Abstract:
A method for assigning conditional access system (CAS) keys to a terminal to thus reduce the number of entitlement management messages (EMMs) required to send contents to a digital broadcast subscriber. To this end, a broadcast center causes transmission of the CAS-related key to limit access to the digital content, over a mobile communication network used for a mobile communication, and causes transmission of the digital content over a digital broadcasting network used for digital broadcasts. Since the CAS-related key is transmitted to the digital broadcast subscriber&#39;s terminal over the mobile communication network, it is possible to effectively utilize the frequency bandwidth of the digital broadcasting network for the digital broadcasting. Furthermore, the security of the CAS-related key can be strengthened by sending the CAS-related key to the terminal over the mobile communication network.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit under 35 U.S.C. § 119 (a) from Korean Patent Application No. 2005-09124 filed on Feb. 1, 2005 in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     Apparatuses and methods consistent with the present invention relate generally to a conditional access system (CAS) key assignment. More particularly, such apparatuses and methods relate to reducing the number of entitlement management messages (EMMs) required to provide content to authorized digital broadcast subscribers.  
         [0004]     2. Description of the Related Art  
         [0005]     Satellite and terrestrial digital broadcasting provides customized services to subscribers. Broadcasters have introduced a subscription model to existing television (TV) terrestrial broadcast services that mainly depends on advertising revenue and provides various programs to a limited number of subscribers who pay a service rate. Digital broadcasting enables offering various productions and services of specialized programs by specialized contents providers.  
         [0006]     As subscription fees, rather than merely advertising revenue, are used to fund broadcasters, subscribers can enjoy specialized channels and customized services of high quality in the multichannel broadcasting era. A conditional access system (CAS) enables the conditional limited reception of a broadcast. With CAS, only authorized subscribers can receive a scrambled signal broadcast from a transmitting end and descramble the received signal to view the program.  
         [0007]     Requirements of the CAS are the scrambling of programs (contents) and data, and the protection of the scrambled data over a communication channel. Additionally, CAS requires use of the subscriber authentication function and the access control function. The two requirements aim for the protection of the resources (programs and data) and the subscribers. The scrambling and the descrambling are for the resource&#39;s protection, and the subscriber authentication function and the access control function are for the subscriber&#39;s protection. Hereafter, the CAS is described in more detail.  
         [0008]     CAS includes scrambling and descrambling, the entitlement control function, and the entitlement management function.  
         [0009]     Scrambling encrypts data to be transmitted using a control word (CW) to protect the data from access by unauthorized subscribers. The descrambling is performed only by a receiver that can acquire the descrambler key CW. Generally, the CW can be decrypted using a direct entitlement key (DEK).  
         [0010]     The entitlement control function sends an entitlement control message (ECM). The ECM includes the encrypted CW and an address parameter, and is transmitted at regular intervals. The receiving end provides the received ECM to a smart card. A microprocessor in the smart card compares the provided address parameter with an authentication parameter held by the smart card. When the two parameters match, the authentication process at the receiving end terminates and the CW is decrypted using the DEK of the authorized smart card. Typically, the CW differs for each program or data.  
         [0011]     The entitlement management function grants access rights to the receiving end, or updates the authentication key. The entitlement management function generates an entitlement management message (EMM). The receiving end decrypts the DEK of the EMM using a user key stored in the smart card. In general, the EMM can be divided as follows: 
    EMM_B: to allow access of all receiving ends     EMM_G: to allow access of a group including at least one receiving end     EMM_P: to allow access of one receiving end    
 
         [0015]     With reference to  FIG. 1 , the following describes how a conventional terminal obtains a plurality of CAS keys.  
         [0016]     The terminal  100  stores a master private key (MPK) in a smart card (S 100 ). Principally, when the smart card is issued, the MPK is stored therein.  
         [0017]     The digital broadcasting network  102  encrypts a private key (PK) using the MPK and transmits the encrypted PK to the terminal  100  using an EMM_P (S 112 ). The terminal  100  acquires the encrypted PK contained in the received EMM_P by use of the MPK (S 114 ).  
         [0018]     The digital broadcasting network  102  encrypts a group key (GK) using the MPK and transmits the encrypted GK to the terminal  100  using an EMM_G (S 116 ). The terminal acquires the encrypted GK contained in the received EMM_G by using the stored MPK (S 118 ).  
         [0019]     The digital broadcasting network  102  encrypts a broadcast key (BK) using the MPK and transmits the encrypted BK to the terminal  100  using an EMM_B (S 120 ). The terminal acquires the encrypted BK contained in the received EMM_B using the MPK (S 122 ). Primarily, the user key includes the MPK, the PK, the GK, and the BK.  
         [0020]     The digital broadcasting network  102  encrypts the DEK using the PK and transmits the encrypted DEK to the terminal  100  using an EMM_P (S 124 ). The terminal  100  acquires the encrypted DEK contained in the received EMM_P using the stored PK (S 126 ).  
         [0021]     The digital broadcasting network  102  encrypts the DEK using the GK and transmits the encrypted DEK to the terminal  100  using an EMM_G (S 128 ). The terminal  100  acquires the encrypted DEK contained in the received EMM_G using the stored GK (S 130 ).  
         [0022]     The digital broadcasting network  102  encrypts the CW using the DEK and transmits the encrypted CW to the terminal  100  using an ECM (S 132 ). The terminal  100  acquires the encrypted CW of the received ECM using the stored DEK (S 134 ). It is noted that the digital broadcasting network  102  encrypts the CW destined for the terminal  100  using the DEK relating to the PK, and the CW destined for the group using the DEK relating to the GK.  
         [0023]     As such, the more subscribers present in the digital broadcasting network, the more EMM_P messages are transmitted over the digital broadcasting network to each terminal for distributing the keys. As discussed above, to provide various content and data to the subscribers, the digital broadcasting network requires a plurality of transmission channels. In response to this, a new method is needed to efficiently utilize the limited bandwidth of the digital broadcasting network.  
       SUMMARY OF THE INVENTION  
       [0024]     Apparatuses and methods consistent with the present invention address the above-mentioned and other problems and disadvantages occurring in the conventional arrangement, and an aspect of the present invention provides a CAS configuration for reducing the number of EMM_P messages transmitted to each terminal from the digital broadcasting network.  
         [0025]     Another aspect of the present invention provides a method for providing various types of content by efficiently utilizing limited radio resources of the digital broadcasting network.  
         [0026]     Still another aspect of the present invention provides a method for reducing the number of EMM_P messages transmitted to each terminal from the digital broadcasting network even when the number of subscribers of the digital broadcasting network increases.  
         [0027]     To achieve the above aspects of the present invention, a method for providing digital content to a terminal and a conditional access system (CAS) related key to limit access to the digital content, includes transmitting the CAS-related key over a mobile communication network used for mobile communications; and transmitting the digital content using a digital broadcasting network used for digital broadcasts.  
         [0028]     A system for providing a conditional access system (CAS) related key and a digital content, includes a terminal; and a broadcast center which causes the CAS-related key for limiting access to the digital content to be transmitted over a mobile communication network used for mobile communications, and causes the digital content to be transmitted over a digital broadcasting network used for digital broadcasts. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES  
       [0029]     Aspects of the invention will become apparent and more readily appreciated from the following description of illustrative, non-limiting embodiments, taken in conjunction with the accompanying drawing figures of which:  
         [0030]      FIG. 1  is a flow diagram illustrating a CAS key assignment for a digital broadcast at a conventional terminal;  
         [0031]      FIG. 2  is a diagram of broadcasting services provided from a broadcast center, which are grouped based on a service type and a service duration;  
         [0032]      FIG. 3  is a schematic block diagram illustrating a CAS key assignment at a terminal according to an embodiment of the present invention;  
         [0033]      FIG. 4  is a flow diagram illustrating the CAS key assignment for a digital broadcast at the terminal according to an embodiment of the present invention; and  
         [0034]      FIG. 5  is a flow diagram illustrating service extension request operations between a terminal, a mobile communication network, and a broadcast center.  
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0035]     Reference will now be made in detail to exemplary embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. These embodiments are described below in order to explain the present general inventive concept by referring to the drawings.  
         [0036]     Hereinafter, descriptions of a conditional access system (CAS) configuration for reducing the number of EMM_Ps transmitted to a terminal from a digital broadcasting network according to an embodiment of the present invention are made with reference to the attached drawings.  
         [0037]     According to an embodiment of the present invention, an EMM_P message transmitted to a terminal is eliminated, subscribers are grouped into a plurality of groups, and a unique key is assigned to each group. The unique key assigned to each group is transferred to each terminal over a mobile communication network rather than over a digital broadcast network.  
         [0038]      FIG. 2  illustrates a plurality of groups of digital broadcast subscribers. In the embodiment of the present invention, the subscribers are grouped based on the type of service to which they subscribe (hereinafter “service type”) and the remaining duration of the subscribed service. It is to be understood that the subscribers can be grouped based on other criteria selected by a user.  
         [0039]     Referring to  FIG. 2 , the services are grouped based on the service type, such as video data  200 , audio data  202 , video and audio data  204 , and a basic service  206 . The video data  200  is grouped based on the remaining service duration, such as 6 days (D) through 1D. The video and audio data  204  is grouped according to its remaining service duration, such as 30D through 1D. Although not shown in  FIG. 2 , the audio data  202  and the basic service  206  are also grouped into a plurality of groups according to the remaining service duration.  
         [0040]      FIG. 3  depicts a digital broadcast subscriber&#39;s terminal  306  and communication networks  302  and  304  connected to the terminal  306  according to an embodiment of the present invention. As shown in  FIG. 3 , the terminal  306  is connected to the digital broadcasting network  302  and the mobile communication network  304 . A broadcast center  300  is connected to the digital broadcasting network  302  and the mobile communication network  304  as well. Although  FIG. 3  depicts a direct connection from the broadcast center  300  to the digital broadcasting network  302  and to the mobile communication network  304 , the broadcast center  300  may be indirectly connected according to a user&#39;s setting.  
         [0041]     The method for assigning a key in order for the terminal  306  to receive the digital broadcast will be elucidated below with reference to  FIG. 4 .  
         [0042]     The terminal  306  stores a public key (PuK) and a PK in its smart card (S 400 ). The mobile communication network  304  encrypts a subscription key (SK) using the PuK and transmits an EMM_P containing the encrypted SK (S 402 ). In the embodiment of the present invention, the key structure is the public key structure. The public key structure includes the PuK of the terminal  306  and the PK corresponding to the PuK. The terminal  306  sends its PuK to the broadcast center  300 . The broadcast center  300  encrypts specific information using the PuK and transmits the encrypted information, and the terminal  306  acquires the specific information using the stored PK. In other words, the terminal  306  cannot acquire the specific information provided from the broadcast center with a different PK.  
         [0043]     Table 1 shows exemplary SKs transmitted to the groups shown in  FIG. 2 .  
                       TABLE 1                       Service Type   Remaining Service Duration   Subscription Key                   Video data   6 D   SK0           . . .   . . .           1 D   SKk       Audio data   . . .   . . .       Video and audio data   30 D    SKm           . . .   . . .           1 D   SKn       . . .   . . .   . . .       Basic service   . . .   . . .                  
 
         [0044]     In Table 1, SK 0  is assigned to the terminal  306  of the subscriber who requests video data with the service duration of 6D remaining, and SKk is assigned to the terminal  306  of the subscriber who requests video data with the service duration of 1D remaining. SKm is assigned to the terminal  306  of the subscriber who requests video and audio data with the service duration of 30D remaining, and SKn is assigned to the terminal  306  of the subscriber who requests video and audio data with the service duration of 1D remaining.  
         [0045]     Although different SKs are assigned to the groups in Table 1, the same SK is assignable to the groups, according to a user&#39;s setting.  
         [0046]     It is exemplified that the user assigns the same SK to the groups. The same SK can be assigned to the terminal  306  of the subscriber who requests video data. In this case, the SK can be encrypted using different encryption algorithms depending on the duration of the remaining service. More specifically, SK 0  is assigned to the terminal  306  of the subscriber requesting video data. Encryption using (SK 0 ) is conducted for the terminal  306  having 6D of service duration remaining, and encryption using (SK 0 ) is conducted for the terminal  306  having the 1D service duration remaining. A detailed explanation as to the hash function (algorithm) will be omitted for brevity. Note that the SK assigned to the groups is changeable by the day.  
         [0047]     The mobile communication network  304  encrypts the BK using the PuK and transmits an EMM_P containing the encrypted BK to the terminal (S 402 ). The terminal  306  acquires the SK and the BK in the received EMM_P using the stored PuK (S 404 ).  
         [0048]     The mobile communication network  304  encrypts information to be transmitted to the terminal  306  using the SK and transmits the encrypted information to the terminal  306  using an EMM_P (S 406 ). In  FIG. 4 , the information to be transmitted to the terminal is an Entitlement message, for example. The terminal  306  acquires the information relating to the Entitlement in the EMM_P using the stored SK (S 408 ).  
         [0049]     The mobile communication network  304  encrypts information to be transmitted to the terminal  306  of the digital broadcast subscriber, using the BK and transmits the encrypted information to the terminal  306  using an EMM_P (S 410 ). In  FIG. 4 , the information transmitted to the terminal  306  of the digital broadcast subscriber is a Delete message, for example. The Delete message instructs to delete digital broadcasting contents, etc. The terminal  306  acquires the Delete message in the EMM_P by using the stored BK (S 412 ).  
         [0050]     The digital broadcasting network  302  encrypts the DEK using the SK and transmits the encrypted DEK to the terminal  306  using an EMM_G (S 414 ). The terminal  306  acquires the DEK in the EMM_G using the stored SK (S 416 ).  
         [0051]     The digital broadcasting network  302  encrypts the CW using the DEK and transmits the encrypted CW to the terminal  306  using the ECM (S 418 ). The terminal  306  acquires the CW in the ECM using the stored DEK (S 420 ).  
         [0052]     Next, the terminal  306  decrypts the contents received from the digital broadcasting network  302  and thus displays the intended contents.  
         [0053]      FIG. 5  illustrates how the subscriber extends its requested services according to an embodiment of the present invention, to be set forth in detail below.  
         [0054]     The broadcast center  300  inquires of the mobile communication network  304  about whether to extend the service or not when the expiration of the service duration is close (S 500 ). The mobile communication network  304  inquires of the terminal  306  about whether to extend the service (S 502 ). Although the broadcast center  300  inquires as to whether to extend the service in  FIG. 5 , operation S 500  and S 502  can be omitted. Accordingly, when the service duration is to expire, the subscriber can send a request for a service extension to the broadcast center  300 .  
         [0055]     If the terminal  306  determines that it will extend the service duration in response to the request of the mobile communication network  304 , it informs the mobile communication network  304  of the service extension (S 504 ). It is to be noted that the terminal  306  can request other services in addition to the existing services. The mobile communication network  304  forwards the service extension request of the terminal  306  to the broadcast center  300  (S 506 ).  
         [0056]     The broadcast center  300  identifies the subscriber who requests the service extension, and updates its stored subscriber information according to the service extension request (S 508 ). Specifically, the broadcast center  300  updates the service type when the subscriber requests additional service, and updates the service duration when the service duration extension is requested. The broadcast center  300  redefines the fee charged to the subscriber according to the subscriber information update.  
         [0057]     The broadcast center  300  transmits to the mobile communication network  304  a SK that corresponds to the service type requested by the subscriber and the remaining service duration (S 510 ). The mobile communication network  304  forwards the new SK to the terminal  306  (S 512 ). The terminal  306  receives the desired broadcast using the received SK (S 514 ).  
         [0058]     Although  FIG. 5  shows that the broadcast center  300  inquires only as to whether to extend the service duration, it can also inquire, according to a user&#39;s setting, whether to change the service type. It is to be understood that the broadcast center  300  can inquire of both a service extension and a service type change.  
         [0059]     As set forth above, the SK, which requires security, is transmitted over the mobile communication network, and the substantial broadcast service is provided over the digital broadcast network.  
         [0060]     Since the CAS-related key is transmitted to the terminal of the digital broadcast subscriber over the mobile communication network rather than over the digital broadcast network, it is possible to effectively utilize the frequency bandwidth for the digital broadcasting. Furthermore, the security of the CAS-related key can be strengthened by providing the CAS-related key to the terminal over the mobile communication network.  
         [0061]     Although a few embodiments of the present invention have been shown and described, those skilled in the art will appreciate that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.