Abstract:
A conditional access system includes an EMM (Entitlement Management Message) analyzer for retrieving partial viewing authorization information. When the partial viewing authorization information permits partial viewing, ECM (Entitlement Control Message) is decrypted using work keys, and scrambling keys are intermittently retrieved from the ECM. This can solve a problem of a conventional conditional access system in that an unauthorized user&#39;s receiver cannot display pictures at all because the receiver cannot retrieve scrambling keys, and hence the user cannot enjoy a program even to such a degree as to perceive its feeling as in analog broadcasting systems, and thus the user cannot decide as to whether to make contract or not with viewing the program to some extent, which prevents the provider of the program from offering effective invitation to the user.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a conditional access system allowing only specific receivers to receive video and audio digital signals which are broadcast or delivered via media such as satellite waves, terrestrial waves or cables. 
   2. Description of Related Art 
     FIG. 12  is a block diagram showing a conventional conditional access system described in Telecommunications Technology Counsel Report, Advisory No. 74. In this figure, the reference numeral  1  designates a system controller for generating and managing various keys and subscriber contract information;  2  designates a scrambling processor for encrypting coded media data output from sources like encoders on a transmitting side using scrambling keys output from the system controller  1 ;  3  designates an ECM (Entitlement Control Message) generator for generating an ECM from the scrambling keys used for encrypting the coded media data and from program associated information including a broadcasting type of a program and charge information: and  4  designates an ECM encryption section for encrypting the ECM using work keys output from the system controller  1 . 
   The reference numeral  5  designates an EMM (Entitlement Management Message) generator for generating an EMM from the work keys used for encrypting the ECM and from subscriber contract information for each receiver;  6  designates an EMM encryption section for encrypting the EMM using a master key output from the system controller  1 ;  7  designates a multiplexer for generating a packet stream by multiplexing encrypted coded media data, ECM and EMM, and for supplying the packet stream to a transmission path  8  for transmitting the packet stream. 
   The reference numeral  9  designates a demultiplexer for demultiplexing the packet stream transmitted from the transmitting site into the encrypted coded media data, ECM and EMM;  10  designates a master key managing section for storing in advance a master key used for decrypting the EMM; and  11  designates an EMM analyzer for retrieving from the EMM the subscriber contract information and work keys using the master key stored in the master key managing section  10 . The EMM analyzer  11  outputs the work keys only when the subscriber contract information authorizes viewing. 
   The reference numeral  12  designates an ECM analyzer for decrypting the ECM using the work keys supplied from the EMM analyzer  11 , and for retrieving the scrambling keys from the ECM;  13  designates a descrambling processor for descrambling the coded media data using the scrambling keys supplied from the ECM analyzer  12 ;  14  designates a media decoding section for decoding the coded media data descrambled by the descrambling processor  13 ; and  15  designates a television receiver for displaying pictures and outputting voices in response to the coded media data decoded by the media decoding section  14 . 
   Next, the operation of the conventional conditional access system will be described. 
   First, on the transmitting side, the scrambling processor  2  in the transmitting site encrypts the coded media data output from encoders or the like when the coded media data is specific coded media data to be scrambled, such as pay programs. 
   The ECM generator  3  places into the ECM the scrambling keys used for the encryption, the program associated information indicating the broadcasting type of the program, etc., and the ECM encryption section  4  encrypts the ECM. However, it usually does not encrypt the program associated information in the ECM. 
   The EMM generator  5  places into the EMM the work keys used in the encryption, the subscriber contract information for each receiver, etc., and the EMM encryption section  6  encrypts the EMM using the master key prepared for the individual receivers. However, it usually does not encrypt the subscriber contract information in the EMM. 
   Then, the multiplexer  7  multiplexes the encrypted coded media data, encrypted ECM and encrypted EMM, and supplies the multiplexed data to the transmission path  8 , through which the various pieces of the information are transmitted as a packet stream. 
   The packet stream sent from the transmitting side passes through the transmission path  8 , and is received by the demultiplexer  9  on the receiving side. On the receiving side, the demultiplexer  9  in the receiving site extracts the encrypted EMM from the packet stream, first. Then, the EMM analyzer  11  decrypts the EMM using the master key stored in the master key managing section  10  for individual receivers, and retrieves from the EMM the subscriber contract information and work keys (since the subscriber contract information is not usually encrypted, it can be referred to without the decryption). 
   The EMM analyzer  11  supplies the work keys to the ECM analyzer  12  only when the subscriber contract information authorizes the viewing. 
   Receiving the work keys from the EMM analyzer  11 , the ECM analyzer  12  decrypts the encrypted ECM using the work keys, retrieves the scrambling keys from the ECM, and supplies the scrambling keys to the descrambling processor  13 . 
   Receiving the scrambling keys from the ECM analyzer  12 , the descrambling processor  13  descrambles the coded media data using the scrambling keys, and the media decoding section  14  decodes the coded media data descrambled by the descrambling processor  13 . 
   The decoded coded media data is transferred to the television receiver  15  that displays pictures and outputs voices. 
   With the foregoing configuration, the conventional conditional access system authorizes only specific users&#39; receivers to descramble the coded media data in accordance with the contents of the subscriber contract information. The receivers of unauthorized users, however, cannot retrieve the scrambling keys, and hence the unauthorized users cannot view programs at all. Thus, they cannot view even fragments of pictures of programs, and hence cannot have slightest feelings about the programs as in analog broadcasts. As a result, they cannot decide as to whether they make a new contract or not after trial viewing of some programs, which prevents the providers from making effective invitation to their programs. 
   SUMMARY OF THE INVENTION 
   The present invention is implemented to solve the foregoing problem. It is therefore an object of the present invention to provide a conditional access system capable of allowing unauthorized users to watch pictures to some extent in order to increase their desire for programs, thereby making effective invitation to the programs. 
   According to one aspect of the present invention, there is provided a conditional access system comprising: a demultiplexer for demultiplexing a packet stream transmitted from a transmitting site into encrypted coded media data, an ECM (Entitlement Control Message) and an EMM (Entitlement Management Message); an EMM decryption section for retrieving work keys and subscriber contract information from the EMM extracted by the demultiplexer; means for retrieving partial viewing authorization information included in one of the EMM and ECM extracted by the multiplexer; an ECM decryption section for decrypting the ECM using the work keys, and for retrieving scrambling keys from the ECM; and outputting means for descrambling and decoding part of the coded media data using the scrambling keys when the partial viewing authorization information permits partial viewing, and for outputting the partially decoded coded media data. 
   Here, the ECM decryption section may comprise: means for intermittently retrieving the scrambling keys from the ECM when the subscriber contract information inhibits viewing and the partial viewing authorization information permits partial viewing, and wherein the outputting means comprises: a media data descrambling section for descrambling the coded media data using the scrambling keys retrieved by the ECM decryption section; and a decoding section for decoding the coded media data descrambled by the media data descrambling section. 
   The means for intermittently retrieving the scrambling keys may comprise: a decryption validity decision section for outputting decryption control information in response to the subscriber contract information and the partial viewing authorization information; and a decryption processor for intermittently retrieving the scrambling keys from the ECM in response to the decryption control information when the subscriber contract information inhibits viewing and the partial viewing authorization information permits partial viewing. 
   The means for intermittently retrieving the scrambling keys may comprises: a decryption processor for retrieving all scrambling keys included in the ECM; a scrambling key output validity decision section for outputting output control information in response to the subscriber contract information and the partial viewing authorization information; and an output controller for supplying the media data descrambling section with only part of the scrambling keys in response to the output control information when the subscriber contract information inhibits viewing and the partial viewing authorization information permits partial viewing. 
   The outputting means may comprise: a media data descrambling section for intermittently descrambling the coded media data using the scrambling keys when the subscriber contract information inhibits viewing and the partial viewing authorization information permits partial viewing; and a decoding section for decoding the coded media data descrambled by the media data descrambling section. 
   The media data descrambling section may comprise: a descramble validity decision section for outputting descramble control information alternately authorizing and inhibiting descrambling when the subscriber contract information inhibits viewing and the partial viewing authorization information permits partial viewing; a descrambler for descrambling part of the coded media data in response to the descramble control information; and a scramble control information modifier for handling part of the coded media data which is not descrambled as unencrypted data. 
   The outputting means may comprise: a media data descrambling section for descrambling the coded media data using the scrambling keys retrieved by the ECM decryption section; and a decoding section for intermittently decoding the coded media data descrambled by the media data descrambling section, when the subscriber contract information inhibits viewing and the partial viewing authorization information permits partial viewing. 
   The decoding section may comprise: a decoding validity decision section for outputting decoding control information in response the subscriber contract information and the partial viewing authorization information; and a decoding processor for decoding only part of frames in a frame sequence constituting the coded media data in response to the decoding control information when the subscriber contract information inhibits viewing and the partial viewing authorization information permits partial viewing. 
   The decoding processor may decode only I frames in response to the decoding control information when the subscriber contract information inhibits viewing and the partial viewing authorization information permits partial viewing. 
   The decoding section may comprise: a decoding processor for decoding all the coded media data descrambled by the media data descrambling section; and a media display controller for supplying only part of the descrambled coded media data to a television receiver when the subscriber contract information inhibits viewing and the partial viewing authorization information permits partial viewing. 
   The outputting means may comprise: a media data descrambling section for descrambling the coded media data using the scrambling keys retrieved by the ECM decryption section; and a decoding section for decoding the coded media data descrambled by the media data descrambling section, for storing the decoded coded media data into a memory on a block by block basis, and for outputting the blocks with changing their sequence when the subscriber contract information inhibits viewing and the partial viewing authorization information permits partial viewing. 
   The EMM decryption section may comprise means for intermittently retrieving work keys from the EMM when the subscriber contract information inhibits viewing and the partial viewing authorization information permits partial viewing, the ECM decryption section may decrypt the ECM using the work keys and retrieves scrambling keys from the ECM, and the outputting means may comprise: a media data descrambling section for descrambling the coded media data using the scrambling keys retrieved by the ECM decryption section; and a decoding section for decoding the coded media data descrambled by the media data descrambling section. 
   The means for intermittently retrieving work keys may comprise: a decryption validity decision section for outputting decryption control information in response to the subscriber contract information and the partial viewing authorization information; and a decryption processor for retrieving only part of the work keys from the EMM in response to the decryption control information when the subscriber contract information inhibits viewing and the partial viewing authorization information permits partial viewing. 
   The means for intermittently retrieving work keys may comprise: a decryption processor for retrieving all the work keys included in the EMM; a work key output validity decision section for outputting output control information about the work keys in response to the subscriber contract information and the partial viewing authorization information; and a work key output controller for supplying only part of the work keys to the ECM decryption section in response to the output control information when the subscriber contract information inhibits viewing and the partial viewing authorization information permits partial viewing. 
   The partial viewing authorization information may include a control parameter indicating a partially authorized viewable range. 
   The partial viewing authorization information may consist of information authorizing viewing only for a specific time period. 
   The subscriber contract information that includes information authorizing partial viewing may be used as the partial viewing authorization information. 
   The EMM may be used into which the work keys are inserted only for specific time periods. 
   The demultiplexer and the decoding section may be based on the MPEG-2 standard. 
   The conditional access system may determine, when a plurality of programs are multiplexed into the packet stream transmitted from the transmitting site, authorization, partial authorization and inhibition of viewing the programs for individual programs independently. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing a configuration of an embodiment 1 of a conditional access system in accordance with the present invention; 
       FIG. 2  is a block diagram showing an internal configuration of the ECM analyzer  32  of the embodiment 1; 
       FIG. 3  is a block diagram showing an internal configuration of the ECM analyzer  32  of an embodiment 2 of the conditional access system in accordance with the present invention; 
       FIG. 4  is a block diagram showing an internal configuration of a descrambling processor  33  of an embodiment 3 of the conditional access system in accordance with the present invention; 
       FIG. 5  is a block diagram showing an internal configuration of the media decoding section  34  of an embodiment 4 of the conditional access system in accordance with the present invention; 
       FIG. 6  is a block diagram showing an internal configuration of the media decoding section  34  of an embodiment 6 of the conditional access system in accordance with the present invention; 
       FIG. 7  is a block diagram showing an internal configuration of the media decoding section  34  of an embodiment 7 of the conditional access system in accordance with the present invention; 
       FIG. 8  is a block diagram showing an internal configuration of the EMM analyzer  31  of an embodiment 8 of the conditional access system in accordance with the present invention; 
       FIG. 9  is a block diagram showing an internal configuration of the EMM analyzer  31  of an embodiment 9 of the conditional access system in accordance with the present invention; 
       FIG. 10  is a block diagram showing a configuration of an embodiment 10 of the conditional access system in accordance with the present invention; 
       FIG. 11  is a block diagram showing a configuration of an embodiment 12 of the conditional access system in accordance with the present invention; and 
       FIG. 12  is a block diagram showing a configuration of a conventional conditional access system. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The invention will now be described with reference to the accompanying drawings. 
   Embodiment 1 
     FIG. 1  is a block diagram showing a configuration of an embodiment 1 of a conditional access system in accordance with the present invention. In this figure, the reference numeral  21  designates a system controller for generating and managing various keys and subscriber contract information;  22  designates a scrambling processor for encrypting coded media data (program signals) output from sources such as encoders on a transmitting side using scrambling keys output from the system controller  21 ;  23  designates an ECM generator for generating an ECM from the scrambling keys used for encrypting the coded media data and from program associated information including a broadcasting type of a program, charge information etc.; and  24  designates an ECM encryption section for encrypting the ECM using work keys output from the system controller  21 . 
   The reference numeral  25  designates an EMM generator for generating EMM from the work keys used for encrypting the ECM, from the subscriber contract information of individual receivers and from partial viewing authorization information;  26  designates an EMM encryption section for encrypting the EMM with the master key output from the system controller  21 ;  27  designates a multiplexer for multiplexing the encrypted coded media data, ECM and EMM into a packet stream, and for supplying the packet stream to the transmission path  28  to be transmitted. 
   The reference numeral  29  designates a demultiplexer for demultiplexing the packet stream sent from the transmitting site into the encrypted coded media data, ECM and EMM;  30  designates a master key managing section for prestoring a master key for decrypting the EMM;  31  designates an EMM analyzer for retrieving from the EMM the subscriber contract information, partial viewing authorization information and work keys using the master key stored in the master key managing section  30 . The EMM analyzer  31  outputs the work keys when the subscriber contract information authorizes viewing or the partial viewing authorization information permits partial viewing. The master key managing section  30  and EMM analyzer  31  constitute an EMM decryption section. 
   The reference numeral  32  designates an ECM analyzer (ECM decryption section) for decrypting the ECM using the work keys supplied from the EMM analyzer  31 , and for retrieving the scrambling keys from the ECM when the subscriber contract information authorizes viewing or the partial viewing authorization information permits partial viewing. In particular, the ECM analyzer  32  intermittently retrieves the scrambling keys from the ECM when the subscriber contract information inhibits viewing and the partial viewing authorization information permits partial viewing. 
   The reference numeral  33  designates a descrambling processor (media data descrambling section) for descrambling the coded media data using the scrambling keys it receives from the ECM analyzer  32 ;  34  designates a media decoding section (decoding section) for decoding the coded media data descrambled by the descrambling processor  33 ; and  35  designates a television receiver for displaying pictures and outputting voices in response to the coded media data decoded by the media decoding section  34 . 
     FIG. 2  is a block diagram showing an internal configuration of the ECM analyzer  32 . In this figure, the reference numeral  32   a  designates a decryption validity decision section for generating, in response to the subscriber contract information and the partial viewing authorization information, decryption control information for controlling a decryption processing of the ECM;  32   b  designates a decryption processor for decrypting the ECM in response to the decryption control information generated by the decryption validity decision section  32   a.    
   Next, the operation of the present embodiment 1 will be described. 
   On the transmitter side, the scrambling processor  22  in the transmitting site encrypts (scrambles) the coded media data output from encoders or the like when the coded media data is specific coded media data to be scrambled such as pay programs. 
   The ECM generator  23  places into the ECM the scrambling key used for the encryption and the program associated information indicating the program broadcasting type and the like, and the ECM encryption section  24  encrypts the ECM. However, it does not usually encrypt the program associated information in the ECM. 
   At the same time, the EMM generator  25  places into the EMM the work keys used for the encryption, the subscriber contract information for each receiver and the partial viewing authorization information, and the EMM encryption section  26  encrypts the EMM. However, it does not usually encrypt the subscriber contract information and the partial viewing authorization information in the EMM. The encryption is carried out using the master keys assigned to each receiver. 
   Then the multiplexer  27  multiplexes the encrypted coded media data, encrypted ECM and encrypted EMM into one stream, and supplies it to the transmission path  28  to be transmitted as a packet stream. 
   The packet stream transmitted from the transmitting side passes through the transmission path  28 , and is received by the demultiplexer  29  on the receiving side. On the receiving side, the demultiplexer  29  in the receiving site extracts the encrypted EMM from the packet stream, first. Then, the EMM analyzer  31  decrypts the EMM using the master key stored in the master key managing section  30  of each receiver, and retrieves from the EMM the subscriber contract information, the partial viewing authorization information and the work keys (the subscriber contract information and the partial viewing authorization information can be referred to without passing through the decryption processing because they are not usually decrypted). 
   The EMM analyzer  31  supplies the work keys to the ECM analyzer  32  when the subscriber contract information authorizes viewing or the partial viewing authorization information permits partial viewing. In contrast, it does not supply the work keys to the ECM analyzer  32  when the subscriber contract information inhibits the viewing and the partial viewing authorization information inhibits the partial viewing. 
   Receiving the work keys from the EMM analyzer  31 , the ECM analyzer  32  decrypts the encrypted ECM using the work keys, and retrieves the scrambling keys from the ECM. 
   Specifically, when the subscriber contract information authorizes viewing, the decryption validity decision section  32   a  in the ECM analyzer  32  supplies the decryption processor  32   b  with the decryption control information which authorizes decryption regardless of the contents of the partial viewing authorization information. 
   In this case, the decryption processor  32   b  continuously retrieves the scrambling keys from the ECM. 
   On the other hand, when the subscriber contract information inhibits viewing; and the partial viewing authorization information permits partial viewing, the decryption validity decision section  32   a  supplies the decryption processor  32   b  alternately with the decryption control information authorizing the decryption and the decryption control information inhibiting the decryption. 
   In this case, the decryption processor  32   b  intermittently retrieves the scrambling keys from the ECM. 
   When both the subscriber contract information and partial viewing authorization information inhibit viewing and partial viewing, the decryption validity decision section  32   a  in the ECM analyzer  32  supplies the decryption processor  32   b  with the decryption control information that inhibits the decryption. 
   In this case, the decryption processor  32   b  does not retrieve the scrambling keys from the ECM. 
   Receiving the scrambling keys from the ECM analyzer  32 , the descrambling processor  33  decrypts (descrambles) the coded media data using the scrambling keys, and the media decoding section  34  decodes the coded media data descrambled by the descrambling processor  33 . 
   The television receiver  35  displays pictures and outputs voices in response to the decoded coded media data supplied thereto. 
   Although the television receiver  35  can display the pictures properly because of the correctly decrypted coded media data when the descrambling processor  33  receives the scrambling keys continuously, it can only display improper pictures when the descrambling processor  33  receives the scrambling keys intermittently. This is because the television receiver  35  receives only the incorrectly decrypted coded media data (because only part of the coded media data is decrypted with the scrambling keys different from the authorized scrambling keys) 
   Thus, by adjusting the frequency of correct decryption of the coded media data, that is, the frequency of the retrievals of the scrambling keys form the ECM by the ECM analyzer  32 , the television receiver  35  can display partially correct pictures so that a user can have only a feeling about the program as a whole. 
   As described above, the present embodiment 1 is configured such that it decrypts the ECM using the work keys when the partial viewing authorization information retrieved by the EMM analyzer  31  permits partial viewing, and intermittently retrieves the scrambling keys from the ECM. This offers an advantage of being able to authorize an unauthorized user to partially display pictures, and hence effectively increase desire of the user to subscribe to the provider. 
   Embodiment 2 
     FIG. 3  is a block diagram showing an internal configuration of the ECM analyzer  32  of an embodiment 2 of the conditional access system in accordance with the present invention, in which the same reference numerals designate the same or like portions to those of  FIG. 2 , and the description thereof is omitted here. 
   In  FIG. 3 , the reference numeral  32   c  designates a decryption processor for retrieving the scrambling keys from the ECM using the work keys retrieved from the EMM analyzer  31 ;  32   d  designates a scrambling key output validity decision section for generating output control information for controlling the output of the scrambling keys in response to the subscriber contract information and partial viewing authorization information; and  32   e  designates a scrambling key output controller for controlling the output of the scrambling keys in response to the output control information generated by the scrambling key output validity decision section  32   d.    
   Next, the operation of the present embodiment 2 will be described. 
   Although the foregoing embodiment 1 controls the ECM decryption processing in response to the subscriber contract information and partial viewing authorization information, this is not essential. For example, the output of the scrambling keys rather than the decryption thereof can be controlled using the subscriber contract information and partial viewing authorization information, achieving the same advantage as the foregoing embodiment 1. 
   More specifically, receiving the work keys from the EMM analyzer  31 , the decryption processor  32   c  retrieves the scrambling keys from the ECM using the work keys regardless of the contents of the subscriber contract information and partial viewing authorization information, that is, retrieves all the scrambling keys from the ECM. 
   When the subscriber contract information authorizes viewing, the scrambling key output validity decision section  32   d  supplies the scrambling key output controller  32   e  with the output control information permitting the output of all the scrambling keys regardless of the contents of the partial viewing authorization information. 
   In this case, the scrambling key output controller  32   e  supplies the descrambling processor  33  with all the scrambling keys output from the decryption processor  32   c.    
   When the subscriber contract information inhibits the viewing and the partial viewing authorization information permits partial viewing, the scrambling key output validity decision section  32   d  supplies the scrambling key output controller  32   e  with the output control information alternately enabling and disabling the output of the scrambling keys. 
   In this case, the scrambling key output controller  32   e  supplies the descrambling processor  33  with only selected ones among the scrambling keys output from the decryption processor  32   c.    
   When neither the subscriber contract information nor the partial viewing authorization information authorizes the viewing or partial viewing, the scrambling key output validity decision section  32   d  supplies the scrambling key output controller  32   e  with the output control information disabling the output of the scrambling keys. 
   In this case, the scrambling key output controller  32   e  supplies the descrambling processor  33  with none of the scrambling keys output from the decryption processor  32   c.    
   Embodiment 3 
     FIG. 4  is a block diagram showing an internal configuration of the descrambling processor  33  of an embodiment 3 of the conditional access system in accordance with the present invention, in which the same or like portions to those of  FIG. 3  are designated by the same reference numerals and the description thereof is omitted here. 
   In  FIG. 4 , the reference numeral  33   a  designates a descramble validity decision section for generating descramble control information for controlling the decryption processing of the coded media data in response to the subscriber contract information and partial viewing authorization information;  33   b  designates a descrambler for descrambling the coded media data using the scrambling keys retrieved from the ECM analyzer  32  only when the descramble validity decision section  33   a  outputs the descramble control information enabling the decryption of the coded media data; and  33   c  designates a scramble control information modifier for rewriting the scramble control information associated with the coded media data which is not decrypted by the descrambler  33   b  when the descramble validity decision section  33   a  outputs the descramble control information disabling the decryption of the coded media data, so that such coded media data will be handled as unencrypted data. 
   Next, the operation of the present embodiment 3 will be described. 
   Although in the foregoing embodiment 2 the ECM analyzer  32  controls the output of the scrambling keys in response to the subscriber contract information and partial viewing authorization information, this is not essential. For example, the descrambling processor  33  can control its decryption processing of the coded media data in response to the subscriber contract information and partial viewing authorization information, offering the same advantage as the foregoing embodiment 2. 
   More specifically, receiving the subscriber contract information and partial viewing authorization information from the EMM analyzer  31 , the descramble validity decision section  33   a  supplies, when the subscriber contract information authorizes viewing, the descrambler  33   b  and scramble control information modifier  33   c  with the descramble control information enabling the decryption of the coded media data regardless of the contents of the partial viewing authorization information. 
   In this case, the descrambler  33   b  decrypts the coded media data using the scrambling keys retrieved by the ECM analyzer  32 , and the scramble control information modifier  33   c  supplies the media decoding section  34  with the coded media data decrypted by the descrambler  33   b  without changing it. 
   On the other hand, when the subscriber contract information inhibits viewing and the partial viewing authorization information permits partial viewing, the descramble validity decision section  33   a  supplies both the descrambler  33   b  and scramble control information modifier  33   c  with the descramble control information alternately enabling and disabling the decryption. 
   In this case, the descrambler  33   b  decrypts part of the coded media data using the scrambling keys retrieved by the ECM analyzer  32 : When the descrambler  33   b  decrypts the coded media data, the scramble control information modifier  33   c  supplies the decrypted coded media data to the media decoding section  34  without change; in contrast with this, when the descrambler  33   b  does not decrypt the coded media data, the scramble control information modifier  33   c  supplies the media decoding section  34  with the coded media data after rewriting the scramble control information associated with the coded media data so that the coded media data is handled as undecrypted data. 
   When both the subscriber contract information and partial viewing authorization information inhibit viewing, the descramble validity decision section  33   a  supplies both the descrambler  33   b  and scramble control information modifier  33   c  with the descramble control information disabling the decryption of the coded media data. 
   In this case, the descrambler  33   b  outputs the encrypted coded media data without decrypting it. The scramble control information modifier  33   c  supplies the undecrypted coded media data to the media decoding section  34  after rewriting the scramble control information associated with the undecrypted coded media data so that the undecrypted coded media data will be handled as encrypted data. 
   Thus, the media decoding section  34  cannot properly decode the coded media data not decrypted by the descrambler  33   b . As a result, the television receiver  35  displays only partially correct pictures as in the foregoing embodiment 2. 
   Embodiment 4 
     FIG. 5  is a block diagram showing an internal configuration of the media decoding section  34  in an embodiment 4 of the conditional access system in accordance with the present invention, in which the same reference numerals designate the same or like portions to those of  FIG. 4 , and the description thereof is omitted here. 
   In  FIG. 5 , the reference numeral  34   a  designates a decoding validity decision section for generating decoding control information for controlling the decoding processing of the coded media data in response to the subscriber contract information and partial viewing authorization information; and  34   b  designates a decoding processor for decoding the coded media data decrypted by the descrambling processor  33  only when the decoding validity decision section  34   a  outputs the decoding control information enabling the decoding of the coded media data. 
   Next, the operation of the present embodiment 4 will be described. 
   Although the descrambling processor  33  restricts the decryption processing of the coded media data in response to the subscriber contract information and partial viewing authorization information in the foregoing embodiment 3, this is not essential. For example, the media decoding section  34  rather than the descrambling processor  33  can restrict the decoding processing of the coded media data in response to the subscriber contract information and partial viewing authorization information, offering an advantage similar to that of the foregoing embodiment 3. 
   More specifically, receiving the subscriber contract information and partial viewing authorization information from the EMM analyzer  31 , the decoding validity decision section  34   a  supplies, when the subscriber contract information authorizes decoding, the decoding processor  34   b  with the decoding control information enabling the decoding of the coded media data regardless of the content of the partial viewing authorization information. 
   In this case, the decoding processor  34   b  decodes the coded media data decrypted by the descrambling processor  33 , and supplies it to the television receiver  35 . 
   When the subscriber contract information inhibits viewing and the partial viewing authorization information permits partial viewing, the decoding validity decision section  34   a  supplies the decoding processor  34   b  with the decoding control information alternately enabling and disabling the decoding of the coded media data. 
   In this case, the decoding processor  34   b  decodes only fragments of the coded media data decrypted by the descrambling processor  33 , and supplies them to the television receiver  35 . 
   On the other hand, when both the subscriber contract information and partial viewing authorization information inhibit viewing, the decoding validity decision section  34   a  supplies the decoding processor  34   b  with the decoding control information disabling the decoding of the coded media data. 
   In this case, the decoding processor  34   b  supplies the television receiver  35  with the coded media data which is decrypted by the descrambling processor  33  without decoding it. 
   Since the television receiver  35  cannot properly display the coded media data not decoded by the decoding processor  34   b , the television receiver  35  displays corrupted pictures as in the foregoing embodiment 3. 
   Embodiment 5 
   Although the decoding processing of the coded media data is restricted in the foregoing embodiment 4, it is also possible to selectively decode part of frames included in the coded media data such as I frames (intra-frame predictive coded frames) selected from a successive coded media data frame sequence. 
   This enables television receiver  35  to display pictures in a frame skipping state. 
   Embodiment 6 
     FIG. 6  is a block diagram showing an internal configuration of the media decoding section  34  in an embodiment 6 of the conditional access system in accordance with the present invention, in which the same reference numerals designate corresponding portions to those of  FIG. 5 , and hence the description thereof is omitted here. 
   In  FIG. 6 , the reference numeral  34   c  designates a decoding processor for decoding the coded media data decrypted by the descrambling processor  33 ; and  34   d  designates a media display controller for controlling the output of the coded media data in response to the subscriber contract information and partial viewing authorization information. 
   Next, the operation of the present embodiment 6 will be described. 
   Although the media decoding section  34  restricts the decoding processing of the coded media data in response to the subscriber contract information and partial viewing authorization information in the foregoing embodiment 4, this is not essential. For example, the coded media data decrypted by the descrambling processor  33  may be decoded in its entirety, and then its output may be restricted in response to the subscriber contract information and partial viewing authorization information, achieving an advantage similar to that of the foregoing embodiment 4. 
   More specifically, receiving the coded media data from the descrambling processor  33 , the decoding processor  34   c  decodes the coded media data and supplies the decoded coded media data to the media display controller  34   d  regardless of the contents of the subscriber contract information and partial viewing authorization information. 
   The media display controller  34   d  receives the decoded coded media data from the decoding processor  34   c , and supplies, when the subscriber contract information authorizes viewing, the decoded coded media data to the television receiver  35  regardless of the content of the partial viewing authorization information. 
   On the other hand, when the subscriber contract information inhibits viewing and the partial viewing authorization information permits partial viewing, the media display controller  34   d  supplies the television receiver  35  with only part of the decoded coded media data supplied from the decoding processor  34   c , such as specific pictures (I pictures, for example). 
   When both the subscriber contract information and partial viewing authorization information inhibit viewing, the media display controller  34   d  does not supply the television receiver  35  with the decoded coded media data output from the decoding processor  34   c.    
   Thus, when part of the decoded coded media data is transferred to the television receiver  35 , it displays only part of the pictures in a frame skipping mode. 
   Embodiment 7 
     FIG. 7  is a block diagram showing an internal configuration of the media decoding section  34  in an embodiment 7 of the conditional access system in accordance with the present invention, in which the same reference numerals designate the same or like portions to those of  FIG. 6 , and the description thereof is omitted here. 
   In  FIG. 7 , the reference numeral  34   e  designates a decoding processor for decoding the coded media data decrypted by the descrambling processor  33 , for storing decoded coded media data into a frame memory  34   f , and for supplying the decoded coded media data stored in the frame memory  34   f  to the television receiver  35 ; and  34   f  designates the frame memory for temporarily storing the decoded coded media data. 
   Next, the operation of the present embodiment 7 will be described. 
   Although the media decoding section  34  controls the output of the decoded coded media data in response to the subscriber contract information and partial viewing authorization information in the foregoing embodiment 6, this is not essential. For example, the media decoding section  34  can once store the decoded coded media data into the frame memory  34   f  on a block by block basis, and then output the decoded coded media data with changing an order of the blocks, achieving an advantage similar to that of the foregoing embodiment 6. 
   More specifically, receiving the coded media data from the descrambling processor  33 , the decoding processor  34   e  decodes the coded media data regardless of the content of the subscriber contract information and partial viewing authorization information, and stores the decoded coded media data into the frame memory  34   f . In this case, the decoding processor  34   e  stores the decoded coded media data into the frame memory  34   f  on a block by block basis. 
   Subsequently, when the subscriber contract information authorizes viewing, the decoding processor  34   e  supplies the television receiver  35  with the decoded coded media data stored in the frame memory  34   f  regardless of the content of the partial viewing authorization information without changing the sequence of the blocks. 
   On the other hand, when the subscriber contract information inhibits viewing and the partial viewing authorization information permits partial viewing, the decoding processor  34   e  supplies the television receiver  35  with the decoded coded media data stored in the frame memory  34   f  with changing the order of the blocks. 
   When both the subscriber contract information and partial viewing authorization information inhibit viewing, the decoding processor  34   e  does not supply the television receiver  35  with the decoded coded media data stored in the frame memory  34   f.    
   Thus, when the decoded coded media data stored in the frame memory  34   f  is supplied to the television receiver  35  with changing the sequence of the blocks, it can display pictures at random on a block by block basis, although individual pictures in each block will be correctly displayed. Thus, by controlling the timing of changing the order of the blocks, the picture display is achieved such that a user can have a feeling of the program as a whole. 
   Embodiment 8 
     FIG. 8  is a block diagram showing an internal configuration of the EMM analyzer  31  in an embodiment 8 of the conditional access system in accordance with the present invention, in which the same reference numerals designate the same or like portions to those of  FIG. 2 , and the description thereof is omitted here. 
   In  FIG. 8 , the reference numeral  31   a  designates a decryption validity decision section for retrieving the subscriber contract information and partial viewing authorization information from the EMM output from the demultiplexer  29 , and for generating decryption control information for controlling the decryption processing of the work keys in response to the subscriber contract information and partial viewing authorization information; and  31   b  designates a decryption processor for decrypting the EMM using the master key only when the decryption control information enables the decryption of the work keys, and for retrieving the work keys from the EMM. 
   Next, the operation of the present embodiment 8 will be described. 
   Although the ECM analyzer  32  controls the decryption processing of the ECM in response to the subscriber contract information and partial viewing authorization information in the foregoing embodiment 1, this is not essential. Instead of this, for example, the EMM analyzer  31  can control the decryption processing of the EMM in response to the subscriber contract information and partial viewing authorization information, achieving an advantage similar to that of the foregoing embodiment 1. 
   More specifically, the decryption validity decision section  31   a  retrieves the subscriber contract information and partial viewing authorization information from the EMM output from the demultiplexer  29 . Since the subscriber contract information and partial viewing authorization information are not usually encrypted, they can be referred to without passing through the decryption processing. 
   When the subscriber contract information authorizes viewing, the decryption validity decision section  31   a  supplies the decryption processor  31   b  with the decryption control information enabling the decryption of the work keys regardless of the content of the partial viewing authorization information. 
   In this case, the decryption processor  31   b  decrypts the EMM using the master key, retrieves the work keys from the EMM, and supplies them to the ECM analyzer  32 . 
   On the other hand, when the subscriber contract information inhibits viewing and the partial viewing authorization information permits partial viewing, the decryption validity decision section  31   a  supplies the decryption processor  31   b  with the decryption control information alternately enabling and disabling the decryption of the work keys. 
   In this case, the decryption processor  31   b  decrypts the EMM intermittently, retrieves some of the work keys included in the EMM, and supplies them to the ECM analyzer  32 . 
   When both the subscriber contract information and partial viewing authorization information inhibit viewing, the decryption validity decision section  31   a  supplies the decryption processor  31   b  with the decryption control information disabling the decryption of the work keys. 
   In this case, the decryption processor  31   b  does not retrieve any work keys from the EMM. 
   Thus, during intervals in which the work keys are not retrieved, the coded media data cannot be properly descrambled because the scrambling keys are not correctly retrieved from the ECM. Accordingly, the television receiver  35  displays only partially correct pictures as in the foregoing embodiment 1. 
   Embodiment 9 
     FIG. 9  is a block diagram showing an internal configuration of the EMM analyzer  31  in an embodiment of the conditional access system in accordance with the present invention, in which the same reference numerals designate the same or like portions to those of  FIG. 8 , and the description thereof is omitted here. 
   In  FIG. 9 , the reference numeral  31   c  designates a decryption processor for retrieving the subscriber contract information and partial viewing authorization information from the EMM output from the demultiplexer  29 , for decrypting the EMM using the master key, and for retrieving the work keys from the EMM;  31   d  designates a work key output validity decision section for generating output control information for controlling the output of the work keys in response to the subscriber contract information and partial viewing authorization information; and  31   e  designates a work key output controller for supplying the ECM analyzer  32  with the work keys retrieved by the decryption processor  31   c  only when the output control information enables the output of the work keys. 
   Although the decryption processing of the EMM is controlled in response to the subscriber contract information and partial viewing authorization information in the foregoing embodiment 8, this is not essential. For example, the output processing of the work keys rather than the decryption processing of the EMM can be controlled in response to the subscriber contract information and partial viewing authorization information, achieving an advantage similar to that of the foregoing embodiment 8. 
   More specifically, when the decryption processor  31   c  retrieves from the EMM the subscriber contract information and partial viewing authorization information, it supplies them to the work key output validity decision section  31   d , and when the decryption processor  31   c  retrieves the work keys from the EMM using the master key, it supplies the work keys to the work key output controller  31   e.    
   When the subscriber contract information authorizes viewing, the work key output validity decision section  31   d  supplies the work key output controller  31   e  with the output control information enabling the output of the work keys regardless of the content of the partial viewing authorization information. 
   In this case, the work key output controller  31   e  supplies the ECM analyzer  32  with all the work keys output from the decryption processor  31   c.    
   On the other hand, when the subscriber contract information inhibits viewing and the partial viewing authorization information permits partial viewing, the work key output validity decision section  31   d  supplies the work key output controller  31   e  with the output control information alternately enabling and disabling the output of the work keys. 
   In this case, the work key output controller  31   e  supplies the ECM analyzer  32  with part of the work keys output from the decryption processor  31   c.    
   When both the subscriber contract information and partial viewing authorization information inhibit viewing, the work key output validity decision section  31   d  supplies the work key output controller  31   e  with the output control information disabling the output of the work key. 
   In this case, the work key output controller  31   e  does not supply the ECM analyzer  32  with any of the work keys output from the decryption processor  31   c.    
   Embodiment 10 
   Although the partial viewing authorization information is inserted into the EMM to be transmitted in the foregoing embodiments 1-7, this is not essential. For example, the partial viewing authorization information can be inserted into the ECM to be transmitted. 
   In this case, the ECM analyzer  32  rather than the EMM analyzer  31  retrieves the partial viewing authorization information from the ECM, making it possible to achieve the view control as in the foregoing embodiments 1-7. 
   The view control can be carried out on a program by program basis for all the receivers simultaneously or for each group of receivers. 
   Embodiment 11 
   Although the partial viewing authorization information transmitted from the transmitting side controls the receiving side EMM analyzer  31 , ECM analyzer  32  and descrambling processor  33  or media decoding section  34  in the foregoing embodiments 1-10, the partial viewing authorization information can further include a control parameter indicating a partial view authorized range. 
   This makes it possible for the transmitting side to control various receiving side operations such as the frequency of decrypting the work keys, that of decrypting the scrambling keys, that of decrypting the coded media data, the degree of changing the order of the blocks of the coded media data or the frequency of decoding the coded media data. As a result, transmitting side can control the information amount obtainable from the pictures displayed on the television receiver  35 . 
   Embodiment 12 
   Although the receiving side displays pictures partially in response to the partial viewing authorization information transmitted from the foregoing embodiments 1-11, this is not essential. For example, the receiving side can display pictures only during a specific time period by using specific time period viewing authorization information (information authorizing viewing for a specific time period) as the partial viewing authorization information as shown in FIG.  11 . 
   More specifically, the transmitting side inserts into the EMM the specific time period viewing authorization information that authorizes viewing only for a specific time period as the partial viewing authorization information and encrypts it. Receiving it, the receiving side retrieves from the EMM the specific time period viewing authorization information, and carries out for only the specific time period the retrieval of the scrambling keys and work keys, and the decryption and decoding of the coded media data. 
   This makes it possible for a noncontract users to view the program normally for only the specific time period. 
   Since the EMM is transmitted for each receiver, inserting the specific time period viewing authorization information into the EMM makes it easier for each receiver to manage the viewing validity control in the specific time period. On the other hand, inserting the specific time period viewing authorization information into the ECM makes it easy for all the receivers or for each group of the receivers to manage the authorization information. 
   Furthermore, since the EMM and ECM are multiplexed into a video signal or voice signal to be transmitted simultaneously, the receiving side can receive the specific time period viewing authorization information without providing any new storage for temporarily holding the specific time period viewing authorization information. This makes it possible to limit the scale of additional devices to a minimum for implementing the viewing validity control in the specific time period. 
   Embodiment 13 
   Although the subscriber contract information and the partial viewing authorization information are separately transmitted in the foregoing embodiments 1-11, this is not essential. For example, it is possible not to transmit the partial viewing authorization information separately from the subscriber contract information, but to transmit subscriber contract information authorizing partial viewing. 
   More specifically, when the transmitting side sends the EMM to noncontract receivers, the system controller  21  inserts into the subscriber contract information not only parameters authorizing or inhibiting viewing, but also parameters authorizing viewing for a specific time period or parameters for authorizing periodic viewing at the time when the system controller  21  supplies the subscriber contract information to the EMM generator  25 . 
   This causes the subscriber contract information to bear on the information corresponding to the partial viewing authorization information, and hence makes it possible for the receiving side to carry out the view control as in the foregoing embodiments 1-11 by retrieving the subscriber contract information from the EMM. 
   Embodiment 14 
   Although the receiving side partially displays pictures by receiving the partial viewing authorization information in the foregoing embodiments 1-11, the transmitting side can send the work keys by inserting them into the EMM only for a specific time period. 
   More specifically, by transmitting the EMM including the work keys only for the time period authorizing the receiving side to display pictures, the receiving side can correctly display pictures only for the time period it can retrieve the work keys from the EMM. In particular, this makes it possible for the receiving side to implement the viewing control as in the foregoing embodiments 1-11 without controlling the EMM analyzer  31 , ECM analyzer  32 , descrambling processor  33  or media decoding section  34 . 
   Embodiment 15 
   Although the embodiments 1-14 do not refer to the specifications of the multiplexer  27 , demultiplexer  29  and media decoding section  34 , they can be based on the MPEG-2 standard. 
   This makes it possible to construct a conditional access system based on the MPEG-2 standard. 
   Embodiment 16 
   Although the number of programs to be transmitted from the transmitting side is not referred to in the foregoing embodiments 1-15, a plurality of programs can be multiplexed into the packet stream transmitted from the transmitting side. In this case, the partial viewing authorization or inhibition can be implemented for individual programs independently. This makes it possible for each program to carry out its own viewing control.