Pre-payment television system using a memory card associated with a decoder.

PAY TELEVISION SYSTEM USING A MEMORY CARD ASSOCIATED WITH A DECODER

The present invention relates to a pay-TV system using a memory card associated with a decoder.

In pay-TV systems, the video and audio signal intended for broadcast is first scrambled, by some method, then broadcast by radio, satellite or cable and finally received and unscrambled at each level. client using a specific device commonly called a decoder.

In the most advanced pay-TV systems currently known, the decoder essentially comprises an electronic device 10 intended for descrambling the video image with which is associated a memory card intended primarily for controlling access to broadcasts and managing the transmission. credit necessary for the purchase of these programs.

In such pay-TV systems, an arbitrary word is edited at the transmission level intended for the initialization of a pseudo-random generator which determines a code table, this code table is used to effect the scrambling. of the video signal, then this initialization word is encrypted according to a predetermined key before sending it thus encrypted with the video signal. On reception, at the client's decoder, the initialization word is decrypted using the same predetermined key so as to restore it in clear to use it as the initialization word of an identical pseudo-random generator which determines consequently an identical code table, then this code table is used to perform descrambling of the video signal which can then be sent to the television set.

Such a pay-TV system using a memory card has already been described in patent application CH 1576/86.

The smart card has many advantages and among others that of allowing simultaneous operation of several providers but may also have certain drawbacks related to the risks of piracy that we will now discuss. In the case of a memory card intended for multi-service provider operation, a service provider A can register its keys and rights on each card and another service provider B can register its keys on the same cards or on other cards. and its rights independently. In this case it is conceivable that a well-informed person obtains a subscriber card and uses it to decrypt the initialization word sent encrypted with the video signal from provider A, then uses a motherboard to encrypt this word. initialization according to the key used by service provider B and thus sends the programs of service provider A transposed into service provider B's card network to accomplices.

The invention aims to remedy the aforementioned drawback.

The present invention therefore relates to a pay-TV system using a memory card associated with a decoder.

According to one characteristic of the invention, on the transmission side, a number generator delivers a random code which is sent to the input 15 of a motherboard which encrypts this code according to a predetermined key, the code thus encrypted is broadcast with the video signal, a first device receives as inputs on the one hand the random code and on the other hand the code encrypted by the motherboard and performs a determined digital combination of these two codes, this combination constituting the initialization word of a pseudo-random generator 20 intended to provide a code table intended to effect the scrambling of the video signal and, on the reception side, the decoder receives the encrypted code which is decrypted by a customer card, a second device receives as inputs on one side the encrypted code and on the other hand the code decrypted by the customer card and performs a numerical combination of these two codes identical to that of the first device, the result of this combination constituting the initialization word of another pseudo-random generator providing a code table identical to that of the transmission and intended to perform descrambling of the video signal.

According to another characteristic of the invention, the first and second devices perform the same numerical combination, the function of which is such that the reciprocal function can only be determined by an exhaustive test.

According to another characteristic of the invention, the first and second devices perform the same numerical combination of binary numbers and whose function is an exclusive OR.

Other characteristics and advantages of the invention will become apparent from the description of an exemplary embodiment which follows, illustrated by the appended figures, among which:

- figure 1 shows the basic principle of a pay-TV system,

FIG. 2 represents the operating principle of a pay-TV system using a memory card currently known and also illustrates a possible pirating mode,

- And FIG. 3 represents the operating principle of a pay-TV system according to the invention and also illustrates the impossibility of piracy.

In FIG. 1, a device 1 can be distinguished, for example a microcomputer, with the aid of which the service provider generates data relating to the 15 emissions planned and useful to the customers. These data are associated at 2 with the video signal delivered by a video tape recorder 3 or any other system. In general, the return lines of the vertical scan of the video signal are used to transmit such data.

The video signal and the data associated with it are then scrambled using a scrambler 4 which renders the image unusable by a non-client. This scrambled signal is transmitted by radio link, by satellite or by cable to the receiver 5 of each subscriber, and a descrambler or decoder 6 allows the broadcast to be reproduced on a television 7 with normal quality.

In FIG. 2, there is a scrambling system 8 on transmission and a scrambling system, also called a decoder, 9 on reception at each customer subscribed to this pay-TV system.

In the scrambling system 8, one distinguishes, among others, a random number generator 10 which delivers an initialization word Xini which is sent to the input of a motherboard 11. This motherboard 11 encrypts the word d Xini initialization according to a key 100, the word thus encrypted being associated with the video signal. At the same time, the initialization word Xini is used to initialize a pseudo-random generator 12 which thus outputs a certain code table which is used to impart a certain interference to the video image and to the sound. At reception 9, there is a client card 13 which is used to decrypt the encrypted initialization word as it is received, the decryption being carried out using the same key 100, in order to find the word d at the output. Xini initialization in clear. This word Xini is then sent to initialize a pseudo-random generator 14 of identical design to that of the pseudo-random generator 12 of the transmission. The generator 14 consequently produces a code table identical to that generated on transmission and thus allowing descrambling of the video image transmitted. The initialization word Xini can be modified at any time, for example every second or every ten seconds, without disturbing an authorized reception by the customer.

This system works very well but can allow some hacks elaborated by initiated people, as we will explain it now. It should be noted first of all that several providers can together use the same network of smart card decoders and even use the same smart cards. A service provider A can register his key 100 and his rights and another service provider B can register his key 200 and his rights independently and on the same network of card decoders. A knowledgeable person, for example working at a service provider, can easily with a client card 13 decrypt the initialization word Xini and extract it in clear, then obtain a motherboard 15 programmed to encrypt the password. initialization according to the key 200 of another provider, then send in association with the video signal of the transmission of provider A in a network, for example cable, to which accomplices are connected, provided with customer cards 16 with key 200, and which can then see the program of service provider A transposed into the system of service provider B.

FIG. 3 shows a pay-TV system with a memory card in accordance with the invention and with which it will be verified that the hacking mode described above is impossible. Certain elements identical to those of the prior art as shown in FIG. 2 and which have been identified by identical references can be distinguished in FIG. 3. The random number generator 10 delivers a number which is hereinafter called the random code Xal. The motherboard 11 encrypts this Xal code according to the key 100 and the customer cards 13 decrypt it according to the key 100 in order to find the same Xal code.

A first device 17 receives as inputs on the one hand the random code Xal and on the other hand the code encrypted by the motherboard 11 and performs a determined numerical combination of these two codes, this combination 5 constituting the initialization word of the generator. pseudo-random 12 intended to provide a code table intended to effect the scrambling of the video signal. Likewise, a second device 18 receives as inputs on the one hand the encrypted code and on the other hand the code decrypted by the customer card 13, identical to Xal, and performs a digital combination of these two codes identical to that of the first device 17 such that the pseudo-random generator 14 provides an identical code table for performing descrambling of the video signal. The digital combination of the devices 17 and 18 can be composed simply of an exclusive OR or can implement any function known per se, the reciprocal function of which can only be determined by an exhaustive test. combination function used identically in the two devices 17 and 18, the two pseudo-random generators 12 and 14 will be initialized similarly and descrambling will always be correctly obtained, even if the Xal code changes. On the other hand, if the hacking process, described previously from a conventional system, is applied here, it will be noticed that it does not work. Indeed, the initiated person may well, from a client card 13, find the code Xal in clear, but the broadcast in the network B whose key is 200, therefore different, will result in the encrypted Xal code. will be different and that, consequently, the numerical combination of the device 19, although identical to the devices 17 and 25 18, will give an initialization word of the pseudo-random generator 20 different from the initialization words of the generators 12 or 14.