EP1214840A1

EP1214840A1 - Multimedia digital terminal and detachable module cooperating with the terminal comprising an interface protected against copying

Info

Publication number: EP1214840A1
Application number: EP00960548A
Authority: EP
Inventors: Stéphane Thomson Multimedia MORCEL
Original assignee: Thomson Licensing SAS
Current assignee: THOMSON LICENSING
Priority date: 1999-09-23
Filing date: 2000-08-30
Publication date: 2002-06-19
Also published as: FR2799075A1; FR2799075B1; WO2001022724A1; AU7281400A; AR028172A1

Abstract

The terminal (1) receives a scrambled data stream (TS(D.EMB.)) in which the control words (CW) having served to scramble the data are transmitted in encrypted form. It transmits, via a first interface (ITTS), this stream to a detachable module (2) containing the conditional access system specific to the service provider supplying the data. The module (2) extracts from the stream and decrypts the control words (CW) before transmitting them to the terminal, via a second interface (ITCOM). These control words are used by a descrambler (16) of the terminal to descramble the data. The data stream leaving the module (TS'(D.EMB.)) contains only scrambled data, thus guaranteeing better protection against copying.

Description

Multimedia digital terminal and detachable module cooperating with the terminal comprising an interface protected against copying

The present invention relates to the field of conditional access systems for multimedia digital terminals. It relates more particularly to a multimedia digital terminal as well as to a detachable module associated with this terminal for implementing a conditional access system, in which the interface between the terminal and the module is protected against illicit copying.

The invention has been embodied within the context of the so-called DVB-CI interface (the initials standing for "Digital Video Broadcasting - Common Interface" which is described in particular in European Standard EN 50221 published by CENELEC (Comite Europeen de Normalisation Electrotechnique) .

This DVB-CI common interface has been defined by the DVB group so as to allow standardization of digital receiving equipment whilst allowing service providers supplying the data, (for example suppliers of Pay TV programmes) to remain proprietors of the conditional access system and of the corresponding security elements. This is because a conditional access system must be envisaged whenever it is necessary to control access to broadcast data. However, the service providers supplying the data wish to preserve a specific conditional access system. By virtue of the DVB-CI interface, the specific proprietary elements can be integrated into a module which is separate from the standardized parts of the terminal which receive and decode the digital video data or the service data. Thus, the proprietary elements can be manufactured and sold separately from the terminals, thereby making their distribution easier. The module is referred to as a "DVB-CI module" by extension. The aforesaid European standard describes in particular the hardware and software architecture of the common interface between a server (the host) and one or more DVB-CI modules connected to the server. We have represented this architecture in Figure

1. The host server 1 is a multimedia digital terminal such as a decoder of programmes broadcast by satellite or by cable, a video recorder or a personal computer, to which may be connected one or more modules 2. The module 2 is a detachable device which cannot operate alone but which is intended to execute certain specific tasks in association with the host server, in particular the implementation of a conditional access system or of an electronic programme guide. To do this, the module can access resources, that is to say software or hardware functional units, of the host server. The aforesaid common interface standard defines for this purpose a certain number of objects able to be exchanged between the module and the host server and allowing the module to use the resources of the host server.

Within the framework of the standards defined by the DVB group, the digital data are transmitted in the form of an information stream coded according to the MPEG 2 standard (ISO/IEC 13818-1), but naturally any other data transport standard can be used within the framework of the invention.

The digital terminal 1 of Figure 1 comprises a tuner/demodulator 10 which receives a signal S, emanating from a satellite antenna or from a cable network, and which outputs a digital data stream transmitted in the form of packets, and referred to as the TS (standing for "Transport Stream") in the aforesaid MPEG 2 standard, and containing the services supplied by providers.

The data stream TS comprises, in a known manner, packets of data supplied by various service providers and, so as to guarantee that the data reach only those users having acquired the right to receive them (for example by means of a subscription to the service) , these data are transmitted whilst being scrambled by control words CW.

In order to descramble the data, the service provider also supplies the users with the control words having served to scramble the data. So as to keep the control words secret, they are supplied after having been encrypted with an algorithm with key K. The various encrypted control words are sent to the users in control messages, commonly denoted ECMs (the abbreviation ECM standing for "Entitlement Control Message") transmitted in the data stream TS . The control words are thereafter decrypted in a secure processor containing the key K, the secure processor being included within a security element, for example a smart card, which is supplied to the users .

Returning to Figure 1, TS(D.EMB.) therefore denotes the data stream TS containing the data in scrambled form. In accordance with the principle defined in the aforesaid standard EN 50221, all the elements of the conditional access system which are specific to the service provider are contained in the detachable module 2 which receives the data stream TS(D.EMB.) from the digital terminal 1 across the DVB-CI common interface. The module 2 comprises a microcontroller 21, in which the access control software application CA specific to the service provider is executed. It furthermore comprises a component 22 referred to as demultiplexer which receives the data stream TS(D.EMB.) so as to extract therefrom the video or audio data packets corresponding to a service which the user wishes to view or so as to extract therefrom data packets containing so- called "service" information, such as control messages ECM. The video or audio data packets are sent to a descrambler 23 contained in the module 2 whilst the messages ECM are sent to a smart card 3 which has been inserted into the module 2 and which contains, in a secure processor 30, the key K having served to encrypt the control words. The decryption of the control words contained in the messages ECM is performed by the secure processor 30 which outputs the control words CW unenciphered to the descrambler 23 situated in the module 2. The descrambler 23 is thus in a position to descramble the video or audio data packets received from the demultiplexer 22 by virtue of the control words CW which it receives from the smart card 3. It outputs a digital data stream TS(D.DES.) in which the data packets corresponding to the service which has been selected by the user are descrambled.

The data stream TS(D.DES.) is transmitted to the digital terminal 1 via the DVB-CI common interface. It is more particularly transmitted to an MPEG decoder 14 of the terminal which supplies the audio and video outputs of the terminal which can be read directly by an apparatus such as a television.

The terminal 1 also comprises a microcontroller 11 in which the various software applications of the terminal are executed.

The DVB-CI common interface in fact comprises two logic interfaces: the first, the interface of the data stream ITTS, constitutes a bi-directional bus and transports the digital data packets of the TS stream in both directions between the terminal 1 and the module 2 according to the

MPEG 2 transport standard. From the terminal to the module, the stream TS(D.EMB.) transmitted via the ITTS interface is a scrambled data stream whilst in the other direction stream TS(D.DES)), and on condition that the module 2 allows access to the data selected by the user

(that is to say that it contains the elements of the conditional access system of the service provider which are necessary for descrambling the said data) , the packets containing the selected data are returned descrambled whilst the remainder of the TS stream remains unchanged; the second, the command interface ITCOM, transmits commands in both directions between the terminal 1 and the module 2 (more particularly between the microcontrollers 11 and 21) by way of the objects (or primitive functionalities) defined in the standard and which were mentioned above.

A notable drawback of the system just described is that certain data flow unenciphered at the DVB-CI common interface level (in the stream TS(D.DES.). In actual fact, data broadcasters are becoming increasingly preoccupied by illicit copies which may be made from the data which these broadcasters transmit, all the more so when these data are transmitted in digital form as is the case in the present instance, and they are asking for means of protection against copying to be set in place.

The current DVB-CI common interface cannot guarantee this protection against copying since, the module 2 being detachable, it is possible for a pirate to connect a digital recorder to a connection port across which the stream TS(D.DES.) travels and to record the data packets which flow unenciphered.

A purpose of the present invention is to solve the aforesaid problems by providing means of ensuring protection against copying at the level of the common interface between a digital terminal and a module which are linked together by an interface of the DVB-CI type.

The invention therefore relates to a multimedia digital terminal intended for receiving a stream of digital data scrambled by control words, the control words being contained in the stream in encrypted form, and intended to cooperate with a detachable module for descrambling the data stream. The terminal is able to transmit the scrambled data stream to the detachable module. According to the invention, the terminal is furthermore capable of receiving from the detachable module the decrypted control words, and it comprises a descrambler capable of descrambling the digital data stream by using the control words received from the detachable module. The descrambling of the data is therefore performed solely in the decoder and no data item now travels unenciphered at the level of the interface between the terminal and the detachable module. The risks of illicit copies are therefore considerably reduced by virtue of the invention.

According to a preferred embodiment of the invention, the digital terminal is furthermore able to receive from the detachable module the scrambled digital data stream, wherein the data stream received by the terminal is not modified relative to the data stream transmitted by the terminal.

According to another embodiment, the digital terminal is able to receive from the detachable module the scrambled digital data stream from which, relative to the data stream transmitted by the terminal, the data packets containing the control words have been removed.

According to another aspect of the invention, the terminal is able to transmit the scrambled data stream to the detachable module across a first interface with the detachable module, and it is furthermore capable of receiving from the detachable module the decrypted control words across a second interface with said module.

According to another particular characteristic of the invention, the terminal is furthermore able to receive from the detachable module, across the first interface, the scrambled digital data stream.

The invention also relates to a multimedia digital terminal intended for receiving a stream of digital data scrambled by control words, the stream of data containing the control words encrypted using a first key, and intended to cooperate with a detachable module for descrambling the data stream. The terminal is able to transmit the scrambled data stream to the detachable module across a first interface with the detachable module. According to the invention, the terminal is furthermore capable of receiving from the detachable module the decrypted control words which are encrypted using a second key before being transmitted across a second interface with the module. The terminal also comprises means for decrypting the control words received from the detachable module using the second key; and a descrambler capable of descrambling the digital data stream by using the decrypted control words.

Thus, in addition to the data which travel at the level of the first interface between the module and the terminal only in scrambled form, the control words are also encrypted so as to travel at the level of the second interface, thereby further increasing the security of the assembly.

The invention also relates to a module intended for cooperating with a digital terminal as described above, this module being able to receive from the terminal a scrambled digital data stream. According to another aspect of the invention, the module is furthermore capable of extracting from the data stream the control words having served to scramble the digital data; of decrypting the control words; and of transmitting the decrypted control words (CW) to the terminal .

According to another aspect of the invention, the module is furthermore adapted for returning the scrambled digital data stream to the terminal without modifying the stream.

According to yet another aspect of the invention, the module is furthermore adapted for returning the scrambled digital data stream to the terminal by removing, relative to the data stream transmitted by the terminal, the data packets containing the control words. According to another characteristic of the invention, the module is able to receive the digital data stream across a first interface with the terminal, and it is furthermore capable of transmitting the decrypted control words to the terminal across a second interface with the terminal.

According to a particular embodiment of the invention, the module is able to cooperate with a detachable security element for decrypting the control words, the module being capable of extracting from the digital data stream messages containing the encrypted control word; of transmitting the messages to a security element inserted into the module; of receiving from the security element the decrypted control words; and of transmitting the decrypted control words to the digital terminal with which it cooperates.

The invention also relates to a module intended for cooperating with a digital terminal as described above, the module being able to receive from the terminal a scrambled digital data stream across a first interface with the terminal. According to the invention, the module is furthermore capable of extracting from the data stream the control words having served to scramble the digital data; of decrypting the control words and encrypting the control words using a second key; and of transmitting the encrypted control words to the terminal across a second interface with said terminal.

The invention also relates to a method for descrambling a stream of digital data, scrambled by control words, which is received by a multimedia digital terminal, the control words being contained in the stream in encrypted form. The method comprises a first step consisting in transmitting the scrambled data stream to a detachable module. It furthermore comprises the steps consisting, for the detachable module, in extracting from the data stream the control words having served to scramble the digital data; in decrypting said control words; and in transmitting the decrypted control words to the terminal.

The invention also relates to a method for descrambling a stream of digital data, scrambled by control words, which is received by a multimedia digital terminal, the stream of data containing the control words encrypted using a first key. The method comprises a first step consisting in transmitting the scrambled data stream to a detachable module. According to the invention, it furthermore comprises the steps consisting, for the detachable module, in extracting from the data stream the control words having served to scramble the digital data; in decrypting the control words; in encrypting the control words with using a second key; and in transmitting the encrypted control words to the terminal. The method furthermore comprises the steps consisting, for the terminal, in decrypting the control words using the second key; and in descrambling the digital data stream by using said decrypted control words. The invention also relates to a method for transferring control words between a module as described above and a digital terminal also described above. This method consists essentially: for the module, in sending a first identification object to the terminal; for the terminal, in sending, in response to the first object, a second identification object indicating whether the terminal is or is not able to receive the control words from the module; and - for the module, in the event of a positive response with the second object, in sending the control words enclosed in a third object.

Other characteristics and advantages of the invention will become apparent on reading a particular, non-limiting embodiment of the invention given with reference to Figures 1 to 4, among which: Figure 1, described above, represents a digital terminal into which is plugged a DVB-CI module according to the prior art;

Figure 2 represents a digital terminal into which is plugged a module according to the principle of the invention;

Figure 3 diagrammatically represents a part of the common interface between a digital terminal and a module such as those of Figure 2; - Figure 4 illustrates a protocol for communication across the part of the common interface represented in Figure 3.

Represented in Figure 2 are a digital terminal 1 together with a module 2 connected to the terminal via an interface which is improved relative to the DVB-CI interface so as ensure better protection against copying at the level of this interface.

The elements which are similar to those of Figure 1 described earlier bear the same reference numbers and will not be described further.

In the embodiment of the invention illustrated in Figure 2, it will be considered that the digital terminal 1 is a decoder receiving programmes from various service providers via cable or via a satellite antenna. A module 2 containing the specific elements of the conditional access system of a first service provider is connected to this decoder.

According to the principle of the invention, the data stream TS'(D.EMB.) leaving the module 2 via the interface ITTS contains only data in scrambled form. The module 2 extracts only the control words CW from the stream TS(D.EMB.) received so as to transmit them, once decrypted, to the decoder. The descrambling of the data is performed only in the decoder 1 by virtue of a descrambler 16 contained in the decoder. Thus, the digital data are no longer available unenciphered at the level of the ITTS interface and illicit copying is, if not eliminated, at least made much more complex than in the prior art.

To do this, as in the prior art, the module 2 comprises a demultiplexer 22 which extracts the ECM control messages from the data stream TS(D.EMB.) received and which transmits them to a smart card 3 inserted into the module. The secure processor 30 of the smart card decrypts the control words CW contained in the ECM messages and it transmits them to the module 2. According to a preferred embodiment of the invention, the control words CW are transmitted from the module 2 to the decoder 1 across the control interface ITCOM. The details of the transmission of the control words via the ITCOM interface will be explained later in conjunction with Figures 3 and 4.

Preferably, the data stream TS(D.EMB.) which is received by the module 2 is retransmitted as is to the decoder (link 29) without being modified (TS'(D.EMB) = TS(D.EMB.)). It is received in the decoder by a demultiplexer 15 which extracts the video or audio data packets corresponding to the programme which the user wishes to view and which transmits them to the descrambler 16 so as to be descrambled before being transmitted to the MPEG decoder 14. For this purpose the descrambler 16 receives the control words CW from the module 2 via the ITCOM interface.

According to a first variant embodiment, the module 2 extracts the ECM control messages from the data stream TS(D.EMB.) and replaces them, in the data stream TS'(D.EMB.) transmitted to the decoder, with stuffing packets. This variant is represented diagrammatically via the link 28 shown dashed.

According to a second variant embodiment, the demultiplex 22, contained in the module 2, extracts the video or audio data packets corresponding to the programme which the user wishes to view and it transmits them in the data stream TS'(D.EMB.) so that they are received directly by the descrambler 16 of the decoder. This variant is represented diagrammatically by the link 27 shown dashed.

Also represented dashed in Figure 2 is a smart card 4 which can be inserted into the decoder 1 and which belongs to the conditional access system of a second service provider. Indeed, the decoder can comprise an integrated conditional access system for accessing the services of the second provider and furthermore comprise several connectors for modules 2 so as to access the services of several other providers.

It is for this reason that the decoder comprises a demultiplexer 15 and a descrambler 16. The access control software application CA' of the second provider is executed in the microcontroller 11. When the user selects a service of the second provider, the stream TS(D.EMB.) is transmitted directly to the demultiplexer 15 via a link 19. This demultiplexer extracts the control messages ECM' and transmits them to the secure processor 40 of the smart card 4 which, with the aid of the key K', decrypts the control words CW' contained in the messages ECM' and sends them unenciphered to the descrambler 16, so as to descramble the services selected by the user.

According to a third variant embodiment of the invention, the data stream TS(D.EMB.) is transmitted from the decoder 1 to the module 2 so that the latter extracts the control words CW and retransmits them unenciphered to the decoder, but unlike the variants mentioned earlier, the stream TS(D.EMB.) is not returned via the ITTS interface to the decoder. Only the control words are transmitted to the decoder after having been decrypted by the module. The stream TS(D.EMB.) is for its part sent directly to the demultiplexer 15 of the decoder via the link 19 inside the decoder. According to a fourth variant embodiment of the invention, the control words CW are not transmitted unenciphered across the ITCOM interface. They are encrypted by the microcontroller 21 of the module 2 using a second key, different from the key K used to encrypt the control words upon their transmission in the data stream TS(D.EMB.), before being transmitted over the ITCOM interface. This variant thus offers increased security since now no element travels unenciphered via the interface between the module and the decoder.

The microcontroller 11 of the decoder 1 will therefore have to decrypt, using this second key, the control words received by the ITCOM interface before transmitting them to the descrambler 16.

In a first embodiment of this variant, the module 2 and the decoder 1 generate, before a single control word travels through the ITCOM interface, a session key SSK by using the Diffie Hellman protocol, which is well known to the person skilled in the art. Thereafter, all the control words are encrypted in the module using the session key SSK, then are transmitted to the decoder in this encrypted form across the ITCOM interface before being decrypted, using the session key, in the decoder.

In this embodiment, a different session key is generated each session, that is to say each time the user selects a programme which needs to be descrambled in the module 2. In a second embodiment of this variant, the decoder 1 and the module 2 each contain a secret key K2 which is stored in a secure area of the decoder or of the module, for example in a secure processor. This secret key is unique for each decoder and if several modules are plugged into the decoder, they all contain the secret key of the decoder.

In this embodiment, the control words are therefore encrypted with the secret key K2 in the module before being transmitted to the decoder via the ITCOM interface; they are then decrypted in the decoder using the key K2. The principle of the invention is particularly advantageous since it makes it possible to harness the demultiplexer and the descrambler which are already contained in the decoder so as to use them even when the conditional access system of another provider, contained in a detachable module, is used.

It should be stated in this regard that the demultiplexer and the descrambler components have been standardized within the framework of the DVB digital broadcasting standard in particular. Only the conditional access system (access control software, decryption algorithm and key, smart card, etc.) remains a proprietary system of the service provider.

We shall now describe in greater detail, in conjunction with Figures 3 and 4, how the control words CW are transmitted via the command interface ITCOM.

The ITCOM interface between the terminal (decoder) 1 and the module 2 uses a communication protocol which is represented diagrammatically in Figure 3 and which comprises several layers: a "Physical" layer (50, 60) which comprises the physical connector and low-level software for initializing the communication; a "Link" layer (51, 61) which forms the link between the Physical layer which deals only with bytes (8 bits of data) and the higher layer which can deal with much larger packets of data; a "Transport" layer (52, 62) which optimizes the occupation of the data stream by the various session in progress and which conveys the messages from the higher layer to the "Link" layer; a "Session" layer (53, 63) which opens, closes and manages the sessions in which the messages of the applications using the ITCOM interface are transmitted. Represented in Figure 3 are on the one hand the access control application CA 65 which is executed in the microcontroller 21 (cf. Fig. 2) of the module 2 and on the other hand a software resource 55 called CAS (standing for "Conditional Access Support") of the terminal 1.

This resource CAS defines a certain number of specific objects intended for use by the access control applications of the modules connected to the terminal. It communicates with the four layers of the ITCOM interface across a " Standard API " (the abbreviation API standing for "Application Programming Interface"), the standard API defining other objects which are used by all the resources of the system.

Likewise, the access control application 65 of the module uses the objects defined by the resources of the terminal through a standard API. It was seen earlier that, according to the principle of the invention, the control words which are decrypted in the module 2 are thereafter transmitted to the terminal 1 across the ITCOM interface. In order that the control words may "cross" the various layers of the ITCOM interface, they must be enclosed in specific objects, comprehensible both to the application CA and to the resource CAS, which will be called "CPTS_CW()" hereinafter. Of course, when the control words are transmitted encrypted with the aid of a second key (SSK or K2) as in the fourth variant set forth above, they are enclosed, in this encrypted form, in the objects CPTS_CW() .

Figure 4 illustrates the protocol for communication between the access control application CA of the module and the resource CAS of the terminal which allows, according to the invention, protection against copying .

The time scale has been represented by two downward vertical arrows. After a first session-opening phase (not represented) , the application CA of the module checks that the resource CAS of the terminal does indeed support protection against copying over the ITCOM interface. To do this, the application CA sends during a step 100 an object "CPTS_identification() " which will be defined later. The resource CAS responds by sending, in step 101, an object "CPTS_identification_answer ( ) " which indicates whether the resource CAS does or does not support protection, against copying. In the event of a positive response, the application CA of the module then regularly sends (steps 102a, 102b, 102i) the control words enclosed in the objects "CPTS_CW()" as and when they are decrypted by the module.

Conversely, in the event of a negative response, the terminal is not considered reliable. Consequently, the control words are not sent to the terminal and the data of the stream TS(D.EMB.) are not descrambled in the terminal. Any appropriate action can be envisaged at that moment (displaying of a message destined for the user by way of the User Interface resource of the terminal for example) . As seen above, it is necessary, in order to implement the protocol of Figure 4, to define additional objects over and above those already envisaged in the existing standards.

Identification objects The function of the identification objects is to allow an access control application CA of a module supporting protection against copying to check whether the terminal to which the module is connected also supports protection against copying. That is to say, whether the resource CAS of the terminal possesses the functionality of protection against copying.

The first object used is the following:

CPTS_identification() { CPTS_identification_tag 24 bits uimsbf Length_field ( ) = 0 8 bits uimsbf

} The notation used here corresponds to that used in the aforesaid standard EN 50221 in which the objects are generally defined by means of a tag (here the "CPTS_identification_tag") , of a length field ("Length_field ( ) ") and of parameters which can take various values .

The CPTS_identification_tag tag is a number defined on 24 bits and the notation "uimsbf" (stemming from "unsigned integer most significant bit first") signifies that this pertains to an unsigned integer whose most significant bits (highest order bits) are always situated first.

The "Length_field() " field defines the length in terms of number of bytes of the parameters forming part of the object. Since here the object

"CPTS_identification ( ) " contains no parameter, the length field is equal to zero.

The second object used is the following:

CPTS_identification_answer ( ) {

CPTS_identification_answer_tag 24 bitsuimsbf Length_field() = 1

Identification_result 8 bits uimsbf }

This "CPTS_identification_answer ( ) " object is used to indicate whether the resource CAS of the terminal does or does not support protection against copying according to the invention. It comprises a "CPTS_identification_answer_tag ( ) " tag and a length field which is equal to 1 since the object also comprises an "Identification_result" parameter defined on 8 bits. This parameter is for example a Boolean indicating whether or not protection against copying is supported by the resource CAS. Other parameters can be envisaged, for example to indicate a version number of the copy protection.

Contro- word transmission objects

These objects enclose the control words which are decrypted in the module. They allow the access control application CA of the module to send them, across the various layers of the ITCOM interface, to the resource CAS of the terminal which can thereafter utilize them by transmitting them to the descrambler 16 (Fig. 2) . These objects have the following format:

CPTS_CW() {

CPTS_CW_tag 24 bits uimsbf

Length_field ( ) MPEG2_stream_type 8 bits uimsbf

MPEG2_pid 16 bits uimsbf

Odd_CW 128 bits uimsbf

Even_CW 128 bits uimsbf

}

They comprise a "CPTS_CW_tag" tag, a length field "Length_field ( ) " and parameters identifying: the type of data packet transported (video, audio or other data) , in "MPEG2_stream_type" defined on 8 bits; the PID (standing for "Programme Identifier"), that is to say the programme identifier associated with the packet transported in "MPEG2_pid", defined on 16 bits; - the odd ("Odd_CW") and even ("Even_CW") control words defined on 128 bits each.

Indeed, it is well known that a succession of even and odd control words is transmitted in the TS data stream, each control word being transmitted in advance relative to the data packets which are scrambled with this control word. The descrambler therefore continuously memorizes the even control word serving to descramble the current data packet and the odd control word serving to descramble the next packet (or vice versa) .

The "CPTS_CW()" object comprises, in a first embodiment, parameters making it possible to send both an even control word (in "Even_CW") and an odd control word (in "Odd_CW"). It can also comprise, in another embodiment, a parameter "CW", on 128 bits, containing the control word and a parameter "Type_CW" on 8 bits, defining the type, even or odd, of the control word transmitted.

Of course, the invention is not limited to the exemplary embodiments which are mentioned hereinabove. In particular, the person skilled in the art will be able to introduce any variant into the definition of the identification objects and control-word transmission objects by adding other parameters to these objects.

Moreover, although the invention has been described with reference to the DVB-CI standard it is not limited to the framework of this standard. The invention can in particular be advantageously applied within the framework of the American NRSS (standing for "National Renewable Security Standard") standard.

Claims

1. Multimedia digital terminal (1) intended for receiving a stream (TS(D.EMB.)) of digital data scrambled by control words (CW) , said control words being contained in said stream (TS(D.EMB.)) in encrypted form, and intended to cooperate with a detachable module (2) for descrambling said data stream, said terminal being able to transmit the scrambled data stream (TS(D.EMB.)) to the detachable module, characterized in that it is furthermore capable of receiving from said detachable module the decrypted control words (CW) , and in that it comprises a descrambler (16) capable of descrambling the digital data stream by using the control words (CW) received from the detachable module (2) .

2. Digital terminal according to Claim 1, characterized in that it is furthermore able to receive from the detachable module (2) the scrambled digital data stream (TS' (D. EMB. ) ) , said data stream received by the terminal not being modified relative to the data stream (TS(D.EMB.)) transmitted by the terminal.

3. Digital terminal according to Claim 1, characterized in that it is furthermore able to receive from the detachable module (2) the scrambled digital data stream (TS' (D. EMB. ) ) from which, relative to the data stream (TS(D.EMB.)) transmitted by the terminal, the data packets containing the control words (CW) have been removed.

4. Digital terminal according to one of Claims 1 to 3, characterized in that said terminal is able to transmit the scrambled data stream (TS(D.EMB.)) to the detachable module across a first interface (ITTS) with said detachable module, and in that it is furthermore capable of receiving from said detachable module the decrypted control words (CW) across a second interface (ITCOM) with said module.

5. Terminal according to Claim 4, in combination with Claims 2 and 3, characterized in that it is furthermore able to receive from the detachable module (2), across said first interface (ITTS), the scrambled digital data stream (TS' (D. EMB. ) ) .

6. Multimedia digital terminal (1) intended for receiving a stream (TS(D.EMB.)) of digital data scrambled by control words (CW) , said stream of data (TS(D.EMB.)) containing the control words encrypted using a first key (K) , and intended to cooperate with a detachable module (2) for descrambling said data stream, said terminal being able to transmit the scrambled data stream (TS(D.EMB.)) to the detachable module across a first interface (ITTS) with said detachable module, characterized in that it is furthermore capable of receiving from said detachable module the decrypted control words (CW) which are encrypted using a second key

(SSK, K2) before being transmitted across a second interface (ITCOM) with the module (2), and in that it comprises: means for decrypting said control words received from the detachable module using said second key (SSK, K2) ; and a descrambler (16) capable of descrambling the digital data stream (TS(D.EMB.)) by using said decrypted control words (CW) .

7. Module (2) intended for cooperating with a digital terminal according to Claim 1, said module being able to receive from the terminal a scrambled digital data stream (TS (D. EMB. ) ) , characterized in that said module is furthermore capable of: extracting from the data stream (TS(D.EMB.)) the control words (CW) having served to scramble the digital data; decrypting said control words; and transmitting the decrypted control words (CW) to said terminal.

8. Module according to Claim 7, characterized in that it is furthermore adapted for returning the scrambled digital data stream (TS' (D. EMB. ) ) to the terminal without modifying said stream.

9. Module according to Claim 7, characterized in that it is furthermore adapted for returning the scrambled digital data stream (TS' (D. EMB . ) ) to the terminal by removing, relative to the data stream (TS(D.EMB.)) transmitted by the terminal, the data packets containing the control words.

10. Module according to one of Claims 7 to 9, characterized in that it is able to receive the digital data stream (TS(D.EMB.)) across a first interface (ITTS) with said terminal (1), and in that it is furthermore capable of transmitting the decrypted control words to said terminal across a second interface (ITCOM) with said terminal.

11. Module according to one of Claims 7 to 10, characterized in that it is able to cooperate with a detachable security element (3) for decrypting the control words, said module being capable of: - extracting from the digital data stream

(TS(D.EMB.)) messages (ECM) containing the encrypted control words; transmitting said messages (ECM) to a security element (3) inserted into said module; receiving from said security element (3) the decrypted control words (CW) ; and - transmitting the decrypted control words to the digital terminal (1) with which it cooperates.

12. Module (2) intended for cooperating with a digital terminal according to Claim 6, said module being able to receive from the terminal a scrambled digital data stream (TS(D.EMB.)) across a first interface (ITTS) with the terminal (1), characterized in that said module is furthermore capable of: extracting from the data stream (TS(D.EMB.)) the control words (CW) having served to scramble the digital data; decrypting said control words; and encrypting said control words using the second key (SSK, K2 ) ; and - transmitting said encrypted control words (CW) to the terminal across a second interface (ITCOM) with said terminal.

13. Module according to one of Claims 10 or 12, characterized in that it pertains to a module according to the DVB-CI standard and in that the first interface (ITTS) of the module is the interface of the transport stream according to the MPEG-2 standard and the second interface (ITCOM) is the control interface.

14. Method for descrambling a stream (TS(D.EMB.)) of digital data, scrambled by control words (CW) , which is received by a multimedia digital terminal (1), said control words (CW) being contained in said stream in encrypted form, the method comprising a first step consisting in: transmitting the scrambled data stream (TS(D.EMB.)) to a detachable module (2), characterized in that it furthermore comprises the steps consisting, for the detachable module (2), in: - extracting from the data stream (TS(D.EMB.)) the control words (CW) having served to scramble the digital data; decrypting said control words; and transmitting the decrypted control words (CW) to said terminal.

15. Method for descrambling a stream (TS(D.EMB.)) of digital data, scrambled by control words (CW) , which is received by a multimedia digital terminal (1), said stream of data containing said control words (CW) encrypted using a first key (K) , the method comprising a first step consisting in: transmitting the scrambled data stream (TS(D.EMB.)) to a detachable module (2), characterized in that it furthermore comprises the steps consisting, for the detachable module (2) , in: extracting from the data stream (TS(D.EMB.)) the control words (CW) having served to scramble the digital data; - decrypting said control words; encrypting said control words using a second key (SSK, K2 ) ; and transmitting the encrypted control words (CW) to said terminal; and, for the terminal (1), in: decrypting said control words using the second key (SSK, K2); and descrambling the digital data stream by using said decrypted control words.

16. Method for transferring control words (CW) between a module according to one of Claims 7 to 13 and a digital terminal according to one of Claim 1 to 6, characterized in that it comprises the steps consisting: for the module (2) , in sending (100) a first identification object ("CPTS_identification ( ) ") to the terminal (1) ; for the terminal (1) , in sending (101) , in response to said first object, a second identification object ("CPTS_identification_answer ( ) ") indicating whether the terminal is or is not able to receive the control words from the module (2); and for the module (2), in the event of a positive response with said second object, in sending (102a, 102b, 102i) said control words (CW) enclosed in a third object ("CPTS CWO") .