Method for providing a secure communication between two devices and application of this method

A method of providing a secure communication between first and second devices is described. The method includes encrypting a random key using an encryption key at a first device and transferring the encrypted random key to the second device for encryption of data communicated from the second device to the first device. The encrypted data received from the second device is decrypted using the random key. The method typically includes transferring a control word encrypted with an encryption key to the second device for decryption, and encryption using the random key. The encrypted control word received from the second device is then decrypted using the random key. The invention extends to a method of enabling a decoder, and to a decoder, to decode a data stream. It also extends, inter alia, to a method of authenticating an enabling device and to an enabling device.

The present invention relates to a method for providing a secure communication between two devices, in particular between devices used in a pay TV system.

In a pay TV system each subscriber generally has a decoder for descrambling the source component signal, wherein said decoder comprises a conditional access module and a smart card for decrypting entitlement control messages and entitlement management messages. In order to prevent unauthorized operation of the decoder for descrambling a source component signal it is important to prevent switching between an authorized and an unauthorized smart card for example.

The invention aims to provide a method of the above-mentioned type wherein the communication between two devices, such as the control access module and the smart card or the decoder and the conditional access module, is arranged in such a manner that switching between authorized and unauthorized devices is not possible.

According to the invention a method is provided, wherein a first device generates a random key (Ci) and transfers said key to a second device in a first message encrypted using a public key, wherein said second device decrypts the first encrypted message by means of a corresponding secret key to obtain said random key (Ci), wherein said random key is used to encrypt and decrypt further transmissions between said devices.

According to the invention this method can be applied in a decoder for a pay TV system, wherein said decoder comprises a conditional access module and a smart card, wherein said method is applied to provide a secure communication between the control access module and the smart card or between the decoder and the conditional access module.

The invention further provides a decoder for a pay TV system, comprising a conditional access module and a smart card, said conditional access module comprising means for generating a random key (Ci), means for encrypting said key in a first encrypted message using a public key encryption method, means for transfering said first encrypted message to the smart card, said smart card comprising means for receiving and decrypting said first encrypted message to obtain said random key, means for encrypting transmissions to the conditional access module under said random key, said conditional access module having means to decrypt said transmissions received from the smart card.

In a further embodiment of the invention, said decoder comprises a conditional access module and a smart card, wherein said decoder comprises means for generating a random key (Ci), means for encrypting said key in a first encrypted message using a public key encryption method, means for transfering said first encrypted message to the conditional access module, said conditional access module comprising means for receiving and decrypting said first encrypted message to obtain said random key, means for encrypting transmissions to the decoder under said random key, said decoder having means to decrypt said transmissions received from the conditional access module.

Referring toFIG. 1there is shown in a very schematical manner a block diagram of a decoder for a pay TV system, wherein digital information signals are scrambled using a control word in accordance with the Eurocrypt standard for example. In this embodiment the decoder comprises a demodulator1, a demultiplexer2and a decompression unit3. The decoder further comprises a conditional access module or CAM4and a smart card5which can be inserted into a connection slot of the conditional access module4. Further the decoder is provided with a microprocessor6for configuration and control purposes.

The conditional access module4is provided with a descrambler unit7and a microprocessor8having a memory9. The smart card5comprises a microprocessor10having a memory11.

As the operation of the above-mentioned parts of the decoder is not a part of the present invention, this operation will not be described in detail. Typically, the signal received by the demodulator1is a modulated data stream between 950 MHz and 2050 MHz. The output of the demodulator1is a scrambled digital data stream which is provided to the CAM4and the descrambler7will be allowed to descramble this scrambled data stream assuming that an authorized smart card has been inserted and the subscriber is entitled to receive the program. The descrambled data stream is demultiplexed by the demultiplexer2and decompressed and converted into the original analogue audio and video signal by the decompression unit3.

In a pay TV system the control word required for descrambling, is transferred to the subscribers in so-called entitlement control messages containing the control word encrypted using a service key. This service key is downloaded in the memory11of the smart card5by means of a so-called entitlement management message for example. During operation the CAM4transfers the entitlement control messages towards the microprocessor10of the smart card5so that the microprocessor10can process the entitlement control message and extract the control word. Thereafter the smart card5returns the decrypted control word towards the CAM4so that the descrambler7is allowed to descramble the digital data stream received from the demodulator1.

In order to prevent the use of an unauthorized smart card5in combination with the CAM4it is important to provide a secure communication between the CAM4and the smart card5. According to the present invention the following method is used to provide such a secure communication. The steps of this method are shown inFIG. 2. When a smart card is inserted into the decoder, the microprocessor8of the CAM4will generate two random numbers Ci and A. The microprocessor8will encrypt in a first message the random numbers Ci and A under a public key of the CAM4. The thus obtained first message is transferred to the smart card5and the microprocessor10will decrypt this first message using the secret key of the CAM4. Thereafter the microprocessor10will return a second message to the CAM4, said second message being the random number A encrypted under the number Ci used as encryption key. The microprocessor8of the CAM4decrypts this second message and verifies whether the random number A is correct. Assuming that the random number A is indeed correct, so that it may be assumed that the inserted smart card5is an authorized smart card, the CAM4will then forward entitlement control messages containing the encrypted control word to the smart card5which will process the entitlement control message and extract the control word in a conventional manner. However, in the return message towards the CAM4, the smart card will forward the extracted control word encrypted under the key Ci and these encrypted control words are decrypted by the microprocessor8using the same key Ci. As soon as one tries to replace the inserted smart card5by an other smart card, for example by switching from the authorized smart card5to an unauthorized smart card, the CAM4will immediately establish such change as the key Ci will not be known to the new smart card, so that the CAM will no longer be able to descramble the return messages containing the control word. Thereby the descrambler unit7will be disabled.

The method described can be used in the same manner for providing a secure communication between the CAM4and the decoder, wherein the same protocol as shown inFIG. 2is followed.

In summary it will be understood that if a new CAM4is connected to the other decoder parts, the microprocessor6of the decoder will generate the two random numbers Ci and A and as soon as the microprocessor6has decrypted the second message received from the microprocessor8of the CAM4, and has verified that the random number A is correct, the key Ci will be used in all transmissions between the CAM4and the microprocessor6.

The invention is not restricted to the above-described embodiments which can be varied in a number of ways within the scope of the claims. As an example for a further embodiment the CAM (i.e. the descrambler) may be part of the decoder. The decoder would now challenge the smart card to authenticate itself to obtain a secure communication between the smart card and the decoder.