encrypted common key from the meta data, and perform the second decryption process, which uses the private key corresponding to the public key, on the common key when the meta data is superposed over the stream data received by the receiver device. The generator may generate the encryption information indicating that the stream data is not encrypted when the meta data is not superposed over the stream data received by the receiver device.  
      The stream data may use a plurality of common keys. An identifier other than a special value may be assigned to each of the plurality of common keys. The identifier for each of the plurality of common keys may be included in the meta data. The generator may prohibit the generation of the encryption information indicating that the stream data is encrypted and generate the encryption information indicating that the stream data is not encrypted when the identifier included in the meta data is the special value in a situation where the meta data is superposed over the stream data received by the receiver device.  
      The receiver device may further include a reproduction controller, which controls the reproduction of first stream data, which is obtained when the decryptor performs the first decryption process on the encrypted stream data, when the decryption controller permits the execution of the first decryption process. The reproduction controller controls the reproduction of second stream data, which is received by the receiver and not encrypted, when the decryption controller prohibits the execution of the first decryption process.  
      When the common key preparation information indicates that the common key is not normally prepared in a situation where the decryption controller permits the execution of the first decryption process, the reproduction controller may prohibit the reproduction of the first stream data but control the reproduction of data generated according to predefined rules. When the common key preparation information indicates that the common key is normally prepared, the reproduction controller may control the reproduction of the first stream data.  
      The data whose reproduction is controlled by the reproduction controller instead of the first stream data may be image data corresponding to a predetermined stationary image.  
      The data whose reproduction is controlled by the reproduction controller instead of the first stream data may be image data corresponding to a predetermined still picture among the first stream data whose reproduction is controlled before the reproduction control of the data.  
      The encrypted stream data may be transmitted to the receiver device via a first route and the common key used by the encrypted stream data may be transmitted to the receiver device via a second route. The receiver may include a first receiver, which receives the unencrypted stream data or the encrypted stream data transmitted via the first route, and a second receiver, which receives the common key transmitted via the second route.  
      According to another embodiment of the present invention, a reception method/program is to be executed by a computer for controlling the reception method/hardware of a receiver device that receives stream data transmitted after being encrypted by a predetermined encryption method or stream data transmitted without being encrypted. The reception method/program includes the step of determining whether the stream data received by the receiver device is encrypted and generating encryption information that indicates the obtained determination result.  
      The receiver device, reception method, and program according to an embodiment of the present invention are applicable to a receiver device/hardware that receives stream data transmitted after being encrypted by a predetermined encryption method or stream data transmitted without being encrypted. The receiver device/hardware determines whether the received stream data is encrypted, and generates encryption information that indicates the obtained determination result.  
      As described above, an embodiment of the present invention makes it possible to receive unencrypted stream data as well as encrypted stream data. Particularly when steam data constituting a digital material is to be played, the receiving end can easily determine whether the digital material sent from the transmitting end is encrypted. Therefore, the receiving end can easily handle the digital material no matter whether it is encrypted.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram illustrating a typical configuration of a transmission/reception system according to an embodiment of the present invention;  
       FIG. 2  is a block diagram illustrating a typical configuration of a cinema server in the transmission/reception system shown in  FIG. 1 ;  
       FIG. 3  illustrates a typical position at which meta data generated by the cinema server shown in  FIG. 2  is superposed over AV data;  
       FIG. 4  shows a typical structure of meta data, which includes the data that is used for encryption in the cinema server shown in  FIG. 4  and for decryption in a projector shown in  FIG. 1 ;  
       FIG. 5  shows a typical structure of meta data, which includes the data that is used for encryption in the cinema server shown in  FIG. 4  and for decryption in the projector shown in  FIG. 1 ;  
       FIG. 6  shows a typical structure of meta data, which includes the data that is used for encryption in the cinema server shown in  FIG. 4  and for decryption in a projector shown in  FIG. 1 ;  
       FIG. 7  is a block diagram illustrating a typical configuration of a sub AV server in the transmission/reception system shown in  FIG. 1 ;  
       FIG. 8  is a block diagram illustrating a typical configuration of a projector in the transmission/reception system shown in  FIG. 1 ;  
       FIG. 9  is a block diagram illustrating the details of a typical configuration of a meta data extraction section in the projector shown in  FIG. 8 ;  
       FIG. 10  is a flowchart illustrating a typical process that is performed by the projector shown in  FIG. 8 ;  
       FIG. 11  is a block diagram illustrating another typical configuration of the transmission/reception system according to an embodiment of the present invention, which differs from the configuration shown in  FIG. 1 ;  
       FIG. 12  is a block diagram illustrating a typical configuration of the cinema server in the transmission/reception system shown in  FIG. 11 , that is, a typical configuration that differs from the configuration shown in  FIG. 2 ;  
       FIG. 13  is a block diagram illustrating a typical configuration of the projector in the transmission/reception system shown in  FIG. 11 , that is, a typical configuration that differs from the configuration shown in  FIG. 8 ;  
       FIG. 14  is a block diagram illustrating the details of a typical configuration of the meta data extraction section in the projector shown in  FIG. 11 , that is, a typical configuration that differs from the configuration shown in  FIG. 9 ; and  
       FIG. 15  is a block diagram illustrating a typical configuration of a personal computer that executes programs according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Embodiments of the present invention will now be described. The relationship between the constituent features of the present invention and the embodiments described under “DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS” is exemplified below. This statement verifies that the embodiments supporting the present invention are described under “DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.” Therefore, even if a certain embodiment is not described here as an embodiment that corresponds to the constituent features of the present invention although the embodiment is described under “DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS,” it does not mean that the embodiment does not correspond to the constituent features. Conversely, even if the embodiment is described here as an embodiment that corresponds to the constituent features, it does not mean that the embodiment does not correspond to the other constituent features.  
      The transmission/reception system (e.g., the transmission/reception system shown in  FIG. 1  or  FIG. 11 ; however, the following description merely deals with the transmission/reception system shown in  FIG. 1 ) according to an embodiment of the present invention includes a first sender device, a second sender device, and a receiver device. The first sender device (e.g., a cinema server  1  shown in  FIG. 1 ) encrypts stream data by a predetermined encryption method and transmits the encrypted stream data. The second sender device (e.g., a sub AV server  2  shown in  FIG. 1 ) transmits stream data without encrypting encryption. The receiver device (e.g., a projector  4  that is shown in  FIG. 1  and configured as indicated in  FIG. 8 ) receives stream data transmitted from the first or second sender device. The receiver device includes a receiver (e.g., a receiver section  71  shown in  FIG. 8 ) and a generator (e.g., a meta data extraction section  72  shown in  FIG. 8 ). The receiver receives the stream data transmitted from the first or second sender device. The generator determines whether the stream data received by the receiver is encrypted, and generates encryption information (e.g., an Encrypted input signal  102  shown in  FIG. 8 ) that indicates the obtained determination result.  
      The reception method according to an embodiment of the present invention is a reception method for the above-mentioned transmission/reception system according to an embodiment of the present invention. The receiver device (e.g., the projector  4  that is shown in  FIG. 1  and configured as indicated in  FIG. 8 ) includes the step of determining whether the received stream data is encrypted and generating encryption information that indicates the obtained determination result (e.g., step S 2  shown in  FIG. 10 ).  
      The receiver device according to an embodiment of the present invention (e.g., the projector  4  that is shown in  FIG. 1  and configured as indicated in  FIG. 8  or a projector  202  that shown in  FIG. 11  and configured as indicated in  FIG. 13 ; however, the following description deals with projector  4  except in a situation where only projector  202  is supported) receives stream data. The stream data are transmitted after being encrypted by a first encryption method (e.g., encrypted AV data transmitted from the cinema server  1  shown in  FIG. 1 ) or are transmitted without being encrypted (e.g., unencrypted AV data transmitted from the sub AV server  2  shown in  FIG. 1 ). The receiver device includes a receiver (e.g., the receiver section  71  shown in  FIG. 8 ) and a generator (e.g., the meta data extraction section  72  shown in  FIG. 8 ). The receiver receives the encrypted or unencrypted stream data. The generator determines whether the stream data received by the receiver is encrypted, and generates encryption information (e.g., the Encrypted input signal  102  shown in  FIG. 8 ) that indicates the obtained determination result.  
      The receiver device further includes a decryptor (e.g., a decryption section  73  shown in  FIG. 8 ) and a decryption controller (e.g., a switching section  75  shown in  FIG. 8 ). The decryptor performs a first decryption process on encrypted stream data by a first decryption method that corresponds to the first encryption method. The decryption controller permits the decryptor to perform the first decryption process on the stream data when the encryption information generated by the generator indicates that the stream data is encrypted, and prohibits the decryptor from performing the first decryption process on the stream data when the encryption information generated by the generator indicates that the stream data is not encrypted.  
      The first encryption method and the first decryption method are an encryption method and a decryption method that are based on a common key (e.g., an LE key  31 -D shown in  FIG. 8 ). The common key used for encrypting the stream data is transmitted to the receiver device. The receiver further receives the common key. The generator further supplies the common key, which is received by the receiver, to the decryptor when the stream data is determined to be encrypted. The decryptor performs the first decryption process on the stream data by using the common key that is supplied from the generator.  
      The generator further determines before the first decryption process of the decryptor whether the common key is normally prepared, and generates common key preparation information (e.g., a Key not found signal  101  shown in  FIG. 8 ) that indicates the obtained determination result. The receiver device further includes a presenter (e.g., a state presentation section  74  shown in  FIG. 8 ) that identifies the state of the receiver device in accordance with the encryption information and the common key preparation information, which are both generated by the generator, and presents the obtained identification result.  
      The presenter identifies and presents a first state (e.g., an “unencrypted input state” mentioned in step S 8 , which is shown in  FIG. 10 ), a second state (e.g., an “encrypted input normal state” mentioned in step S 6 , which is shown in  FIG. 10 ), or a third state (e.g., an “encrypted input error state” mentioned in step S 10 , which is shown in  FIG. 10 ). The first state is where unencrypted stream data is received when the encryption information indicates that the stream data is not encrypted. The second state is where encrypted stream data is received and the first decryption process is normally performed on the stream data when the encryption information indicates that the stream data is encrypted and the common key preparation information indicates that the common key is normally prepared. The third state is where encrypted stream data is received but the first decryption process is abnormally performed on the stream data when the encryption information indicates that the stream data is encrypted and the common key preparation information indicates that the common key is not normally prepared.  
      The common key is transmitted to the receiver device after being further encrypted by a second encryption method. The generator performs a second decryption process on the encrypted common key by a second decryption method, which is related to the second encryption method, when the encrypted common key is received by the receiver (e.g., an LEKP restoration section  122  shown in  FIG. 9  performs an associated process). The generator supplies the common key to the decryptor (e.g., a register  125  shown in  FIG. 9  performs an associated process) and generates the common key preparation information indicating that the common key is normally prepared (e.g., an LEKP table  123  shown in  FIG. 9  performs an associated process) when the second decryption process is successfully performed before the first decryption process of the decryptor. The generator generates in the other situations the common key preparation information indicating that the common key is not normally prepared (e.g., the LEKP table  123  shown in  FIG. 9  performs an associated process).  
      The second encryption method and the second decryption method are a public key cryptosystem (RSA encryption method hereinafter described) based on a public key and a decryption method that is associated with the public key cryptosystem and based on a private key.  
      The common key encrypted with the public key is included in meta data (e.g., meta data  34  shown in  FIG. 8 ; more specifically, meta data  34 - 1  shown in  FIG. 4  or meta data  34 - 2  shown in  FIG. 5 , which are included as Elekp data) of the stream data that is encrypted with the common key. The meta data is superposed over the stream data, and the stream data over which the meta data is superposed is transmitted to the receiver device.  
      The generator generates the encryption information indicating that the stream data is encrypted, extracts the encrypted common key from the meta data, and performs the second decryption process, which uses the private key corresponding to the public key, on the common key when the meta data is superposed over the stream data received by the receiver device. The generator generates the encryption information indicating that the stream data is not encrypted when the meta data is not superposed over the stream data received by the receiver device.  
      The stream data uses a plurality of common keys. An identifier other than a special value (e.g., 0 in a later example) is assigned to each of the plurality of common keys. The identifier for each of the plurality of common keys is included in the meta data.  
      The generator prohibits the generation of the encryption information indicating that the stream data is encrypted and generates the encryption information indicating that the stream data is not encrypted (e.g., a meta data extraction/separation section  121  shown in  FIG. 9  performs an associated process) when the identifier included in the meta data is the special value in a situation where the meta data is superposed over the stream data received by the receiver device.  
      The receiver device further includes a reproduction controller (e.g., a switching section  79  and a reproduction control section  80  shown in  FIG. 8 ). The reproduction controller controls the reproduction of first stream data, which is obtained when the decryptor performs the first decryption process on the encrypted stream data, when the decryption controller permits the execution of the first decryption process. The reproduction controller controls the reproduction of second stream data, which is received by the receiver and not encrypted, when the decryption controller prohibits the execution of the first decryption process.  
      When the common key preparation information indicates that the common key is not normally prepared in a situation where the decryption controller permits the execution of the first decryption process, the reproduction controller prohibits the reproduction of the first stream data but controls the reproduction of data generated according to predefined rules (e.g., changes the input of the switching section  79  to select the switching section  78 ).  
      The data whose reproduction is controlled by the reproduction controller instead of the first stream data is image data corresponding to a predetermined stationary image (e.g., stationary image data stored in a stationary image storage section  76  shown in  FIG. 8 ).  
      The data whose reproduction is controlled by the reproduction controller instead of the first stream data is image data (e.g., frame data stored in a frame storage section  77  shown in  FIG. 8 ) corresponding to a predetermined still picture among the first stream data whose reproduction is controlled before the reproduction control of the data.  
      As regards the receiver device (here, it is assumed that the receiver device is projector  202 , which is shown in an example in  FIG. 11 ), the encrypted stream data (e.g., encrypted AV data transmitted from a cinema server  201  shown in  FIG. 11 ) is transmitted to the receiver device via a first route (e.g., a route via a router  3  shown in  FIG. 11 ). The common key (e.g., an LE key+key ID  211  shown in  FIG. 11 ) used by the encrypted stream data is transmitted to the receiver device via a second route (e.g., a route via a network  203 ). The receiver includes a first receiver (e.g., an AV receiver section  231  shown in  FIG. 13 ), which receives the unencrypted stream data or the encrypted stream data transmitted via the first route, and a second receiver (e.g., a key receiver section  232  shown in  FIG. 13 ), which receives the common key transmitted via the second route.  
      The reception method/program according to an embodiment of the present invention relates to the aforementioned receiver device according to an embodiment of the present invention (e.g., the projector  4  that is shown in  FIG. 1  and configured as indicated in  FIG. 8 ). The reception method/program includes the step of determining whether the received stream data is encrypted and generating encryption information that indicates the obtained determination result (e.g., step S 2  shown in  FIG. 10 ).  
      Embodiments of the present invention will now be described with reference to the accompanying drawings.  
       FIG. 1  illustrates a typical configuration of the transmission/reception system according to an embodiment of the present invention.  
      The transmission/reception system shown in  FIG. 1  is used to play content made of a digital material in a movie theater or the like. The reproduction content according to an embodiment of the present invention includes a movie, which is a main piece of content, and trailers and commercials, which are incidental pieces of content. Therefore, the transmission/reception system shown in  FIG. 1  includes a cinema server  1 , a sub AV server  2 , a router  3 , and a projector  4 .  
      The word “include” is used because the system may include some additional devices. The same also holds true for the subsequent description.  
      In the example shown in  FIG. 1 , the cinema server  1  outputs the stream data for the movie, that is, the main content data, in the form, for instance, of an HD-SDI signal. In such an instance, the cinema server  1  encrypts and outputs the stream data to protect the main content by using the AES (Advanced Encryption Standard), which is one of various common key based encryption methods (hereinafter referred to as common key cryptosystems).  
      Meanwhile, the sub AV server  2  outputs the stream data for trailers and commercials, which are incidental content data, in the form, for instance, of an HD-SDI signal. The stream data output from the sub AV server  2  is not encrypted.  
      The stream data (HD-SDI output signal) output from the cinema server  1  is hereinafter referred to as the encrypted AV data. The stream data (HD-SDI output signal) output from the sub AV server  2  is hereinafter referred to as the unencrypted AV data. The stream data that is still not encrypted within the cinema server  1  (the stream data stored in an AV data storage section  11  that is shown in  FIG. 2  and described later), the encrypted AV data (the stream data output from the cinema server  1 ), and the unencrypted AV data (the stream data output from the sub AV server  2 ) are collectively referred to as the AV data.  
      The router  3  selects either the encrypted AV data from the cinema server  1  or the unencrypted AV data from the sub AV server  2 , and supplies the selected AV data to the projector  4 .  
      The projector  4  receives the AV data from the router  3  and controls the reproduction of the content corresponding to the received AV data. More specifically, the projector  4  handles the video and audio of the movie, trailers, and commercials, projects the video on a screen or the like, and outputs the audio from loudspeakers or the like.  
      The cinema server  1 , sub AV server  2 , and projector  4  of the transmission/reception system shown in  FIG. 1  will now be described in detail in the order named.  
       FIG. 2  illustrates a typical configuration of the cinema server  1 .  
      In  FIG. 2 , solid-line squares represent blocks or component elements of a device or system (the cinema server  1  in the current example), whereas broken-line squares represent predetermined items of information. This selective use of solid lines and broken lines also apply to the other drawings referenced in the subsequent description.  
      In the example shown in  FIG. 2 , the cinema server  1  includes an AV data storage section  11 , an encryption section  12 , a meta data generation section  13 , a superposition section  14 , and a sender section  15 .  
      The AV data storage section  11  stores one or more AV data corresponding to a movie.  
      In the example shown in  FIG. 2 , the encryption section  12  includes an AES encryption data generation section  21  and an AV data encryption section  22 .  
      The AES encryption data generation section  21  uses an LE key  31 -E (this may be referred to as the AES key), which is a common key for the AES, and an AES input  32 -E to generate the data  35  (hereinafter referred to as the AES encryption data  35 ) for directly encrypting predetermined AV data stored in the AV data storage section  11 , and supplies the generated data to the AV data encryption section  22 . The AES input  32 -E will be described later.  
      The AV data encryption section  22  encrypts a predetermined piece of data among one or more pieces of AV data stored in the AV data storage section  11  by using the AES encryption data  35 , which is fed from the AES encryption data generation section  21 , and supplies the resulting encrypted AV data to the superposition section  14 . In this instance, the AV data encryption section  22  encrypts the AV data in the unit of a frame by using a Frame reset  33 -E, which is fed from the meta data generation section  13 .  
      For explanation purposes, it is assumed that the AV data according to an embodiment of the present invention is an HD-SID signal as mentioned above and made of one or more pieces of frame data. It is also assumed that the frame data includes data Y, which indicates the brightnesses of all pixels constituting the frame, and data Cb/Cr, which indicates the colors of all pixels constituting the frame. Further, it is assumed that the LE key  31 -E and AES input  32 -E are made of 128 bits.  
      In the above instance, the AES encryption data generation section  21 , for example, generates 128-bit AES encryption data  35  from 256-bit input data, which has the 128-bit LE key  31 -E and 128-bit AES input  32 -E, and supplies the generated data to the AV data encryption section  22 . More specifically, the AES encryption data generation section  21 , for example, obtains the lowest 120 bits of the 128-bit AES encryption data  35  in 10-bit units as the data that is actually used for AV data encryption, and supplies the obtained bits to the AV data encryption section  22 .  
      The AV data encryption section  22  uses each set of 10-bit data, converts data Y and data Cb/Cr in real time, and supplies the encrypted data to the superposition section  14  on an individual basis.  
      In following explanation, however, data Y and data Cb/Cr are generically referred to as AV data except when they need to be differentiated from each other.  
      The meta data generation section  13  generates various items of information necessary for the encryption process of the encryption section  12  (i.e., various items of information for the projector&#39;s (projector  4 ) decryption process, which will be described later), such as the LE key  31 -E, AES input  32 -E, and Frame reset  33 -E. Further, the meta data generation section  13  generates data called an LEKP (Link Encryption Key Payload) by adding some of the above-mentioned various items of information to the LE key  31 -E as supplementary information. Next, the meta data generation section  13  performs an encryption process on the LEKP by an encryption method that uses the public key of the projector  4  (hereinafter referred to as the public key cryptosystem), more specifically, by, for instance, the RSA (R. Rivest, A. Shamir, L. Adelman) (trademark) 2048-bit encryption method (hereinafter referred to as the RSA encryption method). The data obtained when the LEKP is encrypted by the RSA encryption method is hereinafter referred to as the ELEKP. In other words, the meta data generation section  13  generates the ELEKP. The meta data generation section  13  generates meta data  34 , which includes an element (Le_attribute_data, which will be described later) of the AES input  32 -E, in addition to the ELEKP, and supplies the generated meta data to the superposition section  14 . Examples of meta data  34  will be described later with reference to FIGS.  4  to  6 .  
      The superposition section  14  superposes the meta data  34 , which is transmitted from the meta data generation section  13 , over a predetermined portion of the encrypted AV data, which is transmitted from the AV data encryption section  22 . The encrypted AV data over which the meta data  34  is superposed is supplied from the superposition section  14  to the sender section  15 .  
      More specifically, the present embodiment superposes one meta data  34  over each frame data (one frame) constituting the encrypted AV data during its V-ANC (V-blanking period) as indicated in  FIG. 3 .  
      The left-hand half of  FIG. 3  indicates insertion (superposition) position of the meta data  34  in the case of progressive (P), whereas the right-hand half indicates insertion (superposition) position of the meta data  34  in the case of progressive segmented frame (P) or interface (I) The term “H-ANC” denotes the H-blanking area. The term “active video area” denotes a so-called effective line area.  
      Returning to  FIG. 2 , the sender section  15  transmits to the router  3 , which is shown in  FIG. 1 , the encrypted AV data, which is fed from the superposition section  14 , that is, the encrypted AV data over which the meta data  34  is superposed in the unit of a frame.  
      The present embodiment does not use one LE key  31 -E for one piece of AV data (entire stream data). An update may be performed. In other words, a plurality of LE keys  31 -E are used for one piece of AV data. The reason is stated below.  
      If a malicious third party steals AES-encrypted content (AES-encrypted AV data for a movie in the current example) in a situation where the content is encrypted with one type of AES key (LE key  31 -E in the current example), there will be an increased danger that the AES key might be directly deciphered. To solve this problem, the meta data generation section  13  according to the present embodiment periodically changes the AES key (update the AES key) for the purpose of making it difficult for third parties to directly decipher content.  
      In the present embodiment, the cinema server  1  protects the LE key  31 -E by not directly transmitting the LE key  31 -E as it is. More specifically, the meta data generation section  13  generates the LEKP, which is a combination of the LE key  31 -E and supplementary data, performs an RSA encryption process on the LEKP, and generates the meta data  34 , which contains the resulting ELEKP, as described above. The superposition section  14  then superposes the meta data  34  over the encrypted AV data. The sender section  15  transmits the encrypted AV data over which the meta data  34  is superposed.  
      In other words, the cinema server  1  according to the present embodiment performs an RSA encryption process on each of a plurality of LE keys  31 -E and transmits each of the plurality of RSA-encrypted LE keys  31 -E (after being contained in a predetermined frame in the order of generation) to the projector  4  via router  3 . Therefore, the projector  4  also performs an RSA decryption process on each of the plurality of RSA-encrypted LE keys  31 -E as described later.  
      As described above, the present embodiment uses a plurality of LE keys  31 -E, instead of only one LE key  31 -E, for the AV data corresponding to a movie (an update may be performed). More specifically, the LE key  31 -E used for encrypting each frame (data) constituting a piece of AV data is not limited to one. The LE key  31 -E is updated to use a new LE key  31 -E every certain number of frames.  
      In the present embodiment, therefore, the meta data generation section  13  attaches an LE key identifier (hereinafter referred to as a key ID) to each of the plurality of LE keys  31 -E whenever they are generated. The key ID is also included in the meta data  34  as described later. However, a predetermined special value (the value 0 in the present embodiment) is not attached as the key ID (is excluded).  
      The LE key  31 -E for AV data encryption needs to be generated before the AV data encryption section  22  performs an encryption process on the AV data. In other words, there is a time lag between the instant at which the encrypted AV data is generated and the instant at which the LE key  31 -E for AV data encryption is generated. As a result, in the superposition section  14 , the LE key  31 -E included in the meta data  34  superposed over a predetermined frame (data) of the encrypted AV data (more precisely, the LE key  31 -E included in the LEKP prevailing before RSA encryption) is not used when the predetermined frame (data) is encrypted, but is to be used for encrypting a frame (data) that comes after the predetermined frame.  
      Examples of meta data  34 , which is generated by the meta data generation section  13 , will be described with reference to FIGS.  4  to  6  before describing examples of the sub AV server  2  and projector  4  ( FIG. 1 ) in detail.  
       FIGS. 4 and 5  show typical structures of meta data  34 - 1  and  34 - 2 , which include the aforementioned ELEKP (RSA-encrypted LE key  31 -E and the like).  FIG. 6  shows a typical structure of meta data  34 - 3 , which includes an AES input  32 -E.  
      The packet structures of meta data  34 - 1 ,  34 - 2 , and  34 - 3 , which are employed by the present embodiment, conform to the SMPTE (Society of Motion Picture and Television Engineers) 291M standard (Proposal SMPTE STANDARD for Television-Ancillary Data Packet and Space Formatting). They are generally referred to as meta packets  1 ,  2 , and  3 , respectively. In the packet structures defined by the SMPTE 291M standard, various items of information including an ADF (Ancillary Data Flag (000, 3FF, 3FF for component; 3FC for composite)), DID (Data ID), SDID (Secondary Data ID), DC (Data Count), User Data, and CS (Check Sum) are arranged in the order named, as indicated in FIGS.  4  to  6 . Meta data  34 - 1 ,  34 - 2 , and  34 - 3  have such packet structures. In the present embodiment, the user data includes the following items of information.  
      Within meta data  34 - 1 , the user data includes a Key ID, Type, SHAI digest, Lekp length, Elekp length, and Elekp data. The user data within meta data  34 - 2  includes Elekp data.  
      Explanations will now be given in random order. As described earlier, the Elekp data is the ELEKP, that is, the data obtained when an RSA encryption process is performed on the LEKP, which includes the LE key  31 -E and the like. The Key ID is an identifier for the LE key  31 -E that is encrypted and included in the ELEKP as described earlier. The Elekp length is the data length of the ELEKP. The Lekp length is the data length of the LEKP that corresponds to the ELEKP.  
      The Type is an encryption method (algorithm type) that was used for LEKP encryption. In the present embodiment, the value 0, which represents the RSA encryption method, is substituted as the Type. The SHAI digest is an identifier for the public key that was used when the ELEKP was generated (when the LEKP was RSA-encrypted).  
      The user data in meta data  34 - 1  and meta data  34 - 2 , which include the various items of information described above, is hereinafter referred to as the LEKM (Link Encryption Key Message).  
      Unlike the above LEKM (user data in meta data  34 - 1  and meta data  34 - 2 ), the user data in meta data  34 - 3  includes a Next Key ID, Current Key ID, Current Frame Count, Key Changing Timing, and HD-SDI Link Number.  
      A frame into which the target meta data  34 - 3  is inserted, more precisely, the AV data frame data into which meta data  34 - 3  is inserted, is referred to as the relevant frame. Further, the LE key  32 -E prevailing, for instance, immediately before the start of the encryption of the relevant frame is referred to as the current LE key  32 -E. The LE key  32 -E that is to be generated next to the current LE key  32 -E (to be generated by the meta data generation section  13  at the next update time) is referred to as the next LE key  32 -E. The LE key  32 -E that was generated immediately before the current LE key  32 -E (generated by the meta data generation section  13  at the preceding update time) is referred to as the previous LE key  32 -E.  
      The Next Key ID, Current Key ID, Current Frame Count, Key Changing Timing, and HD-SDI Link Number in the meta data  34 - 3  inserted into the relevant frame are the information described below.  
      The Next Key ID is the key ID of the next LE Key  32 -E. The Current Key ID is the key ID of the current LE key  32 -E.  
      The Current Frame Count is a value indicating what number frame is the relevant frame when counting is performed from the frame (frame No. 0) prevailing when the previous LE key  32 -E is updated to the current LE key  32 -E (this time is hereinafter referred to as the Key Changing Timing).  
      As the Key Changing Timing, 2′b11, 2′b10, 2′b01, or 2′b00 is substituted. These values indicate what frame the next Key Changing Timing represents. For example, the value 2′b11 (=3) indicates that the Key Changing Timing represents the third or subsequent frame from the relevant frame. The value 2′b10 (=2) indicates that the Key Changing Timing represents the second frame from the relevant frame. The value 2′b01 (=1) indicates that the Key Changing Timing represents the first frame from the relevant frame (that is, the next frame). The value 2′b00 (=0) indicates that the Key Changing Timing represents the relevant frame.  
      The HD-SDI Link Number is a value indicating the style of HD-SDI signal transmission between the cinema server  1  and projector  4  (the encrypted AV data transmission style in the present embodiment). When, for instance, the value 0 is substituted as the HD-SDI Link Number, it indicates Link-A, which is a single link (the transmission style based on one HD-SDI interface) or dual link (the transmission style based on two HD-SDI interfaces). When, for instance, the value 1 is substituted as the HD-SDI Link Number, it indicates Link-B, which is a dual link.  
      The LE key  31 -E that is included in the ELEKP for the meta data  34 - 1 ,  34 - 2  inserted into the relevant frame (more precisely, included in the unencrypted LEKP) is used to encrypt a frame that is positioned after the relevant frame. In other words, the LE key  31 -E used for the relevant frame is included in the ELEKP for the meta data  34 - 1 ,  34 - 2  inserted into a frame preceding the relevant frame (more precisely, included in the unencrypted LEKP).  
      In the present embodiment, the encrypted AV data over which meta data  34 - 1  to  34 - 3  are superposed is generated by the cinema server  1  ( FIGS. 1 and 2 ) and may be transmitted to the projector  4  via the router  3  as described above.  
      The sub AV server  2 , which is shown in  FIG. 1 , will now be described in detail with reference to  FIG. 7 .  
       FIG. 7  illustrates a typical configuration of the sub AV server  2 . In the example shown in  FIG. 7 , the sub AV server  2  includes an AV data storage section  51  and a sender section  52 .  
      The AV data storage section  51  stores one or more pieces of AV data corresponding to movie trailers and commercials.  
      The sender section  52  transmits to the router  3 , which is shown in  FIG. 1 , one piece of unencrypted AV data, that is, a predetermined one of one or more pieces of AV data stored in the AV data storage section  51 .  
      The projector  4 , which is shown in  FIG. 1 , will now be described in detail with reference to FIGS.  8  to  10 .  
       FIG. 8  illustrates a typical configuration of the projector  4 . In the example shown in  FIG. 8 , the projector  4  includes a receiver section  71 , a meta data extraction section  72 , a decryption section  73 , a state presentation section  74 , switching sections  75 ,  78 ,  79 , a stationary image storage section  76 , a frame storage section  77 , and a reproduction control section  80 .  
      The receiver section  71  receives AV data from the router  3  ( FIG. 1 ). More specifically, the receiver section  71  receives encrypted AV data from the cinema server  1  ( FIGS. 1 and 2 ) or unencrypted AV data from the sub AV server  2  ( FIGS. 1 and 7 ).  
      The meta data extraction section  72  determines whether the AV data received by the receiver section  71  is encrypted or unencrypted.  
      When the obtained determination result indicates that the AV data is encrypted, the meta data extraction section  72  generates an encryption detection signal  102  (hereinafter referred to as the Encrypted input signal  102  in accordance with  FIG. 8 ) and supplies it to the state presentation section  74  and switching section  75 .  
      Further, the meta data extraction section  72  extracts the meta data  34  (meta data  34 - 1  to  34 - 3  in the present embodiment) and other information from the encrypted AV data. The meta data extraction section  72  then generates an LE key  31 -D, AES input  32 -D, and Frame reset  33 -D from the extracted information, and supplies them to the decryption section  73 . The LE key  31 -D, AES input  32 -D, and Frame reset  33 -D are restored respectively from the LE key  31 -E, AES input  32 -E, and Frame reset  33 -E that were used when the cinema server  1  generated encrypted AV data. Therefore, it can be understood that the meta data extraction section  72  restores the LE key  31 -E, AES input  32 -E, and Frame reset  33 -E and supplies them to the decryption section  73 .  
      If, for some reason, the LE key  31 -D is not normally generated (the LE key  31 -E is not restored), that is, if the LE key  31 -D, which is a decryption key, is not registered (not supplied) for the decryption section  73 , which is described later, the meta data extraction section  72  generates a decryption key unregistered signal  101  (hereinafter referred to as the Key not found signal  101  in accordance with  FIG. 8 ) and supplies it to the state presentation section  74 , frame storage section  77 , and switching section  79 .  
      When, on the other hand, unencrypted AV data is received by the receiver section  71 , the unencrypted AV data does not include meta data  34  so that the meta data extraction section  72  may not be able to extract the meta data  34 . When the meta data  34  is not superposed over the AV data received by the receiver section  71 , the meta data extraction section  72  determines that the AV data is not encrypted, and prohibits the generation of the Encrypted input signal  102 .  
      When predefined conditions are met in a situation where AV data over which the meta data  34  is superposed is received by the receiver section  71 , the AV data may be regarded as unencrypted AV data to prohibit the generation of the Encrypted input signal  102 . The above-mentioned predefined conditions will be described later.  
      It may be assumed that the level of the Key not found signal  101  is either Hi (1) or Lo (0). The Hi level (“1” level) may be regarded as the generation of the Key not found signal  101 . The Lo level (“0” level) may be regarded as the prohibition of Key not found signal  101  generation. When the LE key  31 -D is normally generated (the LE key  31 -E is restored) in this instance, the level of the Key not found signal  101  is Lo (0). When the LE key  31 -D is not normally generated (the LE key  31 -E is not restored), the level of the Key not found signal  101  is Hi (1).  
      Similarly, it may also be assumed that the level of the Encrypted input signal  102  is either Hi (1) or Lo (0). The Hi level (“1” level) may be regarded as the generation of the Encrypted input signal  102 . The Lo level (“0” level) may be regarded as the prohibition of Encrypted input signal  102  generation. When encrypted AV data is received by the receiver section  71  in this instance, the level of the Encrypted input signal  102  is Hi (1). When, unencrypted AV data is received by the receiver section  71 , the level of the Encrypted input signal  102  is Lo (0).  
      The following explanation assumes that the levels of the Key not found signal  101  and Encrypted input signal  102  are either Hi (1) or Lo (0).  
      The meta data extraction section  72  has been outlined above. The meta data extraction section  72  will be described in detail with reference to  FIG. 9 .  
      In the example shown in  FIG. 8 , the decryption section  73  includes an AES decryption data generation section  91  and an AV data decryption section  92 .  
      The AES decryption data generation section  91  uses the LE key  31 -D and AES input  32 -D, which are supplied from the meta data extraction section  72 , to generate the data  103  for directly decrypting encrypted AV data (hereinafter referred to as the AES decryption data  103 ), and supplies it to the AV data decryption section  92 . In other words, the AES decryption data  103  is the decryption data corresponding to the AES encryption data  35 , which is shown in  FIG. 2 .  
      The AV data decryption section  92  performs a decryption process on the encrypted AV data received by the receiver section  71  by using the AES decryption data  103  supplied from the AES decryption data generation section  91 , and supplies the resulting AV data (hereinafter referred to as the restored AV data in order to distinguish it from unencrypted AV data) to the switching section  75 . In this instance, the AV data decryption section  92  decrypts the encrypted AV data in the unit of a frame by using the Frame reset  33 -D supplied from the meta data extraction section  72 .  
      It should be pointed out in this connection that the same AES decryption data  103  is not used for the whole encrypted AV data, which is stream data. As described earlier, the LE keys  31 -D corresponding to a plurality of LE keys  31 -E, which are updated by the cinema server  1  at predetermined intervals, are used, that is, a plurality of LE keys  31 -D that are updated at predetermined intervals are used to generate a plurality of AES decryption data  103 . Further, the plurality of AES decryption data  103  are used respectively for the associated portions of the encrypted AV data (the associated frames).  
      The state presentation section  74  identifies the current state of the projector  4  depending on whether the Key not found signal  101  and Encrypted input signal  102  are supplied (generated) from the meta data extraction section  72 , and presents the obtained identification result to the user of the projector  4  (e.g., the person who reproduces the movie).  
      In the present embodiment, the following first to third states are defined as the current state of the projector  4 .  
      The first state is a state where encrypted AV data is received and normally decrypted (hereinafter referred to as the encrypted input normal state). When the Key not found signal  101  is not supplied (Lo level) and the Encrypted input signal  102  is supplied (Hi level), the current state of the projector  4  is identified as the encrypted input normal state.  
      The second state is a state where encrypted AV data is received but erroneously decrypted (hereinafter referred to as the encrypted input error state). When the Key not found signal  101  is supplied (Hi level) and the Encrypted input signal  102  is supplied (Hi level), the current state of the projector  4  is identified as the encrypted input error state.  
      The third state is a state where unencrypted AV data is received (hereinafter referred to as the unencrypted input state). When the Encrypted input signal  102  is not supplied (Lo level), the current state of the projector  4  is identified as the unencrypted input state.  
      The presentation method used by the state presentation section  74  is not particularly limited. For example, it may display an image or generate an audio output. However, the present embodiment assumes that the state presentation section  74  has a lamp that illuminates in two different colors, more specifically, a lamp that illuminates, for instance, in red and in green, and uses the following presentation method. In the present embodiment, the lamp illuminates in green in the encrypted input normal state, illuminates in red in the encrypted input error state, and becomes extinguished (turns off) in the unencrypted input state. This enables the user of the projector  4  (e.g., the person who reproduces the movie) to monitor three different states easily in real time.  
      The switching section  75  switches its input depending on whether the Encrypted input signal  102  is supplied from the meta data extraction section  72  (depending on whether the level is Hi or Lo).  
      More specifically, when the Encrypted input signal  102  is supplied from the meta data extraction section  72  (when the level is Hi), it means that encrypted AV data is received by the receiver section  71  (the encrypted input normal state or encrypted input error state prevails). In such a situation, the switching section  75  switches to the input from the AV data decryption section  92 . The restored AV data, which is obtained when the encrypted AV data is decrypted by the AV data decryption section  92 , is then input to the switching section  75  and supplied to the frame storage section  77  and switching section  79  (however, if the encrypted input error state prevails, noise data is obtained instead of the restored AV data).  
      On the other hand, when the Encrypted input signal  102  is not supplied from the meta data extraction section  72  (when the level is Lo), it means that unencrypted AV data is received by the receiver section  71  (the unencrypted input state prevails). In such a situation, the switching section  75  switches to the input from the receiver section  71 . The unencrypted AV data is then input to the switching section  75  and supplied to the frame storage section  77  and switching section  79 .  
      As described above, it can be understood that the switching section  75  is capable of inhibiting the decryption section  73  from performing a decryption process (prohibiting the execution of the decryption process) when unencrypted AV data is received by the receiver section  71 .  
      In the encrypted input error state, noise data is output from the AV data decryption section  92  as described above. If the noise data is used for reproduction purposes, an awful image (noise image) appears on the screen or the like. Therefore, the present embodiment projects an alternative image onto the screen or the like in the encrypted input error state. The alternative image (hereinafter referred to as the stationary image) is a predetermined pattern image or an image containing the message “Wait for a while.” 
      The image data for the stationary image (hereinafter referred to as the stationary image data) is stored in the stationary image storage section  76 .  
      In the present embodiment, a predetermined still picture, which is a frame for the movie, trailer, or commercial that was just reproduced, may be adopted as the alternative image that is to be projected onto the screen or the like in the encrypted input error state.  
      The frame data for such a frame is stored in the frame storage section  77 . In other words, the frame storage section  77  stores the last-supplied frame data, which is among the AV data supplied from the switching section  75 .  
      In the encrypted input error state, however, the encrypted data is not normally restored and turns out to be noise data as described above. Further, the resulting noise data is supplied to the frame storage section  77 . As described above, the frame storage section  77  exists for the purpose of storing predetermined frame data (still picture data) as the alternative image to be projected onto the screen or the like instead of noise data in the encrypted input error state. If the frame storage section  77  stores the noise data, it fails to achieve its purpose.  
      As such being the case, the frame storage section  77  checks whether the Key not found signal  101  is supplied (the level is Hi or Lo). When the Key not found signal  101  is supplied (the level is Hi), the frame storage section  77  determines that noise data is supplied from the switching section  75 , and prohibits the storage of such data. As a result, the frame storage section  77  continuously stores normal frame data (normal still picture data), which is supplied immediately before a first point of time at which the Key not found signal  101  is supplied, during the time interval between the first point of time (at which the level changes from Lo to Hi) and a second point of time at which the Key not found signal  101  is no longer supplied (at which the level changes back to Lo from Hi).  
      In accordance with the operation performed by the user of the projector  4  (e.g., the person who reproduces the movie) (the operating control section is not shown), the switching section  78  switches to the input from either the stationary image storage section  76  or frame storage section  77 .  
      When the switching section  78  switches to the input from the stationary image storage section  76 , the stationary image data is input to the switching section  78  and supplied to the switching section  79 . When, on the other hand, the switching section  78  switches to the input from the frame storage section  77 , the frame data is input to the switching section  78  and supplied to the switching section  79 .  
      The switching section  79  changes its input depending on whether the Key not found signal  101  is supplied from the meta data extraction section  72  (whether the level is Hi or Lo).  
      More specifically, when the Key not found signal  101  is not supplied from the meta data extraction section  72  (the level is Lo), it means that the encrypted input normal state or unencrypted input state prevails. In such a situation, the switching section  79  switches to the input from the switching section  75 . The restored AV data for the movie or the unencrypted AV data for the trailers and commercials is then input to the switching section  79  and supplied to the reproduction control section  80 .  
      When, on the other hand, the Key not found signal  101  is supplied from the meta data extraction section  72  (the level is Hi), it means that the encrypted input error state prevails, that is, noise data is output from the switching section  75 . In such a situation, the switching section  79  switches to the input from the switching section  78 . The stationary image data or frame data (the data corresponding to a frame for the movie, trailer, or commercial image) is then input to the switching section  79  and supplied to the reproduction control section  80 .  
      The reproduction control section  80  controls the reproduction of the content that corresponds to the content data supplied from the switching section  79 .  
      In the present embodiment, the content data and content are as described below.  
      When the switching section  79  has switched to the input from the switching section  75 , that is, when the encrypted input normal state or unencrypted input state prevails, the content data denotes AV data, whereas the content denotes the movie, trailer, or commercial corresponding to the AV data. In the above instance, therefore, the reproduction control section  80  projects the video corresponding to the AV data, that is, the video of the movie, trailer, or commercial onto the screen or the like, and causes the loudspeaker to output the audio corresponding to the AV data, that is, the audio of the movie, trailer, or commercial. More precisely, while the encrypted input normal state prevails, the reproduction control section  80  projects the video of the movie onto the screen or the like and causes the loudspeaker or the like to output the audio of the movie. While the unencrypted input state prevails, the reproduction control section  80  projects the video of a movie trailer or commercial onto the screen or the like and causes the loudspeaker or the like to output the related audio.  
      When, on the other hand, the switching section  79  has switched to the input from the switching section  78 , that is, when the encrypted input error state prevails, the content data denotes the stationary image data stored in the stationary image storage section  76  or the frame data stored in the frame storage section  77 . The content denotes a stationary image (a predetermined pattern image or an image containing the message “Wait for a while”) corresponding to the stationary image data or a frame (a frame for the movie, trailer, or commercial image) corresponding to the frame data. In this instance, therefore, the reproduction control section  80  exercises control so that a predetermined pattern image or an image containing the message “Wait for a while” is projected onto the screen or the like or that a frame for the movie, trailer, or commercial image is continuously projected onto the screen or the like (a frozen image is projected).  
      As described above, when the encrypted input error state prevails, the switching section  79  is capable of projecting an alternative image onto the screen or the like until the error vanishes (until the encrypted input normal state prevails) while refraining from reproducing noise data.  
      Further, the switching section  78  is capable of allowing the user of the projector  4  (e.g., the person who reproduces the movie) to freely select a stationary image or the last frame (frozen image) of the video projected onto the screen or the like as the alternative image.  
      The meta data extraction section  72 , which is shown in  FIG. 8 , will now be described in detail with reference to  FIG. 9 .  FIG. 9  shows the details of a typical configuration of the meta data extraction section  72 .  
      In the example shown in  FIG. 9 , the meta data extraction section  72  includes a meta data extraction/separation section  121 , an LEKP restoration section  122 , an LEKP table  123 , a key change trigger generation section  124 , registers  125 ,  126 ,  128 , a counter  127 , and an AES input generation section  129 .  
      The meta data extraction/separation section  121  determines whether the meta data  34  is superposed over a predetermined portion (see  FIG. 3 ) of the AV data supplied from the receiver section  71 .  
      If the obtained determination result indicates that the meta data  34  is not superposed, the meta data extraction/separation section  121  concludes that unencrypted AV data is received from the receiver section  71 , prohibits the generation of the Encrypted input signal  102  (invokes a Lo level), and inhibits the meta data extraction section  72  from performing a process.  
      If, on the other hand, the obtained determination result indicates that the meta data  34  is superposed, the meta data extraction/separation section  121  extracts the meta data  34  and other information from the relevant frame, which is among various frames (data) that constitute the AV data.  
      Next, the meta data extraction/separation section  121  confirms the Current Key ID value written in meta data  34 - 3 , which is a part of the meta data  34  including meta data  34 - 1  to  34 - 3  (see FIGS.  4  to  6 ).  
      If the written Current Key ID value is a special value (e.g., 0), the meta data extraction/separation section  121  concludes that unencrypted AV data is received from the receiver section  71 , prohibits the generation of the Encrypted input signal  102  (invokes a Lo level), and inhibits the meta data extraction section  72  from performing a process.  
      If, on the other hand, the written Current Key ID value is a value other than the special value (e.g., 0), the meta data extraction/separation section  121  concludes that encrypted AV data is received from the receiver section  71 , generates the Encrypted input signal  102  (invokes a Hi level), and supplies the generated signal to the state presentation section  74  and switching section  75  ( FIG. 8 ). Further, the meta data extraction/separation section  121  separates various items of information that constitute the meta data  34 , and supplies them to the associated blocks.  
      In the example shown in  FIG. 9 , the meta data extraction/separation section  121  extracts or separates an LEKM  141 , Current LE_Key ID  142 , Next LE_Key ID  143 , Key Change Timing  145 , Current Frame Count  148 , Frame/line reset  150 , and HD-SDI Link Number  152 .  
      The Frame/line reset  150  includes a Frame reset  146  and a line reset  151 . As described earlier, the LEKM  141  is a written user data value for meta data  34 - 1  and  34 - 2 . The Current LE_Key ID  142  is a written Current Key ID value for meta data  34 - 3 . The Next LE_Key ID  143  is a written Next Key ID value for meta data  34 - 3 . The Key Change Timing  145  is a written Key Change Timing value for meta data  34 - 3 . The HD-SDI Link Number  152  is a written HD-SDI Link Number value for meta data  34 - 3 . The Current Frame Count  148  is a written Current Frame Count value for meta data  34 - 3 .  
      The LEKM  141  is supplied to the LEKP restoration section  122 . The Current LE_Key ID  142  and Next LE_Key ID  143  are supplied to the LEKP table  123 . The Key Change Timing  145  is supplied to the key change trigger generation section  124 . The Current Frame Count  148  is supplied to the register  126 . The Frame reset  146  is supplied to the key change trigger generation section  124 . The line reset  151  is supplied to the counter  127 . The Frame/line reset  150 , which includes the Frame reset  146  and line reset  151 , is supplied to the decryption section  73  as the Frame reset  33 -D, which is shown in  FIG. 8 . The HD-SDI Link Number  152  is supplied to the register  128 .  
      The LEKP restoration section  122  restores the LEKP from the LEKM  141 , and stores the association between the LEKP and its Key ID and the like in the LEKP table  123 . In other words, the LEKM  141  is a written user data value for meta data  34 - 1 , which is shown in  FIG. 4 , and meta data  34 - 2 , which is shown in  FIG. 5 , as mentioned earlier. The user data includes the ELEKP (which is designated “Elekp data” in  FIGS. 4 and 5 ). As mentioned earlier, the ELEKP is the data obtained when an RSA encryption process is performed on the LEKP. Therefore, the LEKP restoration section  122  performs a decryption process on the ELEKP included in the LEKM  141  by using a pair key (private key) for the public key for ELEKP generation, and stores the association between the resulting LEKP (decrypted LEKP) and its Key ID and the like in the LEKP table  123 .  
      As described above, the LEKP table  123  stores the associations between one or more LEKPs and LEKP identification Key IDs and the like. Each LEKP stored in the LEKP table  123  includes the LE key  31 -D (restored LE key  31 -E shown in  FIG. 2 ) and some pieces of supplementary information. The present embodiment assumes that the supplementary information includes Le_attribute_data  144 , which is an element of the AES input  32 -E, which is shown in  FIG. 2 . When the register  125  issues a request (with predetermined timing after the issuance of a key change instruction, which will be described later), the LEKP table  123  causes the register  125  to store the LE Key  31 -D (hereinafter referred to as the Current LE Key  31 -D) included in the LEKP having the same Key ID as the Current LE_Key ID  142  and the Le_attribute_data  144  (hereinafter referred to as the Current Le_attribute_data  144 ).  
      The LE Key  31 -D included in the LEKP that has the same Key ID as the Next LE_Key ID  143  is hereafter referred to as the Next LE Key  31 -D. The Le_attribute_data  144  is hereinafter referred to as the Next Le_attribute_data  144 .  
      When the LEKP having the same Key ID as the Current LE_Key ID  142  does not exist, that is, when the Current LE Key  31 -D does not exist, the LEKP table  123  concludes that the Current LE Key  31 -D is not normally generated by the LEKP restoration section  122  (the associated LE Key  31 -E is not restored), that is, the decryption key is not registered, generates the Key not found signal  101  (invokes a Hi level), and supplies the generated signal to the state presentation section  74 , frame storage section  77 , and switching section  79  ( FIG. 8 ).  
      When, on the other hand, the LEKP having the same Key ID as the Current LE_Key ID  142  exists, that is, when the Current LE Key  31 -D exists, the LEKP table  123  concludes that the Current LE Key  31 -D is normally generated by the LEKP restoration section  122  (the associated LE Key  31 -E is restored), that is, the decryption key is registered, and prohibits the generation of the Key not found signal  101  (invokes a Lo level).  
      Each time the Frame reset  146  is supplied, the key change trigger generation section  124  supplies the key change trigger  147  to the register  125 . More specifically, a key change instruction and a key change prohibition instruction exist as the key change trigger  147 , and either of these instructions is supplied to the register  125 . The key change trigger generation section  124  monitors the value of the Key Change Timing  145 . When the monitored value is 2′b00 (=0), that is, when the relevant frame serves as the Key Change Timing, the key change trigger generation section  124  supplies the key change instruction to the register  125  as the key change trigger  147 . When the monitored value is other than 2′b00 (=0), the key change trigger generation section  124  supplies the key change prohibition instruction to the register  125  as the key change trigger  147 .  
      The register  125  stores the Current LE Key  31 -D and Current Le_attribute_data  144 .  
      As far as the key change prohibition instruction is supplied as the key change trigger  125 , the register  125  supplies the Current LE Key  31 -D to the decryption section  73  and the Current Le_attribute_data  144  to the AES input generation section  129 . The decryption section  73  then uses the Current LE Key  31 -D to decrypt the relevant frame (AES-encrypted frame data).  
      When, on the other hand, the key change instruction is supplied as the key change trigger  147 , the register  125  requests the LEKP table  123  to update the stored contents.  
      The LEKP table  123  then causes the register  125  to store the Current LE Key  31 -D (Next LE Key  31 -D as viewed from the LE Key  31 -D, which is stored in the register  125  as the Current LE Key  31 -D) and Current Le_attribute_data  144  (Next Le_attribute_data  144  as viewed from the Le_attribute_data  144 , which is stored in the register  125  as the Current Le_attribute_data  144 ) that prevail when the request is issued by the register  125 .  
      In other words, when viewed from the Current LE Key  31 -D and Current Le_attribute_data  144  prevailing before a key change, the Next LE Key  31 -D and Next Le_attribute_data  144  are stored in the register  125  as the new Current LE Key  31 -D and new Current Le_attribute_data  144  prevailing after the key change.  
      Next, the new Current LE Key  31 -D (which has been the Next LE Key  31 -D) is supplied to the decryption section  73 , and the new Current Le_attribute_data  144  (which has been the Next Le_attribute_data  144 ) is supplied to the AES input generation section  129 .  
      In the decryption section  73 , therefore, the LE Key  31 -D for decryption is updated from the Current LE Key  31 -D to the Next LE Key  31 -D (new Current LE Key  31 -D) so as to decrypt the relevant frame (AES-encrypted frame data).  
      The register  126  may keep the Current Frame Count  148  and may supply it to the AES input generation section  129 .  
      The counter  127  increments its count by one each time the line reset  151  is supplied, and supplies the resulting count to the AES input generation section  129 .  
      The register  128  keep the HD-SDI Link Number  152  and supply it to the AES input generation section  129 .  
      As described above, the AES input generation section  129  inputs the Le_attribute_data  144  from the register  125 , the Current Frame Count  148  from the register  126 , the Line Number of HD SDI  149  from the meta data extraction/separation section  121 , the count reached by the counter  127 , and the HD-SDI Link Number  152  from the register  128 . The AES input generation section  129  then generates the AES input  32 -D (by restoring the AES input  32 -E shown in  FIG. 2 ), which includes at least the Le_attribute_data  144 , the Current Frame Count  148 , the Line Number of HD SDI  149 , the count reached by the counter  127 , and the HD-SDI Link Number  152 , and supplies the generated AES input  32 -D to the decryption section  73 .  
      A typical configuration of the transmission/reception system shown in  FIG. 1  has been described with reference to FIGS.  1  to  9 . The operation performed by the transmission/reception system will now be described.  
      The cinema server  1  encrypts the AV data for a movie and transmits it to the router  3 .  
      The present embodiment employs the AES encryption method, which performs encryption and decryption processes at a high speed, as the method for encrypting the AV data for a movie. The cinema server  1  is configured as indicated in  FIG. 2 .  
      To generate the AES encryption data  35 , which is a code for AES encryption, the meta data generation section  13  shown in  FIG. 2  sequentially generates different LE Keys  31 -E at predetermined update intervals.  
      The AES encryption data generation section  21  uses the sequentially generated LE Keys  31 -E and the associated AES input  32 -E to sequentially generate a plurality of AES encryption data  35 . In other words, the AES encryption data generation section  21  sequentially updates the AES encryption data  35 .  
      The AV data encryption section  22  performs an AES encryption process on the AV data supplied from the AV data storage section  11  on an individual frame basis by using each of the sequentially updated AES encryption data  35 . The resulting encrypted AV data is supplied from the AV data encryption section  22  to the superposition section  14 .  
      The sequentially updated LE Keys  31 -E are combined with some supplementary data (Le_attribute_data  144 , etc.) to produce the LEKP. The meta data generation section  13  performs an encryption process on the LEKP by the RSA encryption method, which is a public key cryptosystem. The ELEKP is obtained as a result of the encryption process. The meta data  34  containing the ELEKP, more specifically, meta data  34 - 1  to  34 - 3  (FIGS.  4  to  6 ) in the present embodiment, is generated for each frame by the meta data generation section  13  and supplied to the superposition section  14 .  
      The superposition section  14  superposes the meta data  34  over all the frames (data) that constitute the encrypted AV data that is supplied from the AV data encryption section  22 . The resulting encrypted AV data over which the meta data  34  are superposed on an individual frame basis, are supplied from the superposition section  14  to the sender section  15  and then transmitted to the router  3 .  
      As described above, the cinema server  1  shown in  FIG. 1  outputs the encrypted AV data for a movie and transmits it to the router  3 .  
      Meanwhile, the sub AV server  2  outputs the unencrypted AV data for trailers and commercials and transmits it to the router  3 .  
      The router  3  selects either the encrypted AV data, which is supplied from the cinema server  1 , or the unencrypted AV data, which is supplied from the sub AV server  2 . The selected AV data is supplied to the projector  4 .  
      The projector  4  to which the selected AV data is supplied is capable of performing an operation (process) in accordance, for instance, with a flowchart in  FIG. 10 .  FIG. 10  is a flowchart illustrating a typical process that the projector  4  performs.  
       FIG. 10  illustrates a process with particular reference to the relevant frame, which is one of a plurality of frames (data) that constitute the AV data. In other words, the process shown in  FIG. 10  is performed independently for each of the frames (data) that constitute the AV data. More specifically, when, for instance, step S 4  is performed for the first frame, step S 2  may be simultaneously performed for the second frame.  
      Step S 1  is performed to determine whether AV data (more precisely, the relevant frame; however, the term “AV data” will be used in the subsequent explanation of  FIG. 10 ) is received by the receiver section  71  of the projector  4  shown in  FIG. 8 .  
      If the determination result obtained in step S 1  indicates that the AV data is not received, processing returns to step S 1 , which is performed to determine whether AV data is received. In other words, the determination process in step S 1  is repeatedly performed until the AV data is transmitted from the router  3  so that the projector  4  remains in a process standby state.  
      When the AV data is transmitted from the router  3  and received by the receiver section  71 , the determination result obtained in step S 1  indicates that the AV data is received. When the AV data is supplied to the meta data extraction section  72  and the like, processing proceeds to step S 2 .  
      Step S 2  is performed to determine whether the AV data received by the receiver section  71  and supplied to the meta data extraction section  72  is encrypted.  
      When, in the present embodiment, the meta data  34  is included in the AV data and the value written as the Current Key ID (see  FIG. 6 ) is other than a special value of 0 as described earlier, the determination result obtained in step S 2  indicates that the supplied AV data is encrypted. In the other situation, the determination result obtained in step S 2  indicates that the supplied AV data is unencrypted.  
      If the determination result obtained in step S 2  indicates that the supplied AV data is unencrypted (is not encrypted), processing proceeds to step S 7 . The process performed in steps S 7  and beyond will be described later.  
      If, on the other hand, the determination result obtained in step S 2  indicates that the supplied AV data is encrypted, processing proceeds to step S 3 . In this instance, the Encrypted input signal  102  is generated (a Hi level is invoked) and supplied to the state presentation section  74 , switching section  75 , and the like.  
      Step S 3  is performed to determine whether the LE Key  31 -D is prepared normally by the meta data extraction section  72 .  
      As described earlier, if, in the present embodiment, the LE_Key  31 -D corresponding to the Current LE_Key ID  142  is stored in the LEKP table  123  as indicated in  FIG. 9 , the determination result obtained in step S 3  indicates that the LE Key  31 -D is prepared normally. If not, the determination result obtained in step S 3  indicates that the LE Key  31 -D is not prepared normally.  
      If the determination result obtained in step S 3  indicates that the LE Key  31 -D is not prepared normally, processing proceeds to step S 9 . The process performed in steps S 9  and beyond will be described later.  
      If, on the other hand, the determination result obtained in step S 3  indicates that the LE Key  31 -D is prepared normally, processing proceeds to step S 4 . In this instance, the generation of the Key not found signal  101  is prohibited (a Lo level is invoked).  
      In step S 4 , the decryption section  73  performs a decryption process on the encrypted AV data that is supplied from the receiver section  71 .  
      In the above instance, the switching section  75  has switched to the input from the decryption section  73  as described earlier because the Encrypted input signal  102  is continuously supplied to the switching section  75  and the like (the Hi level is maintained) Further, since the generation of the Key not found signal  101  is continuously prohibited (the Lo level is maintained), the switching section  79  has switched to the input from the switching section  75 . Consequently, the restored AV data (the AV data for a movie), which is obtained as a result of step S 4 , is output from the decryption section  73  and supplied to the reproduction control section  80  via the switching sections  75 ,  79 .  
      In step S 5 , the reproduction control section  80  reproduces the movie corresponding to the restored AV data.  
      Further, the Encrypted input signal  102  is continuously supplied to the state presentation section  74  (the Hi level is maintained), and the supply of the Key not found signal  101  is continuously prohibited (the Lo level is maintained). In step S 6 , therefore, the state presentation section  74  indicates the “encrypted input normal state” (the present embodiment causes the lamp to illuminate in green).  
      In reality, however, step S 6  is not performed after completion of step S 5 . Steps S 5  and S 6  are performed independently and virtually simultaneously.  
      Returning to step S 2 , if the determination result obtained in step S 2  indicates that the supplied AV data is unencrypted (is not encrypted), processing proceeds to step S 7  as described earlier.  
      In the above instance, the generation of the Encrypted input signal  102  and Key not found signal  101  is prohibited (a Lo level is invoked). Therefore, the switching section  75  switches to the input from the receiver section  71 , and the switching section  79  switches to the input from the switching section  75 . Consequently, the unencrypted AV data received by the receiver section  71  in step S 1  is supplied to the reproduction control section  80  via the switching sections  75 ,  79 .  
      Thus, in step S 7 , the reproduction control section  80  reproduces trailers and commercials corresponding to the unencrypted AV data.  
      The supply of the Encrypted input signal  102  to the state presentation section  74  is continuously prohibited (the Lo level is maintained). Thus, in step S 8 , the state presentation section  74  indicates the “unencrypted input state” (the present embodiment extinguishes the lamp).  
      In reality, however, step S 8  is not performed after completion of step S 7 . Steps S 7  and S 8  are performed independently and virtually simultaneously.  
      Returning to step S 3 , if the determination result obtained in step S 3  indicates that the LE Key  31 -D is not prepared normally, processing proceeds to step S 9  as described earlier.  
      In the above instance, the generation of the Key not found signal  101  begins and then continues (the Hi level is maintained) as described earlier. The switching section  79  then switches to the input from the switching section  78 . Therefore, the stationary image data stored in the stationary image storage section  76  or the frame data stored in the frame storage section  77  is supplied to the reproduction control section  80  via the switching sections  78 ,  79 .  
      Thus, in step S 9 , the reproduction control section  80  reproduces the predetermined frame (still picture) or stationary image.  
      Both the Key not found signal  101  and Encrypted input signal  102  are continuously supplied to the state presentation section  74  (the levels of both signals are continuously Hi). Thus, in step S 10 , the state presentation section  74  indicates the “encrypted input error state” (the present embodiment causes the lamp to glow red).  
      In reality, however, step S 10  is not performed after completion of step S 9 . Steps S 9  and S 10  are performed independently and virtually simultaneously.  
      The operation performed by the transmission/reception system shown in  FIG. 1  has been described.  
      The present invention is applicable not only to the transmission/reception system shown in  FIG. 1  but also to various other systems. In the example shown in  FIG. 1  (the example shown in  FIG. 2 ), the LE Key  31 -E is included in the meta data  34 , and the meta data  34  is superposed over encrypted AV data and transmitted from the cinema server  1 , which is the transmitting end, to the projector  4 , which is the receiving end. In other words, the LE Key  31 -E is transmitted together with the encrypted AV data. However, the LE Key  31 -E need not be transmitted together with the encrypted AV data. The present invention can also be applied to a transmission/reception system in which the LE Key  31 -E and encrypted AV data are transmitted from the transmitting end to the receiving end through different transmission paths. A typical configuration of such a transmission/reception system is shown in  FIG. 11 .  FIG. 11  is a block diagram illustrating a typical configuration of the transmission/reception system according to an embodiment of the present invention, which is different from the configuration shown in  FIG. 1 .  
      As regards the elements of the transmission/reception system shown in  FIG. 11  that are the same as those in  FIG. 1 , their description is omitted as appropriate with the same reference numerals assigned.  
      In the example shown in  FIG. 11 , the transmission/reception system includes a cinema server  201 , a sub AV server  2 , a router  3 , and a projector  202 .  
      The comparison between the examples shown in  FIGS. 1 and 11  reveals that the cinema server  1  within the example shown in  FIG. 1  (the example shown in  FIG. 2 ) handles the meta data  34  that includes the LE Key  31 -E, superposes the meta data  34  over encrypted AV data, and transmits the resulting combination to the projector  4  via the router  3  as described earlier.  
      On the other hand, the cinema server  201  within the example shown in  FIG. 11  transmits a combination of the LE Key  31 -E and its Key ID (this combination is hereinafter referred to as the LE Key+Key ID  211 ) to the projector  202  via a predetermined network  203 .  
      As described above, the example shown in  FIG. 11  differs from the example shown in  FIG. 1  in that the former transmits the LE Key  31 -E and encrypted AV data through different paths. In other respects, the example shown in  FIG. 11  is basically the same as the example shown in  FIG. 1 . Therefore, the cinema server  201  shown in  FIG. 11 , which transmits the LE Key  31 -E, has a configuration that is slightly different from the configuration example ( FIG. 2 ) of the cinema server  1  shown in  FIG. 1 . By the same token, the projector  202  shown in  FIG. 11 , which receives the LE Key  31 -E, has a configuration that is slightly different from the configuration example ( FIG. 8 ) of the projector  4  shown in  FIG. 1 .  
      For example, the cinema server  201  and projector  202  may be configured as indicated in  FIGS. 12 and 13 , respectively.  FIG. 12  illustrates a typical configuration of the cinema server  201 .  FIG. 13  illustrates a typical configuration of the projector  202 .  
      As regards the elements of the cinema server  201  shown in  FIG. 12  that are the same as those of the cinema server  1  shown in  FIG. 2 , their description is omitted as appropriate with the same reference numerals assigned. Similarly, as regards the elements of the projector  202  shown in  FIG. 13  that are the same as those of the projector  4  shown in  FIG. 8 , their description is omitted as appropriate with the same reference numerals assigned.  
      In the example shown in  FIG. 12 , the cinema server  201  includes the AV data storage section  11 , encryption section  12 , and superposition section  14 , which are the same as those of the cinema server  1  shown in  FIG. 2 . However, the cinema server  201  shown in  FIG. 12  also includes a meta data generation section  221 , an AV sender section  222 , and a key sender section  223  unlike the cinema server  1  shown in  FIG. 2 .  
      The elements different from those of the cinema server  1  shown in  FIG. 2 , that is, only the meta data generation section  221 , AV sender section  222 , and key sender section  223  will be described below.  
      The meta data generation section  221  generates various information for the encryption process of the encryption section  12  (that is, various information for the decryption process of the projector  202 ), more specifically, the LE Key  31 -E, AES input  32 -E, Frame reset  33 - 3 , and the like as mentioned earlier, and supply the generated information to the encryption section  12 .  
      The process performed so far is basically the same as the process performed by the meta data generation section  13  shown in  FIG. 2 . However, the subsequent process is slightly different from the process performed by the meta data generation section  13  shown in  FIG. 2 .  
      More specifically, the meta data generation section  221  stores the LE Key+Key ID  211 , which is a combination of the generated LE Key  31 -E and its Key ID, in the built-in LE key table  221 - 1 . Therefore, the LE key table  221 - 1  stores one more sets of the LE Key+Key ID  211 .  
      The LE Key+Key ID  211  is the key information that concerns the security of the transmission/reception system. Therefore, the LE Key+Key ID  211  needs to be prevented from being stolen although it exists within the cinema server  201 . Under such circumstances, the LE Key+Key ID  211  should be encrypted before being stored in the LE key table  221 - 1 . The method of encrypting the LE Key+Key ID  211  is not particularly defined.  
      The number of sets of the LE Key+Key ID  211  to be prepared and stored in the LE key table  221 - 1  is not particularly defined. It depends on the intervals at which the LE Key  31 -E is updated and on various other transmission/reception system operating policies. Any number of sets of the LE Key+Key ID  211  may be prepared and stored in the LE key table  221 - 1  as far as it conforms to the operating policies.  
      One or more sets of the LE Key+Key ID  211 , which are stored in the LE key table  221 - 1 , are individually transmitted to the key sender section  223 .  
      Further, the meta data generation section  221  supplies the aforementioned various information (e.g., the information included in meta data  34 - 3 , which is shown in  FIG. 6 ) except the LE Key+Key ID  211  to the superposition section  14  as meta data  224 . In this instance, the meta data  224  may be RSA-encrypted or left unencrypted because it does not include the LE Key  31 -E that needs to be protected.  
      The process described above is slightly different from the process performed by the meta data generation section  13  shown in  FIG. 2 .  
      In the superposition section  14 , the meta data is superposed over the encrypted AV data no matter whether superposition occurs in the example shown in  FIG. 2  or  FIG. 12 . However, the meta data superposed in the example shown in  FIG. 12  differs from the meta data superposed in the example shown in  FIG. 2 . As described earlier, the meta data  34  is superposed over the encrypted AV data in the example shown in  FIG. 2 . In the example shown in  FIG. 12 , on the other hand, the meta data  224  is superposed over the encrypted AV data.  
      The AV sender section  222  transmits the encrypted AV data supplied from the superposition section  14 , that is, the encrypted AV data over which the meta data  224  is superposed, more specifically, the encrypted AV data that does not include the LE_Key  31 -E, to the projector  202  via the router  3 .  
      Meanwhile, the key sender section  223  transmits the LE Key+Key ID  211 , which is stored in the LE key table  221 - 1 , to the projector  202  via the network  203 .  
      The LE Key+Key ID transmission timing for the key sender section  223  is the timing preceding the transmission of a portion of the encrypted AV data (a certain number of frame data) that is encrypted with the associated LE Key  31 -E. It is acceptable as far as the decryption process is performed by the projector  202  without delay.  
      To prevent the LE Key+Key ID  211  (associated LE Key  31 -E) from being disclosed to a third party due to network bugging or projector spoofing, the communication between the projector  202  and cinema server  201  (key sender section  223 ) via the network  203  can be established, for instance, by communicating the LE Key+Key ID  211  after the key sender section  223  establishes a TLS (Transport Layer Security) session (secure communication path) with the public key of the projector  202 .  
      In the example shown in  FIG. 13 , the projector  202  that relates to the cinema server  201  includes an AV receiver section  231 , a key receiver section  232 , and a meta data extraction section  233  unlike the projector  4  shown in  FIG. 8 . The projector  202  also includes the decryption section  73 , state presentation section  74 , switching sections  75 ,  78 ,  79 , stationary image storage section  76 , frame storage section  77 , and reproduction control section  80 , which are the same as the elements of the projector  4  shown in  FIG. 8 .  
      The elements that differ from those of the projector  4  shown in  FIG. 8 , that is, only the AV receiver section  231 , key receiver section  232 , and meta data extraction section  233  will be described below.  
      The AV receiver section  231  receives AV data from the router  3  ( FIG. 11 ). In other words, the AV receiver section  231  receives encrypted AV data (encrypted AV data over which the meta data  224  without the LE Key  31 -E is superposed) from the cinema server  201  ( FIGS. 11 and 12 ) or unencrypted AV data from the sub AV server  2 .  
      Meanwhile, the key receiver section  232  receives the LE Key+Key ID  211 , which is transmitted from the cinema server  201  via the network  203  ( FIG. 11 ), and supplies it to the meta data extraction section  233 .  
      The meta data extraction section  233  can be configured as indicated, for instance, in  FIG. 14 .  FIG. 14  is a block diagram illustrating the details of a typical configuration of the meta data extraction section  233 .  
      As regards the elements of the meta data extraction section  233  shown in  FIG. 14  that are the same as those of the meta data extraction section  72  shown in  FIG. 9 , their description is omitted as appropriate with the same reference numerals assigned.  
      In the example shown in  FIG. 14 , the meta data extraction section  233  includes a meta data extraction/separation section  241 , an LEKP table  123 , a key change trigger generation section  124 , registers  125 ,  126 ,  128 , a counter  127 , and an AES input generation section  129 .  
      The comparison between  FIGS. 9 and 14  reveals that the meta data extraction section  72  shown in  FIG. 9  includes the LEKP restoration section  122  because the LE Key  31 -E (the LE Key  31 -D to be generated), which is a restoration target, is included in the meta data (see  FIG. 8 ) that is superposed over the encrypted AV data supplied from the receiver section  71 .  
      On the other hand, the key receiver section  232  supplies the LE Key  31 -E (the LE Key  31 -D to be generated), which is a restoration target, to the meta data extraction section  233  shown in  FIG. 14  as the LE Key+Key ID  211 . Therefore, the meta data extraction section  233  shown in  FIG. 14  is not provided with the LEKP restoration section  122  so that the LE Key+Key ID  211  supplied from the key receiver section  232  is directly stored in the LEKP table  123   
      As described earlier, the LE Key+Key ID  211  is the key information that concerns the security of the transmission/reception system and is transmitted before the transmission of encrypted AV data. Therefore, the LE Key+Key ID  211  needs to be prevented from being stolen although it exists within the projector  202 . Under such circumstances, the LE Key+Key ID  211  should be encrypted before being stored in the LEKP table  123  although the method of encrypting the LE Key+Key ID  211  is not particularly defined.  
      The meta data extraction/separation section  241  determines whether the meta data  224  is superposed over a predetermined portion of the AV data supplied from the AV receiver section  231  (see  FIG. 3 ; however, replace the reference numeral  34  in  FIG. 3  with the reference numeral  224 ).  
      If the obtained determination result indicates that the meta data  224  is not superposed, the meta data extraction/separation section  241  concludes that unencrypted AV data is received from the AV receiver section  231 , refrains from generating the Encrypted input signal  102  (invokes a Lo level), and prohibits the meta data extraction section  233  from performing a process.  
      If, on the other hand, the obtained determination result indicates that the meta data  224  is superposed, the meta data extraction/separation section  241  extracts the meta data  224  and other information from the relevant frame, which is among a plurality of frames (data) constituting the AV data.  
      Next, the meta data extraction/separation section  241  confirms the written Current Key ID value included in the meta data  224  (the written Current Key ID value in meta data  34 - 3  or the value corresponding to it).  
      If the written Current Key ID value is a special value (e.g., 0), the meta data extraction/separation section  241  concludes that unencrypted AV data is received from the receiver section  231 , refrains from generating the Encrypted input signal  102  (invokes a Lo level), and prohibits the meta data extraction section  233  from performing a process.  
      If, on the other hand, the written Current Key ID value is a value other than the special value (e.g., 0), the meta data extraction/separation section  241  concludes that encrypted AV data is received from the receiver section  231 , generates the Encrypted input signal  102  (invokes a Hi level), and supplies the generated signal to the state presentation section  74  and switching section  75  ( FIG. 13 ) Further, the meta data extraction/separation section  241  separates various items of information that constitute the meta data  224 , and supplies them to the associated blocks.  
      In the example shown in  FIG. 14 , the meta data extraction/separation section  241  extracts or separates the Current LE_Key ID  142 , Next LE_Key ID  143 , Key Change Timing  145 , Current Frame Count  148 , Frame/line reset  150 , and HD-SDI Link Number  152 , and the information for generating the Le_attribute_data  144 .  
      The Current LE_Key ID  142 , Next LE_Key ID  143 , and the information for generating the Le_attribute_data  144  are then supplied to the LEKP table  123 . In such an instance, the Le_attribute_data  144  is generated in accordance with the information for generating the Le_attribute_data  144 , combined with the mating Le Key+Key ID  211  (the associated LE_Key  31 -D), and stored in the LEKP table  123 . The Key Change Timing  145  is supplied to the key change trigger generation section  124 . The Current Frame Count  148  is supplied to the register  126 . The Frame reset  146  is supplied to the key change trigger generation section  124 . The Line reset  151  is supplied to the counter  127 . The Frame/line reset  150 , which includes the Frame reset  146  and Line reset  151 , is supplied to the decryption section  73  as the Frame reset  33 -D shown in  FIG. 13 . The HD-SDI Link Number  152  is supplied to the register  128 .  
      Elements of the meta data extraction section  233  shown in  FIG. 14  that are not described above, that is, the LEKP table  123 , key change trigger generation section  124 , registers  125 ,  126 ,  128 , counter  127 , and AS input generation section  129 , are not described here because they have basically the same functions and configurations as the counterparts shown in  FIG. 9 .  
      A typical configuration of the transmission/reception system shown in  FIG. 11  has been described with reference to FIGS.  11  to  14 . The operations performed by the transmission/reception system are not described here because they are basically the same as the operations described with reference to  FIG. 1  except for the operations (processes) performed by the elements that differ from the counterparts shown in  FIG. 1 .  
      As described above, the projector  4  ( FIG. 8 ) and projector  202  ( FIG. 13 ) of the transmission/reception system shown in  FIGS. 1 and 11 , which are receiver devices, are capable of generating the Encrypted input signal  102 . Therefore, the use of the Encrypted input signal  102  enables the receiving end to easily determine whether the digital material (the stream data for the movie, trailers, or commercials in the example described above) transmitted from the transmitting end is encrypted. As a result, a first advantage of permitting the receiving end to easily handle digital materials no matter whether they are encrypted is provided.  
      When unencrypted stream data is input to the projector  4  or projector  202  due to the router&#39;s switching, a process needs to be performed to skip the decryption process (the process of the decryption section  73 ) within the projector  4  or projector  202  in synchronism with the switching timing. The same process also needs to be performed at the time of reverse switching. When a projector in related art is used, external control needs to be exercised to perform the above process. The present embodiment, however, provides a second advantage of not having to exercise such external control because the switching section  75  ( FIGS. 8 and 13 ) is furnished to change the input in accordance with the Encrypted input signal  102 .  
      Since such external control may not be required, it is not necessary to furnish the transmission/reception system with a special element for exercising such external control. Further, a switch or other simple device may be used as the switching section  75 . As a result, a third advantage of constructing the whole transmission/reception system with ease is provided.  
      The second and third advantages can be provided not only for a small-scale reproduction signal switching system (transmission/reception system), which is shown in  FIGS. 1 and 11 , but also for a large-scale transmission/reception system that includes a large number of servers and projectors. Particularly, the third advantage will be enhanced during the use of the latter system, that is, a large-scale transmission/reception system.  
      Further, the projector  4  ( FIG. 8 ) and projector  202  ( FIG. 13 ), which are receiver devices, can generate the Key not found signal  101 , which indicates whether the common key for the decryption of encrypted stream data (encrypted AV data in the example described above) is prepared. Therefore, the use of the Key not found signal  101  (the use of the switching section  79  shown in  FIGS. 8 and 13  in the example described above) makes it possible to provide a fourth advantage. More specifically, even when the common key is not prepared because the signal to be input to a different projector is erroneously input or the decryption process on an encrypted common key (LE Key  31 -D in the example described above) is delayed, it is possible to prevent noise from being output to a projected image (on-screen image).  
      Furthermore, the projector  4  ( FIG. 8 ) and projector  202  ( FIG. 13 ), which are receiver devices, are provided with the state presentation section  74 , which uses the Key not found signal  101  and Encrypted input signal  102 . Therefore, a fifth advantage of being capable of monitoring the status of the projector  4  and projector  202  is provided. More specifically, it is possible, for instance, to check whether received stream data (AV data in the example described above) is encrypted and whether or not encrypted stream data (encrypted AV data in the example described above) is decrypted normally. The fifth advantage (the functionality provided by the fifth advantage) can be effectively used not only for monitoring regular operations but also for checking the system by manually changing the signals (stream data).  
      The transmission/reception system capable of providing the advantages described above, that is, the transmission/reception system according to an embodiment of the present invention, is not limited to the examples shown in  FIGS. 1 and 10 . The present invention is applicable to a transmission/reception system as far as it includes at least a receiver device that incorporates the functionality described below.  
      In other words, the transmission/reception system provides the first to third advantages as far as it is capable of determining whether the stream data received by the receiver is encrypted, and generating encryption information that indicates the obtained determination result (generating the Encrypted input signal  102  in the example described above or generating any other information that indicates the obtained determination result).  
      The transmission/reception system provides the fourth advantage as far as it determines whether the common key is prepared normally before a decryption process is performed on encrypted stream data, which is encrypted with the common key and received by the receiver, when the common key is to be transmitted to the receiver device. The transmission/reception system, as the fourth advantage, generates common key preparation information (Key not found signal  101  in the example described above or any other common key preparation information) that indicates the obtained determination result.  
      The transmission/reception system provides the fifth advantage as far as it identifies the state of the receiver device in accordance with the encryption information and common key preparation information, which are generated by the aforementioned two functions, and presents the obtained identification result.  
      The projector  4  and projector  202 , which are capable of inputting an encrypted HD-SDI signal (encrypted AV data in the example described above), have been described as a receiver device to which the present invention is applicable. In the future, however, such an input capability may be incorporated not only in projectors but also in videotape recorders, disc recorders, switchers, monitors, and various other devices. The devices having such an input capability can serve as the receiver device according to an embodiment of present invention when they additionally incorporate various functions that provide the first to fifth advantages. When various devices are implemented as the receiver device to which the present invention is applicable, it is conceivable that the transmission/reception system will be used with increased ease.  
      In the example described above, the stream data transmitted by the transmission/reception system to which the present invention is applicable is encrypted by a common key cryptosystem and then transmitted from the transmitting end to the receiving end. Therefore, the common key (the LE Key  31 -E or the like in the example described above) is also transmitted from the transmitting end. In this instance, the common key needs to be prevented from being stolen during its transmission because it is the key information that concerns the security of the transmission/reception system. Under such circumstances, the common key in the example described above is RSA-encrypted before being transmitted from the transmitting end to the receiving end.  
      The common key can also be encrypted by various encryption methods other than the RSA encryption method. However, the use of a public key cryptosystem is suitable including the example described above (the use of the RSA encryption method). The reason is that, in a system based on the use of the RSA encryption method or other public key cryptosystem, the private key (e.g., the key for decrypting the encrypted common key) is managed so that it does not leak out of the decryption side (e.g., receiver device) (the private key is presumably managed in such a manner because it should be managed in such a manner). It can therefore be that the private key is difficult to practically be stolen. It is even more difficult to steal the private key that is incorporated in an IC (Integrated Circuit) card or chip. In other words, the possibility of private key stealing can be rendered more close to 0%.  
      For example, ECC (Elliptic Curve Cryptography), ElGamal, Rabin, Williams, EPOC, and NTRU encryption methods exist as public key cryptosystems in addition to the aforementioned RSA encryption method. All of these public key cryptosystems can be used as a method for encrypting the common key (e.g., LE Key  31 -E in the example described above).  
      A series of processes described above (e.g., processes illustrated in  FIG. 10 ) can be executed by hardware and by software. When the series of processes is to be executed by software, the programs constituting the software are installed from a program storage medium onto a computer built in dedicated hardware or a general-purpose personal computer or other computer that can execute various functions when various programs are installed.  
       FIG. 15  is a block diagram illustrating a typical configuration of a personal computer that performs the aforementioned series of processes. When, for instance, the processes shown in  FIG. 10  are to be performed by a program, a personal computer having the configuration shown in  FIG. 15  may constitute the whole or a part of the projector  4  shown in  FIG. 1  or the projector  202  shown in  FIG. 11 .  
      Referring to  FIG. 15 , a CPU (Central Processing Unit)  301  performs various processes in accordance with programs stored in a ROM (Read Only Memory)  302  or in a storage section  308 . The programs and data to be executed by the CPU  301  may be stored in a RAM (Random Access Memory)  303 . The CPU  301 , ROM  302 , and RAM  303  are interconnected via a bus  304 .  
      The CPU  301  is connected to an input/output interface  305  via the bus  304 . The input/output interface  305  is connected to an input section  306 , which includes a keyboard, a mouse, a microphone, and the like, and to an output section  307 , which includes a display, a projection device, a loudspeaker, and the like. The CPU  301  performs various processes in compliance with instructions input from the input section  306 . The CPU  301  outputs processing results to the output section  307 .  
      The storage section  308 , which is connected to the input/output interface  305 , is, for instance, a hard disk and used to store various data and the programs to be executed by the CPU  301 . A communication section  309  communicates with an external device via a network such as the Internet or local area network.  
      Further, a program may be acquired via the communication section  309  and stored in the storage section  308 .  
      When removable medium such as a magnetic disk, optical disk, magnetooptical disk, semiconductor memory, is inserted into a drive  310 , which is connected to the input/output interface  305 , the drive  310  drives the inserted medium and acquires a program or data recorded on the medium. The acquired program or data may be transferred to the storage section  308  and stored.  
      As indicated in  FIG. 15 , the program recording medium for storing programs, which are to be installed on a computer and rendered executable by the computer, is a removable medium  311 , the ROM  302 , or the hard disk. The removable medium  311  is a package medium, including a magnetic disk (flexible disk included), optical disk (CD-ROM [Compact Disc-Read Only Memory] and DVD [Digital Versatile Disc] included), magnetooptical disk, and semiconductor memory. The ROM  302  stores the programs temporarily or permanently. The hard disk constitutes the storage section  308 . The programs may be stored on the program recording medium via the communication section  309 , which is an interface for the router and modem, by using a wired or wireless communication medium such as the Internet, local area network, or digital satellite broadcast.  
      In this document, the steps for writing the programs to be stored on the recording medium not only include processes that are performed in a described chronological order but also include processes that are performed parallelly or individually and not necessarily in chronological order.  
      The term “system,” which is used in this document, represents an aggregate of a plurality of devices.  
      It should be understood by those skilled in the art that various modifications, combinations, sub-combinations, and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.