EP2491714A1

EP2491714A1 - Methods for transmitting and receiving streams of images, recording medium, transmitting device and transformation module for said methods

Info

Publication number: EP2491714A1
Application number: EP10766292A
Authority: EP
Inventors: Emmanuel Pira
Original assignee: Viaccess SAS
Current assignee: Viaccess SAS
Priority date: 2009-10-22
Filing date: 2010-10-20
Publication date: 2012-08-29
Also published as: RU2012120849A; FR2951891A1; TWI415464B; CN102687519B; WO2011048142A1; CN102687519A; FR2951891B1; TW201130307A; RU2513907C2

Abstract

The invention relates to a method for transmitting a stream of images in unscrambled mode, which comprises the transformation (108) of the images from the stream of images in unscrambled mode by means of a secret reversal transform in order to obtain a stream of transformed images which is encoded and transmitted in place of the stream of images in unscrambled mode, said secret reversal transform transforming each image of a series of images in unscrambled mode: by identically swapping the position of pixels of each image of the series of images in unscrambled mode, and/or by identically modifying pixel colors of each image of the succession of images in unscrambled mode, such that the transformation performed on each image in unscrambled mode is immediately perceptible to a human being when the transformed image is displayed on a screen without the reversal transform having been applied in advance.

Description

METHODS OF TRANSMITTING AND RECEIVING IMAGE FLOWS, RECORDING MEDIUM, TRANSMITTING DEVICE,

The invention relates to methods for transmitting and receiving image streams. The invention also relates to an information recording medium, a transmission device and an inverse transformation module for implementing these methods.

[002] Known methods for transmitting a clear image stream include: encoding of the image stream so as to obtain a compressed image stream, and

- Emitting the compressed image stream to at least one receiving device.

[003] Known methods for receiving the image stream emitted according to the method above comprise:

the reception of the image stream, and

the decoding of the received image stream so as to obtain an uncompressed stream of images.

[004] The stream of images in clear is a flow of images directly perceptible and understandable by a human when they are displayed on a screen. For example, it is a sequence of a multimedia program such as a movie or an audiovisual program. The stream of images in clear is not protected. It can therefore be viewed by any reception device equipped with a display and the appropriate decoder. In particular, the stream of clear images is not scrambled to subject its visualization to certain conditions.

Typically, these known methods also include, to protect the stream of images in the clear, the scrambling of the compressed image stream so as to condition the visualization in clear of this stream of images under certain conditions such as the purchase of access title.

Correspondingly, the known reception methods typically include descrambling the stream of images before decoding. The image stream is thus protected during its transfer from a transmitting device to the receiving device.

[007] Scrambling is systematically performed after encoding. Indeed, with the current scrambling methods, inverting these two operations leads to very significantly degrade the compression ratio of the encoder.

[008] The compression ratio is the ratio between the amount of information before compression and the amount of compressed information obtained after compression.

In these known transmission and reception methods, the image flow is no longer protected from the output of the descrambler having descrambled the image stream. Thus, an illegal use is to recover the flow of images descrambled or uncompressed to broadcast to users who have not purchased corresponding access titles.

To remedy this problem, it has already been proposed to integrate the descrambler and the decoder in the same electronic component, known by the acronym SOC (System On Chip), to make the recovery of the flow of images more difficult. However, such a component generally uses an unprotected external Random Access Memory (RAM) to perform the descrambling and decoding operations. It is therefore still possible to recover the descrambled image stream from the contents of this RAM.

[001 1] When the receiving device is a personal computer (PC), it is also possible to recover the descrambled image stream by spying calls to the driver of the graphics card of this computer.

To remedy this drawback, it has also been proposed to encrypt, at the output of the decoder, the decompressed image stream. The stream of images thus encrypted is decrypted just before being displayed on a screen. This method thus makes it possible to protect the link between the reception device and a display. This is for example HDCP (High Band Digital Content Protection) technology. However, in this method, the descrambled image stream can be retrieved before it enters the decoder, or at its output, but before its encryption. The encryption applied at the output of the decoder is then ineffective to prevent this form of illegal recovery of the image flow.

Another difficulty is when encoders, scramblers, descramblers and decoders are proprietary devices. Indeed, in case of a security breach, only the owner of these devices can intervene to remedy it. A third party can not propose a solution that modifies the operation of these devices.

The invention aims to remedy at least one of these disadvantages. It relates to a method for transmitting a clear image stream in which, before encoding, the method comprises transforming, using a secret invertible transform, images of the image stream. in clear to obtain a stream of transformed images which is encoded and emitted in place of the stream of images in clear, this secret invertible transform transforming each image of a succession of images in clear:

by permuting, identically, the position of pixels of each image of the succession of images in the clear, and / or

by modifying, identically, pixel colors of each image of the sequence of images in the clear,

so that the transformation made to each plaintext image is directly perceptible by a human being when the transformed image is displayed on a screen without first having applied the inverse transform. By "invertible" transform is meant that there is an inverse transform to find the stream of images in clear from the scrambled image stream. This inverse transform is the reciprocal of the invertible transform.

Secret means the fact that the transform is not intended to be disclosed to the public.

The above method serves to deter recovery of the compressed image stream at the output of the descrambler or the decompressed image stream at the output of the decoder. Indeed, the image flow recoverable at these two outputs is the stream of transformed images. However, this stream of transformed images is visibly different from the stream of images in the clear. Thus, its visualization, without first applying the inverse transform, remains unsatisfactory for the user. The characteristics of the transform used, and therefore those of the inverse transform, are secret. The transmitted image stream is therefore protected even at the output of the decoder.

Furthermore, to implement this method, it is not necessary to intervene on existing encoders, scramblers, decoders and descramblers.

In addition, since only permutations of the position of the pixels and / or changes in the colors of the pixels are used to transform the image in clear, this limits the degradation of the compression ratio of the existing encoder.

Embodiments of this transmission method may include one or more of the following features:

The secret invertible transform is such that the compression ratio of the transformed image stream is identical, within ± 55%, to the compression ratio that would be obtained if the images were not transformed;

The secret invertible transform is an elementary transform or a combination of elementary transforms selected from the group consisting of:

the construction of a symmetrical image in clear relative to an axis parallel to an edge of the image,

the permutation of blocks of pixels of the image in the clear, and

the addition of an offset to the chrominance values of at least one block of pixels of the clear image;

The transform is capable of transforming the stream of images into a stream of transformed images containing as much information about each image as the stream of images in the clear;

The method comprises modifying, at predetermined time intervals or in response to a command, the secret invertible transform;

The method comprises transmitting a message containing a cryptogram enabling the receiving device to find an inverse transform usable for reconstructing the stream of clear images from the stream of transformed images;

The method comprises inserting the message in the stream of transformed images before encoding it so that this message is transmitted at the same time as the image stream;

The method comprises scrambling the compressed image stream before it is transmitted.

These embodiments of the transmission method also have the following advantages:

- Allow to choose a transformation that degrades the performance of the encoder, and thus retain the bandwidth required to transmit the transformed image stream close to that required to transmit the stream of images in clear;

the construction of a symmetrical image or the permutation of pixel blocks makes it possible to retain the bandwidth required to transmit the stream of transformed images, close to that required to transmit the stream of images in the clear;

using a transformation that does not decrease the amount of information contained in each clear image makes it possible to reconstruct from the stream of transformed images clear images that have exactly the same quality as the transmitted clear images;

regularly modifying the secret transform used makes it possible at the same time to accentuate the unsatisfactory nature for the user of the visualization of the transformed image stream, and to increase the security by limiting the time available for illegally determining the inverse of the transform used;

the transmission of the cryptogram in a message contained in the stream of transmitted transformed images makes it possible to avoid the use of an additional communication channel.

The invention also relates to a method for displaying a stream of images transmitted using the above method in which, after the decoding of the image stream, the method comprises the transformation, using the inverse of the secret transform used during transmission, images of the stream of transformed images to obtain the stream of images in clear then the display of the stream of images thus obtained.

The invention also relates to an information recording medium comprising instructions for the implementation of any of the above methods when these instructions are executed by an electronic computer.

The invention also relates to a device for transmitting a stream of images in clear, this device comprising:

an encoder of the image stream so as to obtain a compressed image stream, and an emitter of the compressed image stream to at least one receiving device, a transformation module capable, by means of a secret invertible transform, of transforming the images of the stream of images into clear to obtain a stream of transformed images which is encoded and transmitted instead of the stream of images in clear, this invertible secret transform transforming each image of a succession of images in clear:

so that the transformation made to each plaintext image is directly perceptible by a human being when the transformed image is displayed on a screen without first having applied the inverse transform.

Finally, another object of the invention is an inverse transformation module for implementing the above reception method, capable of transforming, using the inverse of the secret transform used at the time of the invention. transmission, the images of the transformed image stream to obtain the stream of images in the clear.

The invention will be better understood on reading the description which follows, given solely by way of nonlimiting example and with reference to the drawings in which:

FIG. 1 is a schematic illustration of a system for transmitting and receiving an image stream,

FIG. 2 is a schematic illustration of a clear image,

FIGS. 3 and 4 are schematic illustrations of a transformed image obtained from the clear image of FIG. 2,

FIG. 5 is a schematic illustration of the structure of a message transmitted with the images of the stream,

FIG. 6 is a flowchart of a method for transmitting and receiving an image stream using the system of FIG. 1,

FIGS. 7, 8 and 9 are, respectively, schematic illustrations of alternative embodiments of the system of FIG. 1.

In these figures, the same references are used to designate the same elements.

In the following description, the features and functions well known to those skilled in the art are not described in detail. In addition, the terminology used is that of conditional access systems to multimedia content. For more information on this terminology, the reader can refer to the following document:

- "Functional Model of Conditional Access System", EBU Review, Technical European Broadcasting Union, Brussels, BE, No. 266, December 21, 1995. FIG. 1 represents a system 2 for transmitting and receiving an image stream. The stream of images corresponds, for example, to a sequence of a multimedia program such as a television program, a film or an audiovisual program.

The stream of clear images is generated by a source 4 and transmitted to a device 6 for transmitting this image stream to a multitude of reception devices through a network 8 for transmitting information.

The network 8 is typically a long-distance network for transmitting information such as the Internet or a satellite network or a wireless network such as that used for the transmission of digital terrestrial television (DTT).

To simplify Figure 1, only two devices 10 and 12 of reception are shown.

The device 6 comprises an encoder 16 adapted to compress the stream of images it receives on an input 18 and to output on an output 19 a compressed image stream. The encoder 16 is a digital encoder capable of processing digital image streams. For example, this encoder operates in accordance with MPEG2 (Moving Picture Expert Group - 2) or ITU-T H264.

The compressed image stream is directed to an input 20 of a scrambler 22. The scrambler 22 scrambles the compressed image stream to condition its display under certain conditions such as the purchase of a title of access by users of receiving devices. The scrambled image stream is output on an output 24 connected to the input of a multiplexer 26.

The scrambler 22 scrambles the compressed image stream using a CW control word supplied to it, as well as a conditional access system 28 more known by the acronym CAS (Conditional Access System), by a 32 key generator. Typically, this scrambling complies with a standard such as DVB-CSA (Digital Common Broadcasting - Common Scrambling Algorithm), ISMA Cryp (Internet Streaming Media Alliance Cryp), IPsec (Internet Protocol Security Keying Information Resource Record Working Group), SRTP (Secure Real-Time Transport Protocol), etc. The system 28 generates ECM (Entitlement Control Message) messages containing a cryptogram CW * of the control word CW generated by the generator 32 and used by the scrambler 22. These messages and the scrambled image stream are multiplexed by the multiplexer 26. , to which they are respectively provided by the conditional access system 28 and the scrambler 22, before being transmitted on the network 8.

The device 6 further comprises a module 30 for transforming images in the clear. The module 30 transforms the stream of clear images received from the source 4 into a stream of transformed images. It is connected between the output of the source 4 and the input 18 of the encoder 16. Thus, the images encoded by the encoder 16 are the images transformed by the module 30 and not the images in the clear. This module 30 aims to protect the image flow after it has been descrambled and decoded in the receiving devices 10 and 12. For this, the transformation of the image must be directly perceptible by a user who would not apply the inverse transformation before displaying it on a screen. Preferably, the transformed images are made as different as possible from the clear images from which they are obtained. For this purpose, a secret invertible transform T is applied by the module 30 to each of the images of the received plaintext stream.

Preferably, this transform T is chosen so as not to substantially modify the compression ratio of the encoder 16. It is considered that the compression ratio of the encoder is not substantially modified if it is equal to the compression ratio that would be obtained by directly encoding the stream of images in the clear, within ± 55%, and preferably within ± 15%. To achieve this objective, the transform T is chosen according to the characteristics of the encoder.

In addition, the transform T is chosen so as not to degrade the quality of the image. For this purpose, the transformed images contain as much information as the images in the clear.

The transform T is here produced by combining several invertible elementary transforms Te ,. More precisely, only two families of elementary transforms Te, are used:

the elementary transforms which permute the position of the pixels of the image in the clear, and

- elementary transforms that change the color of the pixels in the image.

The elementary transforms have in common not to modify the correlations that exist between successive images of the stream of images in clear. Because of this, the compression ratio is not substantially changed. Indeed, with reference to the MPEG2 or H264 encoding method, this limits the generation of additional images I (or "I frame" in English) by the encoder.

By way of illustration, we define below six elementary transforms Tei to Te ₆ . The elementary transforms Tei to Te ₃ permute two or two of the pixels of the clear image, Te ₄ performs a circular permutation of at least three blocks of pixels of the clear image, and Te ₅ and Te ₆ modify the color pixels in the image. In this non-limiting example, Te ₅ and Te ₆ are described in the context of the YUV color coding standard. However, elementary transforms that change the color of the pixels of the image can of course be described in an equivalent manner in the context of any other color coding standard, such as, for example, RGB (for Red, Green, Blue). known to those skilled in the art without departing from the scope of the invention.

For example, the Tei transform inverts the left and right pixels. The clear image 34 before the application of this elementary transform is shown in FIG. 2. The corresponding transformed image 36 is shown in FIG. 3. The image 36 is the symmetrical image 34 with respect to an axis 38 parallel to the vertical edge of the image 34.

The Te ₂ transform inverts the top and bottom of the clear image. The transformed image 40 obtained by applying this elementary transform to the image 34 is shown in FIG. 4. The image 40 is the symmetrical image 34 with respect to an axis 42 parallel to the horizontal edge of the image 34 .

The Te ₃ transform permutes two by two blocks of pixels in the clear image. Preferably, the blocks exchanged by the Te ₃ transform are the same as those used in the encoding method. For example, they are blocks of 16 pixels by 16 pixels identical to those used in standards MPEG2 and H264. Thus, this permutation of pixel blocks limits the degradation of the compression ratio of the encoder 16.

The elementary transform Te ₄ performs a circular permutation of at least three blocks of pixels of the clear image. As for the elementary transform Te ₃ , the permuted blocks are the same as those used in the encoding algorithm.

Preferably, for transforms Te ₃ and Te _4, the identity of the permuted blocks is a parameterizable value.

The transform Te ₅ consists in adding a digital offset to the values of the components U and V of the chrominance of the image, called chrominance values. The value of this offset is configurable. In encoding processes according to H264 and MPEG2 standards, the main information used to compress the image flow is the luminance denoted Y. On the other hand, the U and V components of the chrominance are not taken into account by the encoding algorithm. It is therefore possible to modify the value of the components U and V without disturbing the encoder 16 and therefore without modifying the compression ratio.

The elementary transform Te ₆ consists of inverting the values of the components U and V for each of the pixels of the clear image.

These elementary transforms are recorded in a memory 50 connected to the device 6.

The transform T is obtained by combining one or more of the elementary transforms Te, previous. The combination consists in successively applying to the same image in clear several elementary transforms Te, in a predetermined order. Transform T is therefore a composition of several of the elementary transforms Te ,.

To construct the inverse transform T ^"1 , it is necessary to know the combination of elementary transforms used and also, when these elementary transforms used are parameterizable, the values used of the parameters.This information is kept secret so as to prevent the construction of the inverse transform T ^"1 by unauthorized persons. Here, the module 30 also generates a message containing a secret information cryptogram for constructing the inverse transform T ¹ . By cryptogram, it is meant that this message does not in itself contain enough information to allow the construction of the transform T ¹ . The information contained in this message must indeed be combined with information to be obtained in the receiving device, whether they have been previously recorded, or that they must be measured or calculated.

Thus, the cryptogram may be an identifier of a particular combination of elementary transforms Te ,. The receiving device must then comprise means, such as a correspondence table, for associating with each identifier that can be received an inverse transform T ^"1 to be used to perform the inverse transformation of the image flow.

The cryptogram may also contain an identifier of the parameters. In this case, the receiving device must contain means, such as a correspondence table, to associate this identifier and the parameters to be used.

The cryptogram can also be obtained by encrypting the secret information, for example the executable code, of the inverse transform T ^"1 by means of a symmetric or asymmetric encryption, The reception device then constructs the transform T. ^{" 1} by decrypting the cryptogram received using a prerecorded secret key. Similarly, the identifiers of the parameters of the elementary transforms can be encrypted.

By way of example, FIG. 5 shows the structure of a message 54 generated by the module 30 and containing the secret information enabling the receiving device to construct the inverse transform T ^"1. The message 54 comprises a field 56 containing a cryptogram T * secret information to build the inverse transform T ^"1 . The structure of this message is the same as or similar to that of an ECM message. It will not be described in detail.

The message 54 also comprises a field 58 containing CA access conditions intended to be compared with pre-recorded access titles in the reception device so as to allow and, alternately, to inhibit the construction of the transform. inverse T ^"1 .

The message 54 also includes:

a field 60 containing information making it possible to authenticate the message 54 such as a signature S, and

a field 62 containing information making it possible to check the integrity of the message 54 such as a CRC (Cyclic Redundancy Check) code.

Finally, the module 30 is also able to insert the message 54 in the stream of transformed images before encoding so that it is transmitted to the receiving device at the same time as the stream of transformed images. . The insertion of the message 54 into the stream of transformed images is performed so as not to significantly change the compression ratio of the encoder 16. For this purpose, different methods are possible. For example, the message 54 is transmitted as teletext. This therefore amounts to adding at least one line to the transformed image on which the message 54 is coded using a color code.

The message 54 can also be transmitted in the stream of transformed images by implementing a digital image tattooing method more known by the English term "Watermarking". However, this method is preferably simplified to take into account that it is not necessary to ensure the robustness of the message inserted into the image vis-à-vis subsequent transformations. Indeed, if a subsequent transformation of the image had to modify the message 54, then it would lead to the impossibility of constructing the inverse transform T ^"1 , which does not constitute a security flaw.

The module 30 is for example made from a programmable electronic calculator capable of executing instructions recorded on an information recording medium. For this purpose, the module 30 is connected to the memory 50 and the memory 50 contains the instructions necessary for carrying out the method of FIG. 6.

The receiving device 10 comprises a receiver 70 of the transmitted image stream. This receiver 70 is connected to the input of a demultiplexer 72 which transmits on one side the stream of images to a descrambler 74 and another the messages ECM and EMM (Entitlement Management Message) to a security processor 76 . The processor 76 is generally a hardware component containing confidential information such as cryptographic keys or access titles that only legitimate users can use. To maintain the confidentiality of this information, it is designed to be as robust as possible to attack attempts by hackers. It is therefore more robust with respect to these attacks than the other components of the device 10. For example, a security processor is a smart card equipped with an electronic processor. The security processor can also be a software module executed by an electronic computer.

For example, the processor 76 includes a memory 78 containing different cryptographic keys and access titles for descrambling the scrambled images.

The descrambler 74 descrambles the scrambled image stream from the control word CW in clear transmitted by the processor 76. The descrambled image stream is transmitted to a decoder 80 which decodes it. The decompressed or decoded image stream is transmitted to a graphics card 82 which controls the display of this stream of images on a display 84 equipped with a screen 86. Here, the graphics card 82 comprises a module 88 of inverse transformation. . This module 88 is suitable for constructing the inverse transform T ^"1 and applying this inverse transform to obtain the stream of clear images For this purpose, the card 82 comprises an electronic calculator 90 connected to a memory 92 comprising the instructions necessary for executing the method of Figure 6.

The display 84 is able to clearly display the stream of images received on the screen 86.

For example, the device 12 is identical to the device 10 and will not be described in detail.

The operation of the system 2 will now be described in more detail with regard to the method of FIG. 6. This method essentially consists of a phase 100 for transmitting the image stream and a reception phase 102. of the transmitted image stream.

At the beginning of phase 100, during a step 104, a transform T is constructed. Here, the triggering of the construction of a new transform T is caused at predetermined intervals. The predetermined intervals are for example regular intervals less than 20 seconds and preferably less than 10 seconds.

Then, during a step 106, the message 54 corresponding to the transform T is constructed. In parallel, during a step 108, the stream of clear images is transformed by applying the transform T.

Then, during a step 1 10, the message 54 is inserted into the stream of transformed images, then the resulting stream transmitted to the encoder 16.

In a step 1 12, the encoder 16 encodes the stream of transformed images to obtain a compressed image stream.

In a step 1 14, the compressed image stream is scrambled by the scrambler 22 using the control word CW generated by the generator 32.

In a step 1 16, the scrambled image stream and corresponding ECM messages generated by the system 28, are multiplexed. Finally, this multiplexed information is transmitted during a step 1 18 to the different reception devices.

Phase 102 begins with a step 120 of receiving and demultiplexing the multiplexed information transmitted. Then, the image stream is transmitted to the descrambler 74 while the ECM and EMM messages are transmitted to the processor 76.

In a step 122, the processor 76 decrypts the cryptogram CW * of the control word and sends the control word CW to the descrambler 74.

In a step 124, the descrambler 74 descrambles the scrambled image stream with the received control word.

In a step 126, the descrambled image stream is transmitted to the decoder 80 which decodes it. The decompressed image stream obtained is then transmitted to the inverse transformation module 88. In a step 128, the module 88 extracts from the image stream received the message 54.

Then, in a step 130, it compares the CA access conditions to TdA prerecorded access titles. If the received AC access conditions do not correspond to the access tickets TdA then, during a step 132, the construction of the inverse transform ¹ is inhibited. In the opposite case, during a step 134, the cryptogram T * is used to construct the inverse transform T ^"1 .

In a step 136, the inverse transform T ^"1 is applied to the stream of transformed images to obtain the clear image stream which is displayed, during a step 138, on the screen 86.

FIG. 7 represents a system 150 for transmitting and receiving an image stream. This system 150 is identical to system 2 except that:

the module 88 is implemented in a housing 152 that is mechanically independent of the device 10 and the display 84, and

- The graphics card 82 is replaced by a graphics card 154 without module 88.

This housing 152 is interposed between the graphics card 154 and the display 84.

This embodiment makes it possible to secure an image flow transmission system that would have been a victim of cryptanalysis without having to modify the reception device 10. Indeed, it suffices to add the box 152 to give additional protection to the stream of transmitted images

FIG. 8 represents a system 160 identical to system 2 except that:

the display 84 is replaced by a display 162 containing the module 88, and

- The graphics card 82 is replaced by a graphics card 164 without the module 88.

For example, in this embodiment, the device 10 is a central unit of a personal computer. The descrambler 74 and the encoder 80 are implemented on a removable USB stick (Universal Serial Bus) connected to this central unit. This embodiment makes it possible to secure the transmission of the image flow inside the personal computer between the USB key and the display 162.

9 shows a system 170 identical to the system 2 except that the device 10 is replaced by a 172 receiving device. The device 172 is identical to the device 10 except that it additionally comprises a module 174 for encrypting the decompressed image stream generated at the output of the decoder 80. The device 172 is connected via a local area network 178 to different user terminals. To simplify the figure, only two terminals 180 and 182 have been shown.

The terminal 180 includes a deciphering module 184 for decrypting the stream of images encrypted by the encryption module 174. The module 184 then transmits the decrypted stream of images to the module 88 which builds the stream of images in clear before it is displayed.

The terminal 182 is for example identical to the terminal 180.

In this embodiment, the method is used to protect the image flow transmitted between the decoder 80 and the encryption module 174 and between the decryption module 184 and the inverse transformation module 88.

[0090] Many other embodiments are possible. For example, the images in clear can come from a source 4 itself containing a decoder of a satellite link or an Internet link.

The application of the secret transform and the inverse transform can be activated and, alternately, deactivated, at the head of the network.

The module 88 may be implemented in the form of additional hardware equipment or in the form of a software module executed by an electronic computer.

In a variant, the message 54 is transmitted outside the stream of transformed images. It can be transmitted via a point-to-point, point-to-multipoint or other connection via a separate channel on the network 8 or on a different network. For example, it is transmitted via a specific service of a transport layer of the image stream.

The message 54 can also be multiplexed with the image stream and the ECM messages. In this case, on the side of the receiving device, it is transmitted at the same time as the decompressed image stream to the inverse transform module 88. The message 54 may also be an ECM message, constructed by inserting the cryptogram T * in an ECM as known in the prior art. The module 30 then provides for this purpose the cryptogram T * to the system 28.

The generator 32, the system 28, the scrambler 22, the processor 76 and the descrambler 74, which protect the content according to the prior art, can be omitted or deactivated, as a whole or not, so known to those skilled in the art.

In a variant, the transform T can be constructed using an elementary transform chosen in another family of elementary transforms than the two families described here.

In another embodiment, a new transform T is coded using a predetermined law and certain characteristics of a clear image such as the color of certain pixels of this image. Similarly, a new inverse transform T ^"1 is constructed from a law corresponding to this predetermined law and from the same characteristics of the same plaintext, for example, the plaintext image used to encode the new transform T is transformed using not the new transform but the old transform.On the side of the receiving device, the clear image is found using the old inverse transform. Then, the new transform T ^"1 is constructed from the clear image obtained by applying the old inverse transform, so in this embodiment, it is no longer necessary to insert a specific message, such as the message 54, in the stream of images transmitted to the receiving device.

Alternatively, the construction of a new transform T is triggered in response to the receipt of a command. For example, the command is automatically generated by the encoder 16 at each change of scene or at each generation of a new image I.

In a variant, the graphics card also comprises an encryption module for the image stream transmitted on the link connecting the reception device 10 to the display 84. Correspondingly, the display 84 is equipped with a module decryption of the encrypted stream of images. For example, these modules are built to comply with the High Band Digital Content Protection (HDCP) standard. The inverse transformation is then performed either before the entry of the image stream into the encryption module or at the output of the decryption module.

In this description, the terms "scramble" and "descramble" are considered equivalent, respectively, to the terms "encrypt" and "decipher".

[00101] What has just been described also applies to digital rights management or DRM (Digital Rights Management).

Claims

1. A method of transmitting a clear image stream, said method comprising:

encoding (1 12) the image stream so as to obtain a compressed image stream, and

- transmitting (1 18) the compressed image stream to at least one receiving device,

characterized in that, prior to encoding, the method comprises transforming (108), using a secret invertible transform, images of the plaintext image stream to obtain a transformed image stream that is encoded and emitted instead of the stream of clear images, this secret invertible transform transforming each image of a succession of images in clear:

2. Method according to claim 1, in which the secret invertible transform is such that the compression ratio of the transformed image stream is identical, within ± 55%, to the compression ratio that would be obtained if the images were not transformed.

The method according to any one of the preceding claims, wherein the secret invertible transform is an elementary transform or a combination of elementary transforms selected from the group consisting of:

the permutation of blocks of pixels of the image in the clear, and

the addition of an offset to the chrominance values of at least one block of pixels of the clear image.

4. Method according to any one of the preceding claims, in which the transform is capable of transforming the stream of images into a stream of transformed images containing as much information about each image as the stream of images in the clear. .

The method of any of the preceding claims, wherein the method comprises modifying, at predetermined time intervals or in response to a command, the secret invertible transform.

The method according to claim 5, wherein the method comprises transmitting (1 18) a message containing a cryptogram enabling the receiving device to find an inverse transform that can be used to reconstruct the stream of clear images from the stream of transformed images.

7. The method of claim 6, wherein the method comprises the insertion (1 10) of the message in the stream of transformed images before encoding for this message to be issued at the same time as the image stream.

The method of any one of the preceding claims, wherein the method comprises scrambling (1 14) the compressed image stream prior to transmission.

A method of displaying an emitted image stream according to a method according to any one of the preceding claims, wherein the method comprises:

the reception (120) of the image stream, and

the decoding (126) of the received image stream so as to obtain an uncompressed stream of images,

characterized in that, after the decoding of the image stream, the method comprises transforming (136), using the inverse of the secret transform used in the transmission, images of the transformed image stream to obtain the stream of images in clear, then the display of the stream of images thus obtained.

10. An information recording medium, characterized in that it comprises instructions for the execution of a method according to any one of the preceding claims, when these instructions are executed by an electronic computer.

1 1. Device for transmitting a clear image stream, this device comprising:

an encoder (16) of the image stream so as to obtain a compressed image stream, and

a transmitter of the compressed image stream to at least one receiving device, characterized in that the device comprises a module (30) capable of transforming, by means of a secret invertible transform, transforming the images of the stream Image in clear to obtain a stream of transformed images which is encoded and emitted in place of the stream of images in clear, this secret invertible transform transforming each image of a succession of images in clear:

12. Module (88) of inverse transformation for the implementation of a display method according to claim 9, characterized in that the module is able to transform, using the inverse of the secret transform. used during transmission, the images of the stream of transformed images to obtain the stream of images in clear.