Abstract:
In a system comprising a broadcasting subsystem and a radiocommunication subsystem each connected to a mediation platform, said transmission being ciphered and arranged for being deciphered by using a key, the following steps are performed at the mediation platform, relatively to a user or group of users having terminals: determining one first and at least one second sets of values from said key, so that the knowledge of all the determined sets of values is required to retrieve the key; and controlling the broadcasting subsystem so that it transmits the first set of values to the user or group of users and the radiocommunication subsystem so that it transmits the at least one second set of values to the user or group of users.

Description:
RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 11/475,328, filed Jun. 27, 2006, now U.S. Pat. No. 8,010,794. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to secure transmission between a system and at least one user having a terminal. 
     It is very common to cipher transmission, so that only allowed receiving users, who have a specific key, can decipher the transmitted data. 
     Such ciphering can take place for instance, but of course not exclusively, in a broadcasting system, such as a DVB (Digital Video Broadcasting) network. The DVB system, and more particularly its DVB-H version adapted to handheld terminals, is fully described in the European standard EN 302 304, V1.1.1, “DVB (Digital Video Broadcasting); Transmission System for Handheld Terminals (DVB-H)”, published by the ETSI (European Telecommunications Standards Institute) in November 2004. 
       FIG. 1  illustrates a DVB-H system in which a transmitter  1  multiplexes, through MUX  3 , a plurality of streams s 1 , s 2 , . . . , s n , which can concern different services, e.g. a video service, a data service, an audio service, etc. Before being transmitted over a radio broadcasting channel  6  from an antenna  5 , the resulting signal is multiplied by a ciphering key Kc (see multiplier  4 ). 
     A receiver  2  incorporated in a handheld terminal and listening to the channel  6  can get the ciphered signal through its antenna  7 . If the user of the terminal is allowed to receive the services, he must have a deciphering key Kd capable of deciphering the signal. Such deciphering key can be a key dual to Kc, and can be public or private as well known by one skilled in the art. The signal resulting from the multiplication between the received signal and Kd (see multiplier  8 ) is finally demultiplexed by DEMUX  9  in order to obtain streams r 1 , r 2 , . . . , r n  substantially corresponding to the streams broadcasted by the transmitter  1 . Further detail of the operations hold in transmitter  1  or receiver  2  is in conformity with the above-mentioned European standard EN 302 304. 
     Of course, ciphering can apply to all the streams as in  FIG. 1 , but it can also apply to some of the streams only. This can happen e.g. when the different streams correspond to different broadcasted TV channels, some of which are with charge for admission. In this case, only the users having a subscription for these channels should be provided with the corresponding deciphering key. 
     An easy way of providing the subscribers with deciphering key Kd is to send them a key K equal to Kd or from which Kd can be derived. However, when transmitting such key K over a radio channel, there is a risk that a hacker can listen to the transmission channel and intercept the key and then have access to the service without having a subscription. 
     In other respects, hybrid systems including a broadcasting subsystem and a radiocommunication subsystem are developing. For example, there are currently some efforts to propose hybrid systems combining a DVB subsystem and a cellular interactive communication subsystem, e.g. a GSM (Global System for Mobile communications), GPRS (General Packet Radio Service) or UMTS (Universal Mobile Telecommunication System) subsystem. 
     Such systems are of great interest because both subsystems have complementary advantages: DVB can transmit identical data to a large number of users with high data rates, whereas GSM or UMTS provides a return channel making the transmission interactive. Some terminals can be equipped to operate with both subsystems. 
     Although the hybrid systems offer improved services compared to the separate subsystems, such as video on demand or e-commerce applications, they do not resolve the above-mentioned problem, since the key used for deciphering the information sent from the DVB subsystem for instance is still transmitted over a radio channel which could be spied by hackers. Therefore, in hybrid systems, transmission of information is still subject to attacks. 
     An object of the present invention is to secure transmission in a hybrid system. 
     Another object of the invention is to limit the access to certain services to allowed users only in a hybrid system. 
     Another object of the invention is to limit the access to certain services to some areas only in a hybrid system. 
     SUMMARY OF THE INVENTION 
     The invention thus proposes a method for securing a transmission between a system and at least one user having a terminal, the system comprising a broadcasting subsystem and a radiocommunication subsystem each connected to a mediation platform, said transmission being ciphered and arranged for being deciphered by using a key. The method comprises the following steps performed at the mediation platform, relatively to a user or group of users having terminals:
         determining one first and at least one second sets of values from said key, so that the knowledge of said first and at least one second sets of values is required to retrieve the key; and   controlling the broadcasting subsystem so that it transmits the first set of values to the user or group of users and the radiocommunication subsystem so that it transmits the at least one second set of values to the user or group of users.       

     The splitting of the key and its transmission according to different paths make it more difficult for a hacker to intercept the key, since more than one channel must be listened to for retrieving the key. It is taken advantage of the fact that the hybrid system offers several channels to secure the transmission of the key. 
     Upon reception of every set of values, only the allowed users can retrieve the key and then decipher later transmission from the system. 
     If the radiocommunication subsystem includes a plurality of base stations, at least two second sets of values can be determined and transmitted from respective base stations to the user or group of users. 
     The number of base stations which can transmit sets of values to the user or group of users is advantageously indicated to the mediation platform, so that the latter can take it into account when determining the sets of values. 
     When this number of base stations is at least two, every set of values can be transmitted by radiocommunication subsystem, while no set of values would be transmitted by the broadcasting subsystem. Indeed, even in this case, there is a space diversity which makes it difficult for hackers to retrieve the key. 
     The number of values in each set of values can be identical or different. Particularly, it can depend on radio conditions encountered on the various radio channels involved. 
     In another embodiment, the radiocommunication subsystem transmits at least one second set of values only via at least one base station covering an area in which said transmission is allowed. This allows having a geographically restricted access for later transmission, since only the users present in said area can get the key necessary for deciphering. 
     The invention also proposes a mediation platform connecting a broadcasting subsystem and a radiocommunication subsystem of a system arranged for performing a transmission to at least one user having a terminal, said transmission being ciphered and arranged for being deciphered by using a key. The mediation platform comprises, relatively to a user or group of users having terminals:
         means for determining one first and at least one second sets of values from said key, so that the knowledge of said first and at least one sets of values is required to retrieve the key; and   means for controlling the broadcasting subsystem so that it transmits the first set of values to the user or group of users and the radiocommunication subsystem so that it transmits the at least one second set of values to the user or group of users       

     The invention also proposes a system comprising a broadcasting subsystem and a radiocommunication subsystem each connected to a mediation platform, the system being arranged for transmitting to at least one user having a terminal, said transmission being ciphered and arranged for being deciphered by using a key. The system comprises means to implement the above-mentioned method, relatively to a user or group of users having terminals. 
     The invention also proposes a terminal comprising means for receiving a transmission from a system comprising a broadcasting subsystem and a radiocommunication subsystem each connected to a mediation platform, said transmission being ciphered, and means for deciphering said transmission by using a key. The terminal further comprises:
         means for receiving one first set of values from the broadcasting subsystem;   means for receiving at least one second set of values from the radiocommunication subsystem; and   means for retrieving said key from said first and at least one second sets of values.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 , already described, represents a transmission chain in a broadcasting system; 
         FIG. 2  is a schematic architecture example of a hybrid system according to the invention; 
         FIG. 3  schematically represents a key transmission in a simple system; 
         FIG. 4  schematically represents a key transmission in a more complex system; 
         FIG. 5  schematically represents a key transmission in a more complex system with geographical restriction. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 2  shows an example of a hybrid system comprising a broadcasting subsystem  11 , here a DVB-H subsystem, and a radiocommunication subsystem, here a cellular subsystem  12  e.g. GSM, GPRS or UMTS. The cellular subsystem  12  comprises a core part  14 , including a mesh of switches or routers and a radio part  15  including base stations. A mediation platform  10  ensures a connection between both subsystems. 
     With this architecture, the system is capable of transmitting information from the DVB-H subsystem  11 . In this case the information, which can be provided by a broadcast service provider  16  for instance, is generally broadcasted. The DVB-H subsystem  11  can also be able to transmit some information to a particular user or a group of users, such as the user having the mobile terminal  13 . 
     The system is also capable of transmitting information from the cellular subsystem  12  to a user or a group of users. The transmitted information can come from a service provider  17  which can be the same or different from the broadcast service provider  16 . The cellular subsystem  12  can also receive information from a user or a group of users. For example, the cellular subsystem  12  and the terminal  13  can exchange information in both directions. 
     The mediation platform  10  controls the transmission of both the DVB-H subsystem  11  and the cellular subsystem  12 . It can also control the transmission from the service providers  16  and  17 . 
     This architecture is suitable especially for providing an interactive broadcast service. For example, the user having the terminal  13  could request a video broadcasting from the cellular subsystem  12  through an uplink channel and this request could be processed by the mediation platform  10  which could then control the DVB-H subsystem  11  so that the user receives the requested video broadcasting through the broadcast channel provided by the DVB-H subsystem  11 . 
     As for security aspects, some transmission from the DVB-H subsystem  11  can be ciphered. As explained above, the user  13  is capable of deciphering the transmitted information only if it has the adequate deciphering key, which can be a public or a private key, or a combination thereof. The ciphering/deciphering operations can be symmetrical or dissymmetrical as well known in the art. 
     As indicated above, the transmission, over a broadcast channel provided by the DVB-H subsystem  11 , of a key K with which the broadcasted information can be deciphered would be too easy to intercept, since hackers would only have to listen to the broadcast channel and decode it if necessary. 
     In order to secure the transmission in the system, key K should be sent in a more secured way, so that only allowed users can get it. 
       FIG. 3  illustrates a simple example of a secured transmission of K. In this example, key K must be sent to the user having the terminal  13  and who is an allowed user (e.g. because he has a subscription for a particular broadcast service). In this respect, the mediation platform of the system can maintain a list of the allowed users. Of course, key K could also be transmitted to a group of allowed users. 
     The terminal  13  can be reached by radio signals coming from one transmitter  18  of the DVB-H subsystem  11  and one transmitter of the cellular subsystem  12  which is part of a base station  19 . 
     Key K is split into two sets of values, so that the knowledge of both sets of values is required to retrieve the key. And each one of the sets of values is sent by one of the transmitters  18  and  19 . The splitting of key K can be of any type. It could simply be a division of the key consisting in a number N of bits into two parts, such as the N 1  least significant bits and the N 2  most significant bits of K, where N 1 +N 2 =N. It should be noted that N 1  could be equal to N 2  or different from it. 
     In a more sophisticated example, key K could be defined as a number k of coefficients of a polynomial P of degree k−1, where k is an integer. Lagrange interpolation theorem allows retrieving P when knowing k couples (a m ,P(a m )), where a m  represents an integer and 1≦m≦k. 
     This method is the one used in the example of  FIG. 3 , where the values a 1 , . . . , a i  and P(a 1 ), . . . , P(a i ), with i&lt;k, are transmitted to the terminal  13  from the DVB-H transmitter  18 , whereas the values a i+1 , . . . , a k  and P(a i+i ), . . . , P(a k ) are transmitted to the terminal  13  from the base station  19 . This splitting and distribution are done by the mediation platform which controls the transmission of both subsystems. 
     Upon reception of the 2k values a 1 , . . . , a k  and P(a 1 ), . . . , P(a k ), the terminal  13  can retrieve the key K. 
     By contrast, a hacker cannot retrieve key K when listening only to the broadcast channel coming from the DVB-H transmitter  18  or to the radio channel coming from the base station  19 . To intercept K, the hacker should thus monitor both channels, which is more difficult, especially because the channels use different paths in space and hackers usually spy channels with directive antennas pointed at one transmitter. Moreover, the DVB-H and cellular subsystems generally use different frequency bands, so that the quasi simultaneous reception of both channels by hackers is more complex. 
     To make the transmission of the sets of values more secured, the sets can be ciphered using respective private ciphering functions, like F 1  and F 2  in  FIG. 3 . Such functions can be Kasumi functions as well known and used in the UMTS system. Of course, any type of ciphering function can apply in this respect. 
       FIG. 4  illustrates another embodiment of the present invention, in which the cellular subsystem is able to communicate with a terminal through several channels provided by different base stations at the same time. The three base stations  20 - 22  constitute an active set with which the terminal  13  can communicate. 
     In this case, the mediation platform of the system controls each one of the three base stations  20 - 22  as well as the DVB-H transmitter  18 , so that they each transmit one set of values from which the key K can be retrieved. To this end, key K is split into four different sets of values, according to any splitting method as explained above. In the example shown in  FIG. 4 , each set of values correspond to a number of different values a m  and the corresponding P(a m ), where m is an integer such that 1≦m≦k and P is a polynomial of degree k−1. The total number of different values a m  transmitted equals k. As apparent in  FIG. 4 , each transmitted set of values can advantageously be ciphered with a respective private function F 1 -F 4 . 
     Each set of values can substantially have the same number of values. But it is also possible to have sets of values with different size. Advantageously, the number of values in each set depends on radio conditions on the corresponding channel. For example, if the downlink channel between the base station  20  and the terminal  13  encounters radio conditions not as good as the ones between the base stations  21 - 22  and the terminal  13 , e.g. because there is more interference with it, it will transmit less values than the other ones. In other terms, with the references used in  FIG. 4 , we will thus have: l−j&lt;i and also l−j&lt;j−i. 
     The radio conditions on each channel can be measured by a radio controller of the radio part of the cellular subsystem, controlling the base stations  20 - 22 . An indication of the measured conditions can be sent by the radio controller to the mediation platform of the system, so that it can take account of them in the key splitting operation. 
     The terminal  13  of  FIG. 4  needs to receive the four different sets of values to retrieve key K and to be able to decipher later transmissions, e.g. from the DVB-H subsystem. 
     The high number of different transmitters involved in the transmission of the split key K makes it even more difficult for hackers to intercept the key. 
     Moreover, the base stations  20 - 22  will generally use different radio resource, such as different frequencies when FDMA (Frequency Division Multiple Access) is implemented or different codes when CDMA (Code Division Multiple Access) is implemented for instance. This also makes the task of hackers more complex. 
     The number of different transmitters involved should preferably be indicated to the mediation platform of the system, so that it can split key K in a appropriate number of sets of values and control the two subsystems accordingly for the transmission of the key. For the cellular subsystem, a radio controller can inform the mediation platform of the number of base stations from which sets of values can be transmitted to the terminal  13 . Such radio controller can be a BSC (Base Station Controller) if the cellular subsystem is of the GSM type or a RNC (Radio Network Controller) if the cellular subsystem is of the UMTS type. 
     In a case different from the one illustrated in  FIG. 4 , when the mediation platform is informed that at least two base stations can transmit sets of values to the terminal, it can control the cellular subsystem so that it transmits every set of values from respective base stations, whereas the DVB-H transmitter transmits no set of values. In other terms, in the example of  FIG. 4 , the values a l+1 , . . . , a k  and P(a l+1 ), . . . , P(a k ) could be transmitted by a further base station instead of the DVB-H transmitter  18 , if the mediation platform of the system is informed that four base stations of the cellular subsystem can transmit sets of values to the terminal  13 . 
       FIG. 5  illustrates a further embodiment of the present invention in which the access to a broadcasting service is limited not only to certain users but also to certain areas. As shown in  FIG. 5 , key K is split into four sets of values, one being transmitted from the DVB-H transmitter  18  and the three others from the base stations  23 - 25 . The base station  25  covers a limited area. For example, its transmitter can have an indoor antenna, such that the covered area is a picocell  26 . 
     The system is arranged so that only the base station  25  can transmit a particular set of values relating to key K. This implies that only the users present inside the picocell  26 , like the user having the terminal  13 , can receive said set of values and thus retrieve key K, since all sets of values are necessary for this. If the terminal  13  is outside the picocell  26 , it will only receive the sets of values coming from the DVB-H transmitter  18  and the base stations  23 - 24  and thus will not be able to reconstruct key K. Further transmission from the system will thus not be deciphered by the terminal  13 . 
     In the example described above, the radiocommunication subsystem was a cellular system. Of course, this is not exclusive. For instance, the radiocommunication subsystem could comprise a radio network of access points (APs), such as an Ad-hoc network. The access points are thus capable of providing a terminal communication node with different sets of values split in flows according to different routes, by using a layer  3  routing protocol such as MPLS (“Multi Protocol Label Switching”) for example.