WO2004032508A1

WO2004032508A1 - Method for the secure transmission of audiovisual files

Info

Publication number: WO2004032508A1
Application number: PCT/FR2003/002916
Authority: WO
Inventors: Daniel Lecomte
Original assignee: Medialive
Priority date: 2002-10-03
Filing date: 2003-10-03
Publication date: 2004-04-15
Also published as: FR2845554B1; EP1547383A1; FR2845554A1; AU2003283497A1

Abstract

The invention relates to a method of distributing audiovisual sequences according to a nominal stream format comprising a series of frames. According to the invention, before transmission, a stream analysis is performed in order to generate (i) a first modified stream having the format of a nominal stream and comprising images modified by the substitution of certain data of the same type but which were calculated in a random manner or in relation to an algorithm and (ii) a second stream of any format comprising the substituted data and the digital information which enables said modified stream to be reconstructed. Subsequently, the two streams thus generated are transmitted separately in real or delayed time from the server to the recipient device and a synthesis of a nominal format stream is calculated according to the aforementioned first and second streams.

Description

METHOD FOR SECURE TRANSMISSION OF AUDIOVISUAL FILES

The present invention relates to controlling the processing, broadcasting, transmission and secure viewing of audiovisual data and television programs, or more generally of any multimedia program or sequence using a nominal stream format of the type MPEG, by authorized users and offers a secure system for the control of processing, broadcasting, delivery, recording, private copying and viewing of interactive audiovisual or multimedia programs and sequences. The general problem is to provide a device capable of securely transmitting a set of films of high visual quality in a digital format of MPEG type (MPEG-1, MPEG-2, MPEG-4, etc.) or another wavelet-based type, direct to the screen of a personal computer, a television screen and / or to be recorded on the hard disk of a box connecting the remote transmission network to the display screen, all preserving the audiovisual quality but avoiding any fraudulent use such as the possibility of making pirated copies of films or audiovisual programs recorded on the hard drive of the set-top box. The invention also allows secure control of the use of copies and of the rights to the works broadcast. In particular, the invention relates to securing communications between the client equipment and the server intended to prevent a person capable of reading the communication between the client and the server from reading or copying the multimedia content. With current solutions, it is possible to transmit films and audiovisual programs in digital form via radio, cable, satellite, etc. distribution networks. or via telecommunication networks such as DSL (Digital Subscriber Line) or BLR (local radio loop) or via DAB (Digital Audio Broadcasting) networks. Furthermore, to avoid pirating of the works thus distributed, the latter are often encrypted by various means well known to those skilled in the art. However, the main drawback of all current solutions (TiVo Inc., WO00165762) is that it is necessary to transmit not only the encrypted data to the users, but also the decryption keys. The transmission of the decryption keys can be done before, at the same time or after the transmission of the audiovisual programs. To increase the security and therefore the protection of audiovisual works against malicious use, the decryption keys as well as the decryption functions of audiovisual decoders can include improved security means such as smart cards or other physical keys which can optionally , be updated remotely.

Thus, current solutions applied to a set-top box with the possibility of local recording of audiovisual programs in digital form on any medium such as hard disk or other type of memory, offer an ill-intentioned user the possibility of making unauthorized copies. authorized programs thus recorded, since at a given moment, this user has with his digital decoder unit, associated or not with smart card systems, all the information, software programs and data allowing the complete decryption of audiovisual programs. Due to the fact that he has all the data, the user the intended party will have the possibility of making illegal copies without anyone noticing this fraudulent copy at the time it is made.

One solution would therefore consist in transmitting all or part of a digital audiovisual program only on demand (video and program services on demand) through a broadband telecommunications network, for example of the optical fiber, ADSL, cable or satellite type. , without authorizing the local recording of audiovisual programs. Here, the disadvantage is quite different and comes from the performance of these networks which do not guarantee continuous flows of a few megabits per second for each user, as required by MPEG flows which require bandwidths of a few hundred kilobits at several megabits per second.

Under these conditions, a solution consists in separating the flow into two parts, none of which would be usable alone. In this perspective, several patents have been filed. Thus, document WO09908428 (Gilles Maton) discloses a multi-application processing method for an active localizable terminal in which at least one connection is made with an identifiable program dedicated to the execution of an application, said program dictating its operating conditions at the terminal for the provision of functions. The terminal dialogues punctually, by the use of a link, with the management center for the realization, if necessary, of the inputs and outputs of the latter's capacities, the management center becoming slave or not of the terminal at the level of the application to the incoming program. This invention also relates to the method for identifying the program and the terminal in operation. This prior art method divides the flow into a part used to identify the user and a part which contains the program itself. In particular, said program is not unusable but only locked by the first part.

On the other hand, document EP0778513 (Matsushita) describes a process making it possible to prevent the illegal use of information by adding control information to it in order to verify the rights of the user. The system makes it possible to constantly know which part of the information is used and by which user and thereby to know whether this user is in an illegal position or not. This process therefore secures the data by adding additional information which distorts the initial information.

The document WO0049483 (Netquartz) also offers us methods and systems for creating a link between the users and an editor of digitized entities. The method comprises at least one of the following steps: the step of subdividing said digital entity into two parts; the step of storing a part in a memory area of a server connected to a computer network; the step of transmitting the other party to at least one user having computer equipment; 1 step of connecting said computer equipment to said computer network; the step of establishing a functional link between said first part and said second part. These methods and systems do not specify whether the part stored on the server can be stored by the user, which would allow the latter to hack said digital entity. Still in this approach, the closest prior art is found in the HyperLOCK Technologies patents, the most relevant of which is document US05937164. This invention uses the solution which consists in separating the flow into two parts, the most small holds information necessary for the use of the largest. However, this patent is not sufficient to respond to the problem identified. Indeed, the deletion of part of the stream distorts the format of the stream, and therefore cannot be recognized as a standard stream, usable with general software applications. This prior art method requires both specific server-side software for the separation of the two parts, and other specific software ensuring not only the reconstruction of the stream, but also the acquisition of the main stream and its exploitation according to a proprietary format to the solution. This proprietary format is not the initial format of the stream before separation into two parts, in this known solution.

This company has also filed three other patents: document US5892825 includes the previous patent but in a narrower context because the streams are still encrypted there; the document US6035329 is based on the same principle, it relates to a method allowing the reading of a disc of CD-ROM or DVD-ROM type conditionally to the identification of the rights by the insertion of a smart card on which the information necessary for reading are stored. This process is not yet sufficient for our problem because it does not guarantee that the modified stream is of the same format as the original stream. Document US6185306 relates to a method of transmitting encrypted data from a website to a requesting computer. This process however allows the user to have at any given time all the tools necessary to copy the data.

WO 01/97520 also discloses methods, methods and devices for controlling the transmission and recording of digital content from MPEG-2 type. However, this patent has no specificity for audiovisual documents of the MPEG-4 type. In addition, the method is totally ineffective for low speed telecommunications networks, because it substitutes all or part of the I images whose weight in bytes is very expensive during the transmission of the second stream.

Finally, the document "Cryptography of telecommunications" by Henri Gilbert and Marc Girault, review "for science", Special Edition July-October 2002, pages 80 to 85, presents a system for securing a smart card with a dynamic certificate: a chip identifies itself with an access manager by providing an identifier and a certificate calculated according to the identifier and a "question" variable sent by the access manager. This system guarantees that if an individual clones a smart card in order to use it fraudulently, the clone will not be able to identify himself to the access manager. This system is however limited by its asymmetry. In order to correct these various defects, the invention relates, in its most general sense, to a process for the distribution of audiovisual sequences according to a nominal stream format consisting of a succession of frames, said stream on which we proceed, before transmission to the customer equipment, to an analysis of the flow to generate a first modified flow, presenting the format of a nominal flow, and presenting images modified by the substitution of certain data by data of the same nature but calculated randomly or compared to an algorithm, and a second stream of any format, comprising the substituted data and the digital information able to allow the reconstruction of said modified stream, then to transmit the two streams separately, in real time or in deferred time thus generated from the server to the recipient equipment, and for which a summary of a stream in nominal format is calculated on the recipient equipment as a function of said first stream and said second stream. In this method, the step of transmitting said second stream is secured according to the following method:

a step of initialization of the communication in which the client identifies himself with the server which responds to the client who in turn verifies that the server is communicating with him;

a step of exchanging information between the client and the server where each message from the client is identified with the server by a client identifier sent by the server and each message from the server is identified with the client by a server identifier issued by the client.

According to a preferred embodiment, said client identifier is a random number generated by the server and transmitted by the server to the client and the server identifier is a random number generated by the client and transmitted by the client to the server.

Preferably, the server generates a new random number for each message sent, said random number then also being an identifier of the client's response and the client generates a new random number for each message sent, said random number then also being an identifier from the server response.

Advantageously, the client equipment comprises a chip card reader and that said synthesis is carried out in part on an electronic chip connected to the client equipment by said reader.

In one embodiment of the invention, said electronic chip prohibits the continuation of said synthesis. Advantageously, the chip uses a random number associated with each message from the client equipment to identify the response of the server to said message and to prohibit said further synthesis. According to a particular embodiment of the invention, all communications between the client equipment and the server are encrypted.

Advantageously, said encryption is an encryption with public and private keys involving the identifier of the client with the server.

Preferably, the client identifies himself to the server using reference data concerning the serial number of the equipment, the identifier of the smart card and the client's network identifier. In a particular embodiment of the invention, the smart card and the synthesis device included in the client equipment communicate by a secure link in the same way as the link between said server and said client: each message from the said synthesis device is identified with the chip card by a synthesis device identifier issued by the chip card and each message from the chip card is identified with the synthesis device by a chip identifier issued by the synthesis device.

Advantageously, part of the process for synthesizing the original flow is carried out on the smart card.

The present invention will be better understood on reading the description of a nonlimiting exemplary embodiment which follows, referring to the appended drawings in which:

• Figure 1 describes the overall architecture of a system for implementing the method according to the invention; • Figure 2 shows a particular embodiment of the MPEG-type flow analysis and synthesis system according to the invention;

A description of the general operation of the communication between the server and the client is described below, with reference to FIGS. 1 and 2.

The invention relates to a nominal format data stream, in particular but not exclusively to an MPEG type stream (MPEG-1, MPEG-2, MPEG-4).

The general principle of a process for securing an audiovisual stream is set out below. The objective is to authorize video-on-demand and pay-per-view services across all these broadcasting networks and local recording in the user's digital set-top box. The solution consists in permanently keeping a part of the recorded audiovisual program outside the user's home, in fact in the broadcasting and transmission network, this part being essential for viewing said audiovisual program on a screen. television or monitor type, but being of a very low volume compared to the total volume of the digital audiovisual program recorded by the user. The missing part will be transmitted via the broadcast transmission network when viewing said pre-recorded digital audiovisual program at the user's.

Most of the audiovisual flow will therefore be transmitted via a conventional broadcasting network, while the missing part will be sent on demand via a narrowband telecommunications network such as conventional telephone networks or cellular networks of GSM, GPRS or UMTS type or by using a small part of a DSL or BLR type network, or by using a subset of the shared bandwidth on a cable network. In FIG. 1, the audiovisual interfacing arrangement (8) is adapted to connect at least one display device, for example a monitor, a video projector or a television screen type device (6), to at least a broadband transmission and broadcasting network interface (4) and at least one telecommunications network interface (10). According to the present invention, this arrangement is composed of a module (8) mainly comprising, on the one hand, a processing unit adapted to process, in particular decode and descramble any audiovisual stream of MPEG type according to a decoding software program and preloaded descrambling, so as to display it, in real time or deferred, to store it, to record it and / or to send it on a telecommunications network and, on the other hand, at least one screen interface (7) and an interface for connection to a local or wide area network (5) and / or (9). The broadband transmission and broadcasting network (4) and the telecommunications network (10) can be combined into a single network.

The hard disk of the module (8) can be used as a buffer memory to temporarily store at least part of the program or of the audiovisual sequence to be displayed, in the event of delayed viewing or of limitation in the bandwidth of the transmission network. Viewing can be delayed or delayed at the request of the user or the portal (12).

As shown in Figure 1, the connection interface (5) is connected to a broadband transmission and broadcasting network (4) such as a modem, a satellite modem, a cable modem, a line interface fiber optic or radio or infrared interface for wireless communication. It is through this classic audiovisual broadcasting link that the content of audiovisual programs such as films will be transmitted. However, so as not to allow pirated copies to be made, before transmitting the audiovisual content from the server (1) or the portal (12) it is planned to keep a small part of the audiovisual content in the portal (12).

When viewing an audiovisual program in real time, this small part of the audiovisual content stored in the portal (12) will also be sent to the module (8) via the telecommunications network (10).

As shown in FIG. 1, the connection interface (9) is connected to an extended telecommunications network (10), directly or by a local network serving as an access network and consists for example of a line interface subscriber (Analog or digital telephone network, DSL, BLR, GSM, GPRS, UMTS, etc.).

Thus, the audiovisual programs are conventionally broadcast in multicast mode (“broadcast”) via the broadband transmission network (4) of the radio type, cable, satellite, digital radio, DSL, etc. from the server (1) directly via the link (3bis) or via the portal (12) via the link (2) and (3) to the decoder module (8) through the link (5). Each audiovisual program thus broadcast can be encrypted or not, and, in accordance with the present invention, the MPEG-type streams include modifications in certain images as described above. Depending on the parameters chosen by the user or the information transmitted by the broadcasting server, certain audiovisual programs thus modified and incomplete are recorded in the hard disk of the box (8).

When the user wishes to view an audiovisual program thus recorded in the sound hard drive box (8) it requests it in a conventional manner by means of a remote control connected to its box (8) which then automatically connects to the portal (12) via the link (9) of the local network or direct access type and to through the telecommunications network (10) itself connected to the portal (12) via the link (11). Throughout the visualization of the audiovisual program, the links (9) and (11) remain established and allow the box (8) to receive the functions and parameters for reconstituting the modified stream or the modified images. The functions and parameters for reconstituting the modified stream or transmitted modified images are never saved in the hard disk of the box (8) because the images of the reconstructed audiovisual stream are directly displayed on the display screen (6) via the link. (7) after having been processed by the Reader of the box (8) from its volatile local memory. Once processed and displayed, the functions and parameters for reconstituting the modified flow or the modified images just transmitted by the portal (12) will be erased from the local volatile memory of the box (8).

According to a particular embodiment, the box (8) comprises a smart card reader which will allow the portal (12) to authenticate the user who owns the box (8).

According to a particular embodiment, for a given MPEG content, the smart card contains said second stream which has been stored by the portal (12).

If authorized, the smart card also allows the user to make private copies of audiovisual programs recorded on the hard disk of his decoder box (8). For this, if the user wants to make a private copy of an audiovisual program, he will do so from conventionally on a video recorder via the link (7) which connects the housing (8) to the display screen (6).

However, if he wishes to keep a private copy in the hard disk of his box, he will indicate it in his box (8) which will record the "private copy" information as well as the contact details of the user on the card. chip, in a particular field (84) of this audiovisual program recorded on the hard disk (85) of the decoder unit (8). Then, each time the user wants to view this private copy, the box (8) will automatically connect to the portal (12) and will indicate to the latter that the user wants to read his private copy; in return, if the reading of the private copy is possible for this user who has this smart card connected to this box (8), the decoder box (8) will then receive modified and / or missing information from the first stream as well as all the other parameters allowing the viewing of the audiovisual program constituting the private copy. The present invention also relates to the physical box (8) used by the consumer to access the data. This physical box is located at the user's home. It provides a set of functionalities which manage the appropriate information to be presented according to the selection of the audience and manages the connection and communication with the remote server.

According to a particular embodiment, the physical box corresponding to the audiovisual interfacing arrangement (8) is produced as a fixed autonomous device with integrated hard disk.

According to a particular embodiment, the physical box corresponding to the audiovisual interfacing arrangement (8) is produced as a stand-alone device portable (mobile) with integrated hard drive and / or disc player (CD, DVD, etc.).

According to a particular embodiment, the autonomous physical box (8) comprises a smart card reader. According to another particular embodiment, the audiovisual interfacing arrangement (8) is produced as an additional card which will be installed in a PC type computer and will be connected to at least one broadband transmission and broadcasting network interface ( 4) and at least one telecommunication network interface (10). This card will use the hard drive of the PC computer for recording the first stream, but will include its own calculator and its own volatile memory so as not to leave the malicious PC user with the means of accessing information. additional functions such as the functions and parameters for reconstituting the modified stream or modified images of the second stream.

According to the present invention, the audiovisual and multimedia servers (1) and / or (12) comprise means for coding, transcoding and scrambling audiovisual data, in particular means for adding cryptographic and security information at the start and throughout the sequences. Finally, it should be noted that the invention degrades the MPEG flow from the visual point of view until it no longer allows the recognition of the transmitted and displayed scenes without having access to additional data and characteristics, but completely reconstructs the MPEG flow in the arrangement. audiovisual interfacing (8) without any loss.

Although the present invention is more particularly focused on audiovisual data, it is understood that any interactive multimedia information and all interactive data can be processed by the present arrangement and the present system, audiovisual data of MPEG type being the most elaborate. The present invention will be better understood thanks to the following description presenting the physical basis of the present invention and with reference to FIG. 2 of the appended drawing representing a preferred embodiment of the latter as a non-limiting example of particularly good embodiment suitable for cable and satellite networks.

Editing the MPEG stream is described here as an example and may take another form. However, for an effective implementation of the invention, the part used to reconstruct the initial flow should be very small compared to the total flow, in order to allow it to be delivered on the fly. On the other hand, the first stream generated by the analysis device (121) is an MPEG stream, so that the user can view it without appreciating the content of the stream due to the degradation induced by said stream. analysis.

The other part of the modified MPEG stream will be stored in the buffer memory (123) of the portal (12). For each MPEG stream thus broadcast, the portal (12) will keep in a buffer memory (123) the modifications which have been made to this MPEG stream by the analyzer (121) of the portal (12). It is specified that, for the same MPEG input stream (101), the processing of the stream can be different for each user (8) and / or for each group of users (8). Thus, the buffer memory (123) of the portal (12) includes a different memory area for each user.

The phase described above corresponds to the first phase of preparation of the MPEG stream by the portal (12), at its transmission via the broadband network (4) and its recording in a decoder (8). This decoder can then display this MPEG stream recorded in its hard disk (85). For this, the synthesis system (87) of the decoder (8) will read the MPEG file from its hard disk (85) and will send it to a conventional MPEG player (81). If no additional data is received by the synthesis system (87), then the MPEG stream which reaches the player (81) is processed and displayed as it is, which causes a significant distortion of the display on the screen of display (6). On the other hand, since the recorded stream is indeed an MPEG type stream, the player (81) makes no difference and displays the information on the output screen (6) which appears well as data from an MPEG video stream but totally inconsistent with the human being who looks at the screen (6). Any copy of the MPEG stream coming from the hard disk (85) of the box (8) will produce the same visual effect when it is restored by any MPEG player; any use of this copy which is ill-intentioned is therefore doomed to failure.

In a particular arrangement, the device (8) comprises a cellular link to a GSM network (10).

In the device described above, the invention relates more particularly to the means of securing the connection between the client equipment and the server. Indeed, a malicious person can connect on the communication channel between the client and the server and with the appropriate algorithm, reconstruct the original flow.

For this, the invention involves two mechanisms: a mechanism for encrypting communication data with parameters known only to the server; a mechanism for decrypting data by the client equipment on a smart card, said decryption being prior to any reconstitution of the original flow.

These two mechanisms make it possible to verify both the identity of the client at each request of the latter for the server and the non-storage of the information sent by the server. For this last point, if the user stores the data sent by the server to use it later, he will not be able to perform the decryption operation in the chip because it uses data relating to the date of the data.

The invention will be better understood on reading the description of an embodiment of the invention.

A first step is to initialize the connection between the client and the server. For this, when the program executed on the client needs additional information to reconstruct the original flow, it automatically calls the server, providing the references of the audiovisual file it wants to display. It also transmits to the server the equipment serial number (“Set Top Box” or card integrated into a computer) and an identifier of the smart card. A “Set Top Box” is an equipment which is connected to a display device such as a television for example and which makes it possible to display audiovisual content. The client also provides the server with its address on the network, this address can be the number of its physical telecommunications line (ADSL, BLR, Cable, etc.).

To start the process, the customer's smart card sends a random number Ni whose start of calculation depends on a function of pressing a key key on its remote control (the "read" key preferably) and the hour. Thus, it is certain that the number Ni is completely random, since the time of the press is unpredictable.

The equipment serial number, the smart card identifier, the Ni number and the client's network identifier constitute the client's reference data with the server. This uses the customer's reference data to determine whether the customer is authorized to read the original feed. For this, in a particular embodiment of the invention, the server is connected to a database comprising the reference data of all the clients and the audiovisual content that each client is entitled to obtain.

If the client is authorized to download the missing part of the stream, the server prepares a response by: - choosing a random number Ns which is a function of Ni and the arrival time of the client's message; sending a message containing the number Ns encrypted with the number Ni, the customer reference data and a customer public key;

The server responds on the physical line that corresponds to the number it has in memory, but it does not respond on the telecommunication line that was used by the client to contact the server. This classic method helps prevent certain hacker attacks.

The random number Ns generated by the server is then in turn used by the client to respond to the server, and allows the server to control the origin of each request received. The client receives the response and decrypts it with his private key like PGP ("Pretty Good Privacy", public key encryption algorithm), his reference data and the Ni number. This decryption is carried out in part in the smart card (part of the program is and is executed in the smart card).

If the decryption is valid, the client generates a new random number Ne calculated according to the arrival time of the server message and responds to the server by encrypting its response with the server's public key, the client's reference data, the random number Ns it has just received from the server; the client also indicates in his message the references of said 1 ^st flow and the time position that he is decrypting.

Once this initialization step has been completed, the client and the server interact to allow the audio-visual content to be decrypted as it is read. This secure dialog loop is described below.

The server receives the encrypted data sent by the client and decrypts it with its private key and the client's reference data, the last random number Ns it issued and the client's time and date.

The decryption allows the server to identify the request as coming from the expected client (thanks to the reference data) and in response to the last message from the server (thanks to the random number Ns). In addition, the server decrypts the last random number Ne generated by the client. If the verification is positive, the server sends the client the data of said second stream expected by the client. In addition, it generates a new random number Ns calculated according to the arrival time of the last message from the client.

The server then constructs a message where the data of the second stream and the new number Ns are encrypted with the client's public key, the data of client reference and the last random number Ne generated by the client and received by the server.

The server replies on the physical line which corresponds to the number in its memory, but it does not respond on the telecommunication line which was used by the client to contact the server

The client receives the response from the server, and decrypts it using its private key, the last random number sent to the server, the client's reference data.

The client reconstructs the original stream with said first stream previously loaded and the part of the second stream which it has just decrypted.

For this, at least part of the program for combining said first and second streams is executed in the smart card so that it is not possible for a malicious user (a pirate) to reproduce all or part of the stream of 'origin. This part can be the verification of the random number Ne or the execution of certain instructions which make it possible to recombine said streams to reconstruct coherent audio-visual signals.

The client can stop the decryption procedure by the client (the smart card detects inconsistencies) or by the server, which stops sending data to the client. This termination of the procedure can be done by the non-execution of the programs in the smart card. The decision to stop the procedure can be decided by the smart card, either because the dialogue between the client and the server presents inconsistencies or does not exist (because of the disconnection of the telecommunications line between the client and the server ), or because the dialogue between the server and the client presents inconsistencies at the level of exchanges (for example at the level of the random numbers Ne and Ns). In the event that the procedure is stopped by the smart card, the execution of the programs in the client's processor will be disturbed since part of the processing (co-processing) that was to be executed in the smart card will not be more .

The client then generates a new message to request the continuation of said second flow, encrypting this message with the server's public key, the client's reference data, the random number Ns that it has just received from the server. It includes in the message a new random number Ne generated according to the arrival time of the last message from the server and it again indicates in its message the references of said first stream and the time position that it is decrypting . If the client stops playing multimedia content (by pressing the "Pause" key for example), the process is again initialized and resumes in its initialization phase when the user presses the key key mentioned above again ( "Reading" for example). Optionally, the step of identifying the client with the server is not carried out and the cycle begins with a request for audiovisual content accompanied by a random number generated as a function of the instant of pressing the key key. According to an embodiment of the invention, illustrated in FIG. 2, the multimedia content is broadcast as follows.

During a preparatory phase, the MPEG stream (101) is analyzed by the analysis device (121) in order to generate two streams, the first stream being of the same nature as an MPEG stream and broadcast by the output (122) and the second flow consisting of information allowing the reconstruction of the original flow. The first flow is transmitted to the client by the communication channel (4) which can be a broadband communication network or a physical medium (CD) for example. The first stream is stored on a medium (85) connected to the client equipment (a hard disk or a CD for example). The second stream is stored in a buffer

(123) on the server. It is transmitted at the request of the client through the communication channel (10). The client receives the second stream through the input buffer (86). During the reception of the second stream, the synthesis device (87) receives the first stream through the read buffer (83) and uses the two streams to reconstruct the initial stream. The synthesis device (87) is controlled by the chip located on the smart card (82). For this, the data arriving in the input buffer (86) are transmitted to the smart card (82) by the link (88). The communication of the second stream according to the invention comprises an initialization phase and a dialogue phase. During the initialization phase, the client equipment (8) transmits to the portal server (12) its reference data, the identifier of the sequence requested, the time position in said sequence and a random number Ni calculated by the card. chip (82) as a function of the instant of pressing the “Play” button on the remote control of his equipment by the user; the server (12) compares the client's reference data with data recorded in a database (124) connected to the server (12); it also records the random number Ni; if the client has the right to view the requested sequence, the server (12) generates a first random number Nsl according to the time of arrival of the message of the client and encrypts this number with the client's public key, the client's reference data and the number Ni; the client (8) receives the message from the server (12) and has it processed by the smart card (82), the latter decrypts the random number Nsl with the client's private key, its reference data and the number Ni; if the decryption is successful, this means that the client-server dialogue works perfectly since the number Ni transmitted by the server (12) is the same as the last number Ni generated by the smart card (82); the client therefore takes into account the number Nsl sent by the server; the customer's smart card (82) (8) generates a new random number Ncl calculated from the time of arrival of the server message and encrypts this number with the number Nsl that it has decrypted, the public key of server and client reference data. The message thus formed is sent to the server (12). - The server (12) decrypts the client's message and checks that the Nsl sent by the client corresponds to the Nsl it had generated. If this is not the case, the server interrupts the transmission of audiovisual content. If this is the case, the process enters a loop described below which can be interrupted by the client and / or the server.

The dialogue loop between the client and the server is described below: the server (12) generates a message comprising the part of the second stream expected by the client, the number Nci that it has decrypted and a new random number Nsi + 1 calculated based on the date of arrival of the client's last message. This message is encrypted with the client's public key, the client's reference data and the random number Nci that the server has decrypted in the last message from the client;

The client receives the message, the smart card (82) decrypts the message and checks the value of Nci in the same way as for the number Ni described above. If the verification is positive, the chip (82) authorizes the synthesis device (87) to process the data of the second stream in order to reconstruct the original stream. The chip (82) also decrypts the number Nsi + 1 and stores it for the transmission of the following message.

The chip (82) generates a new random number Nci + 1 and the client (8) encrypts a message containing the number Nci + 1, the data of the second flow necessary for the synthesis device (87), encrypted with the public key of the server , the customer reference data and the number Nsi + 1. This message is sent to the server (12).

The server (12) receives the client's message and decrypts it with its private key, the client's reference data and the number Nsi + 1. If the decryption is successful, the number Nsi + 1 is then correct and the process is authorized to continue.

In a particular embodiment of the present invention, the method further comprises a step of securing communications between the chip card integrated in the client equipment and the reading module (“Reader”) executed by a processor (86) connected to the smart card by an internal link (88). For this, part of the program for redialing the initial flow from said two flows is carried out on the smart card. For this, the "Reader" (87) and the smart card are connected by the link (89). The calculations are carried out by the chip as long as the smart card communicates with the remote server (12). Thus, if a verification of one of the random numbers Nci generated and verified by the chip fails, the chip interrupts the communication with the server (12). The interruption of this communication results in the interruption of the processing of re-composition of the flows by the chip, and the re-composition of said two flows is thus not carried out since the co-processing which the smart card (82) must execute is not done and that the data expected by the module (86) and the reader (87) via the links (89) and (88) will not be transmitted by the chip (82).

In addition, the module (86) and the chip (82) on the one hand and the “Reader” (87) and the chip (82) on the other hand communicate on a secure link (89) and (88) of the same way as the connection between the server (12) and the client (8):

With each message, the smart card generates a random number Ncp calculated according to the arrival time of the last message from the "Reader" (87) and sends a message containing information on the processing of flows and the last number random Ncp, these two pieces of information being encrypted with the public key of the "Reader", so-called reference data, known to the chip and to the "Reader" and the last random number Ni received from the "Reader". On the next message from the "Reader", the chip decrypts the message with its private key, the reference data and the last Ncp number it issued. As long as the decryption is successful, the chip maintains communication with the "Reader". If the decryption fails, the chip interrupts the communication. In a particular embodiment, if the decryption fails, the chip requests the message from the "Reader" one or more times before interrupting the communication. With each message, the "Reader" generates a random number Ni calculated according to the arrival time of the last message coming from the smart card and sends a message containing information on the processing of flows and the last random number Ni , these two pieces of information being encrypted with the public key of the smart card, so-called reference data, known to the chip and to the “Reader” and the last random number Ncp received from the smart card. At the next message from the smart card, the "Reader" decrypts the message with its private key, the reference data and the last Ni number it sent. As long as the decryption is successful, the "Reader" maintains communication with the chip. If the decryption fails, the "Reader" interrupts the communication. In a particular embodiment, if the decryption fails, the “Reader” requests the message again from the chip one or more times before interrupting the communication.

Claims

1. Method for the distribution of audiovisual sequences according to a nominal flow format consisting of a succession of frames, said flow on which a flow analysis is carried out before transmission to the client equipment to generate a first modified flow, presenting the format of a nominal stream, and presenting images modified by the substitution of certain data by data of the same kind but calculated randomly or with respect to an algorithm, and a second stream of any format, comprising the substituted data and digital information able to allow the reconstruction of said modified stream, then to transmit separately, in real time or in deferred time the two streams thus generated from the server to the recipient equipment, and for which one calculates on the equipment recipient a summary of a stream in nominal format according to said first stream and said second characterized stream in that the step of transmitting said second stream is secure according to the following method:

2. Method for the distribution of audiovisual sequences according to claim 1, characterized in that that said client identifier is a random number generated by the server and transmitted by the server to the client and the server identifier is a random number generated by the client and transmitted by the client to the server.

3. Method for the distribution of audiovisual sequences according to claim 2, characterized in that the server generates a new random number for each message sent, said random number then also being an identifier of the response of the client and that the client generates a new random number for each message sent, said random number then also being an identifier of the server response.

4. Method for the distribution of audiovisual sequences according to one of the preceding claims, characterized in that the client equipment comprises a chip card reader and that said synthesis is carried out in part on an electronic chip connected to the client equipment by said reader.

5. Method for the distribution of audiovisual sequences according to claim 4, characterized in that said electronic chip prohibits the continuation of said synthesis.

6. Method for the distribution of audiovisual sequences according to one of claims 4 or 5, characterized in that the chip uses a random number associated with each message from the client equipment to identify the response of the server to said message and to prohibit said tracking of synthesis.

7. Method for the distribution of audiovisual sequences according to any one of the preceding claims, characterized in that all the communications between the client equipment and the server are encrypted.

8. Method for the distribution of audiovisual sequences according to claim 7, characterized in that said encryption is an encryption with public and private keys involving the identifier of the client with the server.

9. Method for the distribution of audiovisual sequences according to any one of the preceding claims, characterized in that the client identifies himself with the server by means of reference data concerning the serial number of the equipment, the identifier of the customer's smart card and network identifier.

10. Method for the distribution of audiovisual sequences according to any one of the preceding claims, characterized in that the smart card and the synthesis device included in the client equipment communicate by a secure link in the same way as the link between said server and said client: each message from said synthesis device is identified with the smart card by a synthesis device identifier issued by the smart card and each message from the smart card is identified with of the synthesis device by a chip identifier issued by the synthesis device.

11. Method for the distribution of audiovisual sequences according to any one of the preceding claims, characterized in that part of the process for synthesizing the original stream is carried out on the smart card.