[001]            The invention pertains to methods for sending and receiving image streams. The invention also pertains to an information-recording medium  a sending device and an inverse transformation or conversion module to implement these methods.
[002]            Prior-art methods for sending an image stream in plain form comprise:
- the encoding of the image stream so as to obtain a compressed image stream  and
- the sending of the compressed image stream to at least one reception device.
[003]            Prior-art methods for receiving image streams sent according to the above method comprise:
- the reception of the image stream  and
- the decoding of the received image stream so as to obtain a decompressed image stream.
[004]            The plain or unencrypted image stream is a stream of images that are directly perceptible and comprehensible to a human being when displayed on a screen. For example  it is a sequence of a multimedia program such as a film or an audio-visual program. The plain stream of images is not protected. It can therefore be viewed by any reception device whatsoever equipped with a display unit and the appropriate decoder. In particular  the plain image stream is not scrambled in such a way as to subject its viewing to certain conditions.
[005]            Typically  in order to protect the plain image stream  these prior-art methods also include the scrambling of the compressed image stream so that the plain viewing of this image stream is made subject to certain conditions such as the purchase of an access entitlement.
[006]            Correspondingly  the prior-art reception methods typically comprise the descrambling of the image stream before it is decoded. The stream of images is therefore protected while being transferred from a sending device to the reception device.
[007]            Scrambling is routinely done after encoding. Indeed  with present-day scrambling methods  inverting these two operations very significantly lowers the compression rate of the encoder.
[008]            The compression rate is the ratio between the quantity of information before compression and the quantity of compressed information obtained after compression.
[009]            In these known sending and receiving methods  the image stream is no longer protected as of output from the descrambler that has descrambled the image stream. Thus  illicit use consists in retrieving the unscrambled or decompressed image stream in order to broadcast it to users who have not purchased corresponding access entitlements.
[0010]        To overcome this problem  it has already been proposed to integrate the descrambler and the decoder into one and the same electronic component  known as an SOC (System On Chip) to make it harder to retrieve the image stream. However  such a component generally uses an unprotected external RAM (Random Access Memory) to perform descrambling and decoding operations. It is therefore still possible to retrieve the descrambled image stream from the content of this RAM.
[0011]        When the reception device is a personal computer (PC)  it is also possible to retrieve the descrambled image stream by snooping on the calls made to the driver of the graphic card of this computer.
[0012]        To overcome this drawback  it has also been proposed to encipher the decompressed image stream at output from the decoder. The image stream thus enciphered is deciphered just before being displayed on a screen. This method therefore protects the link between the reception device and a display unit. It is for example made with HDCP (High Band Digital Content Protection) technology. However  in this method  the descrambled image stream may be retrieved before input into the decoder or at output but before its encipherment. The encipherment applied at output of the decoder is then ineffective to prevent this form of illicit retrieval of the image stream.
[0013]        Another difficulty arises when the encoders  scramblers  descramblers and decoders are proprietary apparatuses. Indeed  in the event of a security flaw  only the proprietor of these apparatuses can take action to remedy the flaw. A third party cannot propose a solution to modify the working of these apparatuses.
[0014]        The invention seeks to remedy at least one of these drawbacks. An object of the invention is a method for sending a stream of plain or unencrypted images in which  before encoding  the method comprises the conversion  by means of a secret invertible transform  of the images of the plain image stream to obtain a transformed or converted image stream which is encoded and sent instead of the plain image stream  this secret invertible transform converting each image of a succession of plain images:
- in permutating  identically  the position of pixels of each image of the succession of plain images  and/or
- in modifying  identically  colors of pixels of each image of the succession of plain images  so that the conversion made to each plain image is directly perceptible by a human being when the converted image is displayed on a screen without having the inverse transform applied preliminarily to it.
[0015]        The term “invertible” transform designates the fact that there is an inverse transform used to retrieve the plain image stream from the scrambled image stream. This inverse transform is the reciprocal of the invertible transform.
[0016]        The term “secret” designates the fact that the transform is not intended for disclosure to the public.
[0017]        The above method is used to dissuade action to retrieve the compressed image stream at output of the descrambler or the decompressed image stream at output of the decoder. Indeed  the image stream retrievable at these two outputs is the converted image stream. Now  this converted image stream is visibly different from the plain image stream. Thus  viewing it without pre-application of the inverted transform remains unsatisfactory to 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 output from the decoder.
[0018]        Furthermore  to implement this method  it is not necessary to act on the existing encoders  scramblers  decoders and descramblers.
[0019]        Furthermore  that fact that only permutations of the position of the pixels and/or modifications of the colors of the pixels are used to convert the plain image limits the deterioration of the compression rate of the existing encoder.
[0020]        The embodiments of this sending method may comprise one or more of the following characteristics:
¦     the secret invertible transform is such that the rate of compression of the converted image stream is identical  +55%  to the compression rate that would be obtained if the images were not converted;
¦     the secret invertible transform is an elementary transform or a combination of elementary transforms chosen from the group comprising:
- the building of an image symmetrical to the plain image relative to an axis parallel to an edge of the image 
- the permutation of the blocks of pixels of the plain image  and
- the adding of an offset to the chrominance values of at least one block of the pixels of the plain image;
¦     the transform is capable of converting the plain image stream into a converted image stream containing as many pieces of information on each image as does the plain image stream;
¦     the method comprises the modification  at predetermined time intervals or in response to a command  of the secret invertible transform;
¦     the method comprises the sending of a message comprising a cryptogram enabling the reception device to find an inverse transform that can be used to rebuild the plain image stream from the converted image stream;
¦     the method comprises the insertion of the message into the converted image stream before it is encoded so that the message is sent at the same time as the image stream;
¦     the method comprises the scrambling of the compressed image stream before it is sent.
[0021]        These embodiments of the sending method furthermore have the following advantages:
- they make it possible to choose a conversion which causes little deterioration in the performance of the encoder and hence make it possible to keep the bandwidth required to transmit the converted image stream at a level close to that required to transmit the plain image stream;
- building an image symmetrical to the image or permutating pixel blocks maintains the bandwidth required to send the stream of converted images at a level close to the bandwidth required to send the plain image stream;
- using a conversion which does not diminish the quantity of information contained in each plain image enables the rebuilding  from the converted image stream  of plain images which have exactly the same quality as the sent plain images;
- regularly modifying the secret transform used makes it possible both to accentuate the unsatisfactory character  for the user  of the display of the converted image stream and to increase security by limiting the time available to illicitly determine the inverse of the transform used;
- transmitting the cryptogram in a message contained in the converted image stream sent averts the need to resort to an additional communications channel.
[0022]        An object of the invention is also a method for displaying an image stream sent using the above method in which  after the decoding of the image stream  the method includes the conversion  by means of the inverse of the secret transform used during sending  of the images of the transformed or converted image stream to obtain the plain image stream and then the display of the image stream thus obtained.
[0023]        An object of the invention is also an information-recording medium comprising instructions to implement any one of the above methods when these instructions are executed by an electronic computer.
[0024]        An object of the invention is also a device for sending a stream of images in plain form  this device comprising:
- an encoder of the image stream so as to obtain a compressed image stream  and
- a sender of the compressed image stream to at least one reception device 
- a conversion module capable  by means of a secret invertible transform  of converting the images of the plain image stream to obtain a converted image stream that is encoded and sent instead of the plain image stream  this secret invertible transform converting each image into a succession of plain images:
- in permutating  identically  the position of pixels of each image of the succession of plain images and/or
- in modifying  identically  colors of pixels of each row of the succession of plain images  so that the conversion made to each plain image is directly perceptible by a human being when the converted image is displayed on the screen without preliminarily having had the inverse transform applied to it.
[0025]        Finally  an object of the invention is also an inverse conversion module for the implementation of the above reception method which  using the inverse of the secret transform used when sending  is capable of converting the images of the converted image stream to obtain the plain image stream.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026]        The invention will be understood more clearly from the following description  given purely by way of a non-restrictive example and made with reference to the appended drawings  of which:
- Figure 1 is a schematic illustration of a system for sending and receiving a stream of images 
- Figure 2 is a schematic illustration of a plain image 
- Figures 3 and 4 are schematic illustrations of a converted image obtained from the plain image of figure 2 
- Figure 5 is a schematic illustration of the structure of a message transmitted with the images of the stream 
- Figure 6 is flowchart of a method for sending and receiving an image stream by means of the system of figure 1 
- Figures 7  8 and 9 are respectively schematic illustrations of alternative embodiments of the system of figure 1.
[0027]        In these figures  the same references are used to designate the same elements.
[0028]        Here below in this description  the characteristics and functions well known to those skilled in the art are not described in detail. Furthermore  the terminology used is that of systems of conditional access to multimedia contents. For further information on this terminology  the reader may refer to the following document:
- “Functional Model of Conditional Access System”  EBU Review  Technical European Broadcasting Union  Brussels  BE  n° 266  21 December 1995.
[0029]        Figure 1 shows a system 2 for sending and receiving a stream of images. The stream
of images corresponds for example to a sequence of a multimedia program such as a television broadcast  a film or an audio-visual program.
[0030]        The stream of plain images is generated by a source 4 and transmitted to a device 6 for sending this stream of images to a multitude of reception devices through an information-transmission network 8.
[0031]        The network 8 is typically a long-distance information-transmission network such as the Internet or a satellite network or again a wireless network such as the one used for transmitting digital terrestrial television (DTTV).
[0032]        To simplify figure 1  only two reception devices 10 and 12 are shown.
[0033]        The device 6 comprises an encoder 16 capable of compressing the image stream that it receives at one input 18 and of restoring a compressed image stream at an output 19. The encoder 16 is a digital encoder capable of processing digital image streams. For example  this encoder works according to the MPEG2 (Moving Picture Expert Group – 2) standard or the UIT-T H264 standard.
[0034]        The compressed image stream is directed towards an input 20 of a scrambler 22. The scrambler 22 scrambles the compressed image stream to make its viewing conditional on certain conditions  such as the purchase of an access entitlement by users of the reception devices. The stream of scrambled images is restored by an output 24 connected to the input of a multiplexer 26.
[0035]        The scrambler 22 scrambles the compressed image stream by means of a control word CW that is given to it  as well as to a conditional access system 28 better known as a CAS  by a key generator 32. Typically  this scrambling is compliant with a standard such as the DVB-CSA (Digital Video Broadcasting – Common Scrambling Algorithm) standard  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 ECMs (Entitlement Control 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 stream of scrambled images are multiplexed by the multiplexer 26 to which they are respectively given by the conditional access system 28 and by the scrambler 22 and then transmitted on the network 8.
[0036]        The device 6 furthermore has a module 30 for converting plain images. The module 30 converts the stream of plain images received from the source 4 into a stream of converted 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 converted by the module 30 and not the plain images. This module 30 is aimed at protecting the stream of images after it has been descrambled and decoded in the reception devices 10 and 12. To this end  the conversion of the image must be directly perceptible by a user who does not apply the inverse conversion before displaying it on a screen. Preferably  the converted images are made as different as possible from the plain images from which they are obtained. To this end  a secret invertible transform T is applied by the module 30 to each of the images of the received stream of plain images.
[0037]        Preferably  this transform T is chosen so as not to appreciably modify the compression rate of the encoder 16. The compression rate of the encoder is deemed to be not appreciably modified if it is equal  ±  55%  and preferably ±  15%  to the compression rate that would be obtained by directly encoding the stream of plain images.  To achieve this goal  the transform T is chosen as a function of the characteristics of the encoder.
[0038]        Furthermore  the transform T is chosen so as not to downgrade the quality of the image. To this end  the transformed images contain as many pieces of information as the plain images.
[0039]        The transform T is herein obtained by combining several invertible elementary transforms Tei. More specifically only two families of elementary transforms  Tei are used:
- the elementary transforms that permutate the position of the pixels of the plain image  and
- the elementary transforms that modify the color of the pixels of the image.
[0040]        The elementary transforms have the common feature of not modifying the correlations that exist between the successive images of the plain stream of images. Because of this  the compression rate is not appreciably modified. Indeed  in the MPEG2 or H264 encoding method  this limits the generation of supplementary I frames by the encoder.
[0041]        By way of an illustration  a definition is given here below of six elementary transforms Te1 to Te6. The elementary transforms Te1 to Te3 permutate pixels of the plain image with other pixels of the plain image  Te4 carries out a circular permutation of three blocks of pixels of the plain image and Te5 and Te6 modify the color of the pixels of the image. In this non-restrictive example  Te5 and Te6 are described in the context of the YUV color encoding standard. Elementary transforms modifying the color of the pixels of the image however can be described of course equivalently in the context of any other color encoding standard  for example RGB (Red Green Blue) well known to those skilled in the art  without departing from the field of the invention.
[0042]        For example  the transform Te1 inverts the left-hand and right-hand pixels. The plain or unencrypted image 34 before this elementary transform is applied is shown in figure 3. The corresponding converted image 36 is shown in figure 3. The image 36 is symmetrical with the image 34 relative to an axis 38 parallel to the vertical edge of the image 34.
[0043]        The transform Te2 inverts the top and the bottom of the plain image. The converted image 40 obtained by applying this elementary transform to the image 34 is shown in figure 4. The image 40 is symmetrical with the image 34 relative to an axis 42 parallel to the horizontal edge of the image 34.
[0044]        The transform Te3 permutates at least one block of pixels of the plain image with another block of pixel of this plain image. Preferably  the blocks permutated by the transform Te3 are the same as those implemented in the encoding method. For example  they are blocks of 16 pixels by 16 pixels identical to those used in the MPEG2 and H264 standards. Thus  this permutation of blocks of pixels limits the degradation of the compression rate of the encoder 16.
[0045]        The elementary transform Te4 performs a circular permutation of at least three blocks of pixels of the plain image. As in the case of the elementary transform Te3  the permutated blocks are the same as those used in the encoding algorithm. 
[0046]        Preferably  for the transforms Te3 and Te4  the identity of the permutated blocks is a parametrizable value.
[0047]        The transform Te5 adds a digital offset to the values of the U and V components of chrominance of the image  known as chrominance values. The value of this offset can be parametrized. In the encoding operations compliant with the H264 and MPEG2 standards  the main piece of information used to compress the stream of images is the luminance denoted as Y. Conversely  the U and V components of the chrominance are not taken into account by the encoding algorithm. It is therefore possible to make modifications of the value of the U and V components without disturbing the encoder 16 and therefore without modifying the compression rate.
[0048]        The elementary transform Te6 for its part inverts the values of the U and V components for each of the pixels of the plain image.
[0049]        These elementary transforms are recorded in a memory 50 connected to the device 6.
[0050]        The transform T is obtained by combining one or more of the above elementary transforms Tei. The combination is done by successively applying several elementary transforms Tei to the same plain image in a predetermined order. The transform T is therefore a composition of several of the elementary transforms Tei.
[0051]        To build the inverse transform T-1  it is necessary to know the combination of elementary transforms used and also  when these elementary transforms used are parametrizable  to know the parameter values used. These pieces of information are kept secret so as to prevent the building of the inverse transform T-1 by unauthorized individuals.
[0052]        Here  the module 30 also generates a message containing a cryptogram of the pieces of secret information enabling the building of the inverse transform T-1. The term “cryptogram” designates the fact that this message does not itself contain sufficient information to enable the building of the transform T-1. The pieces of information contained in this message must indeed be combined with pieces of information to be obtained in the reception device  whether they have been pre-recorded therein or need to be measured or computed therein.
[0053]        Thus  the cryptogram may be an identifier of a particular combination of elementary transforms Tei. The reception device must then include means  such as a correspondence table  to associate  with each identifier liable to be received  an inverse transform T-1 to be used to carry out the inverse conversion of the image stream.
[0054]        The cryptogram can also contain an identifier of the parameters. In this case  the reception device must contain means such as a correspondence table to associate this identifier and the parameters to be used.
[0055]        The cryptogram can also be obtained by enciphering the secret information  for example the executable code  of the inverse transform T-1  by means of a symmetrical or asymmetrical encipherment. The reception device then builds the transform T-1 by deciphering the received cryptogram using a pre-recorded secret key. Similarly  the identifiers of the parameters of the elementary transforms can be enciphered.
[0056]        For example  figure 5 shows the structure of a message 54 generated by the module 30 and containing the pieces of secret information enabling the reception device to build the inverse transform T-1. The message 54 comprises a field 56 containing a cryptogram T* of the secret information enabling the building of the inverse transform T-1. The structure of this message is identical or similar to that of an ECM message. It shall therefore not be described in detail.
[0057]        The message 54 also includes a field 58 containing conditions of access CA designed to be compared with access entitlements pre-recorded in the reception device so as to permit and  in alternation  inhibit the building of the inverse transform T-1.
[0058]        The message 54 also comprises:
- a field 60 containing information to authenticate the message 54 such as a signature S  and
- a field 62 containing information to verify the integrity of the message 54 such as a CRC (Cyclic Redundancy Check) code.
[0059]        Finally  the module 30 is also capable of inserting the message 54 into the stream of converted images before it is encoded so that this message is transmitted to the reception device at the same time as the stream of converted images. The message 54 is inserted into the stream of converted images in such a way as not to appreciably modify the compression rate of the encoder 16. To this end  different methods are possible. For example  the message 54 is transmitted as teletext. This therefore amounts to adding at least one line to the converted image on which the message 54 is encoded by means of a color code.
[0060]        The message 54 can also be transmitted in the stream of converted images by applying a digital image watermarking method. However  this method is preferably simplified to take account of the fact that it is not necessary to ensure the robustness of the message inserted into the image for subsequent conversions. Indeed  if a subsequent conversion of the image were to modify the message 54  that would make it impossible to build the inverse transform T-1: this would not be a security flaw.
[0061]        The module 30 is made for example out of a programmable electronic computer capable of executing instructions recorded on an information-recording medium. To this end  the module 30 is connected to the memory 50 and the memory 50 contains the instructions needed to execute the method of figure 6.
[0062]        The reception device 10 has a receiver 70 of the stream of images sent. This receiver 70 is connected to the input of a demultiplexer 72 which transmits firstly the image stream to a descrambler 74 and secondly the ECMs and EMMs (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 entitlements which only legitimate users can use. To preserve the confidentiality of this information  it is designed to be as robust as possible relatively to attenpted attacks by computer hackers. It is therefore more robust against these attacks than other components of the device 10. For example  a security processor is a chip card equipped with an electronic processor. The security processor can also be a software module executed by an electronic computer.
[0063]        For example  the processor 76 has a memory 78 containing different cryptographic keys and access entitlements for descrambling the scrambled images.
[0064]        The descrambler 74 descrambles the scrambled image stream using the plain control word CW transmitted by the processor 76. The descrambled image stream is transmitted to a decoder 80 which decodes it. The decompressed image stream or decoded image stream is transmitted to a graphic card 82 which drives the display of this image stream on a display unit 84 equipped with a screen 86. Here  the graphic card 82 has an inverse conversion module 88. This module 88 is capable of building the inverse transform T-1 and applying this inverse transform to obtain the plain image stream. To this end  the board 82 has an electronic computer 90 connected to a memory 92 comprising the instructions needed to execute the method of figure 6.
[0065]        The display unit 84 is capable of the plain or unencrypted display of the received image stream on the screen 86.
[0066]        For example  the device 12 is identical to the device 10 and will not be described in detail.
[0067]        The working of the system 2 shall now be described in greater detail with reference to the method of figure 6. This method is formed essentially by a phase 100 for sending the image stream and a phase 102 for receiving the image stream sent.
[0068]        At the beginning of the phase 100  at a step 104  a transform T is built. Here  the activation of the building of a new transform T is prompted at predetermined intervals. The predetermined intervals are for example regular intervals of less than 20 seconds and preferably less than 10 seconds.
[0069]        Then  at a step 106  the message 54 corresponding to the transform T is built. At the same time  at a step 108  the stream of plain images is converted by applying the transform T.
[0070]        Then  at a step 110  the message 54 is inserted into the converted image stream and then the resulting stream is transmitted to the encoder 16.
[0071]        At a step 112  the encoder 16 encodes the converted image stream to obtain a compressed image stream.
[0072]        At a step 114  the compressed image stream is scrambled by the scrambler 22 by means of the control word CW generated by the generator 32.
[0073]        At a step 116  the scrambled image stream and the corresponding ECM messages generated by the system 28 are multiplexed. Finally  these pieces of multiplexed information are sent at a step 118 to the different reception devices.
[0074]        The phase 102 starts with a reception step 120 followed by the demultiplexing of the multiplexed information sent. Then  the image stream is transmitted to the descrambler 74 while the ECMs and EMMs are transmitted to the processor 76.
[0075]        At a step 122  the processor 76 deciphers the cryptogram CW* of the control word and sends the code word CW to the descrambler 74.
[0076]        At a step 124  the descrambler 74 descrambles the scrambled image stream with the received code word.
[0077]        At 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 conversion module 88.
[0078]        At a step 128  the module 88 extracts the message 54 from the received image stream.
[0079]        Then  at a step 130  it compares the conditions of access CA to pre-recorded access entitlements TdA. If the conditions of access CA received do not correspond to the access entitlements TdA then  at a step 132  the building of the inverse transform T-1 is inhibited. If not  at a step 134  the cryptogram T* is exploited to build the inverse transform T-1.
[0080]        At a step 136  the inverse transform T-1 is applied to the converted image stream to obtain the plain image stream which is displayed at a step 138 on the screen 86.
[0081]        Figure 7 represents a system for sending and receiving an image stream. This system 150 is identical to the system 2 except that:
-          the module 88 is implemented in a package 152 that is mechanically independent of the device 10 and the display unit 84  and
-          the graphic card 82 is replaced by a graphic card 154 lacking the module 88.
[0082]        This pack 152 is interposed between the graphic card 154 and the display unit 84.
[0083]        This embodiment is used to secure an image stream transmission system which has suffered crypto-analysis without any need to modify the reception device 10. Indeed  it is enough to add the pack 152 to give additional protection to the transmitted image stream.
[0084]        Figure 8 represents a system 160 identical to the system 2 except that:
-          the display unit 84 is replaced by a display unit 162 containing the module 88  and
-          the graphic card 82 is replaced by a graphic card 164 lacking the module 88.
[0085]        For example  in this embodiment  the device 10 is a central processing unit of a personal computer. The descrambler 74 and the encoder 80 are implemented on a detachable USB stick (Universal Serial Bus) connected to this central processing unit. This embodiment secures the transmission of the image stream within the personal computer itself between the USB and the display unit 162.
[0086]        Figure 9 represents a system 170 identical to the system 2 except that the device 10 is replaced by a reception device 172. The device 172 is identical to the device 10 except that it has in addition a module 174 for enciphering the decompressed image stream generated at output of the decoder 80. The device 172 is connected by means of a local area network 178 to different user terminals. To simplify the figure  only two terminals 180 and 182 have been shown.
[0087]        The terminal 180 includes a deciphering module 184 capable of deciphering the enciphered image stream by means of the encipherment module 174. The module 184 then transmits the deciphered image stream to the module 88 which builds the plain image stream before it is displayed.
[0088]        The terminal 182 is for example identical to the terminal 180.
[0089]        In this embodiment  the method is used to protect the stream of images transmitted between the decoder 80 and the encipherment module 174 and between the decipherment module 184 and the inverse conversion module 88.
[0090]        Many other embodiments are possible. For example  the plain images may come from a source 4 itself containing a decoder of a satellite link or an Internet link.
[0091]        The application of the secret transform and of the inverse transform may be activated and alternately deactivated at the network end point.
[0092]        The module 88 may be implemented in the form of an additional hardware apparatus or in the form of a software module executed by an electronic computer.
[0093]        As a variant  the message 54 is transmitted outside the converted image stream. It may be transmitted through a point-to-point connection or point-to-multipoint connection or other using a distinct channel on the network 8 or on a different network. For example  it is transmitted through a specific service of a layer for transporting the image stream.
[0094]        The message 54 can also be multiplexed with the image stream and the ECM messages. In this case  on the reception device side  it is transmitted at the same time as the decompressed image stream to the inverse conversion module 88. The message 54 can also be an ECM message built by inserting the cryptogram T* in an ECM in a manner known in the prior art. The module 30 then  to this end  provides the cryptogram T* to the system 28.
[0095]        The generator 32  the system 28  the scrambler 22  the processor 76 and the descrambler 74 which provide for the protection of the content according to the prior art  can be omitted or deactivated  in their totality or not  in a manner known to those skilled in the art.
[0096]        As a variant  the transform T may be built by using an elementary transform chosen from family of elementary transforms other than the two families described here above.
[0097]        In another embodiment  a new transform T is encoded by means of a predetermined law and certain characteristics of a plain image such as the color of certain pixels of this image. Similarly  a new inverse transform T-1 is built out of a law corresponding to this predetermined law and the same characteristics of the same plain image. For example  the plain image used to encode the new transform T is converted by using not the new transform but the old transform. On the reception device side  the plain image is retrieved by using the old inverse transform. Then  the new transform T-1 is built out of the plain image obtained by applying the old inverse transform. Thus  in this embodiment  it is no longer necessary to insert a specific message such as the message 54 into the image stream transmitted to the reception device.
[0098]        As a variant  the building of a new transform T is activated in response to the reception of a command. For example  the command is automatically generated by the encoder 16 at each change in scene or at each generation of a new image I.
[0099]        As a variant  the graphic card also has a model for enciphering the stream of images sent on the link connecting the reception device 10 to the display unit 84. Correspondingly  the display unit 84 is equipped with a module for deciphering the enciphered image stream. For example  these modules are built to comply with the HDCP (High Band Digital Content Protection) standard. The inverse conversion is then performed either before the input of the image stream into the encipherment module or at output from the decipherment module.
[00100]    In this description  the terms “ to scramble” and “to descramble” are considered to be equivalent respectively to the terms “to encipher” and “to decipher”.
[00101]    The content of this description can also be applied to digital rights management (DRM).

We Claim:
1. Method for sending a stream of plain images  this method comprising:
the encoding (112) of the image stream so as to obtain a compressed image stream  and
the sending (118) of the compressed image stream to at least one reception device 
characterized in that  before encoding  the method comprises the conversion  by means of a secret invertible transform  of the images of the plain image stream to obtain a converted image stream that is encoded and sent instead of the plain image stream  this secret invertible transform converting each image of a succession of plain images;
in permutating  identically  the position of pixels of each image of the succession of plain images  and/or
in modifying  identically  colors of pixels of each image of the succession of plain images  so that the conversion made to each plain image is directly perceptible by a human being when the converted image is displayed on a screen without having the inverse transform applied preliminarily to it.
2. Method according to claim 1  wherein the secret invertible transform is such that the rate of compression of the converted image stream is identical  +55%  to the compression rate that would be obtained if the images were not converted.
3. Method according to any one of the above claims  wherein the secret invertible transform is an elementary transform or a combination of elementary transforms chosen from the group comprising:
the building of an image symmetrical to the plain image relative to an axis parallel to an edge of the image 
the permutation of the blocks of pixels of the plain image  and
the adding of an offset to the chrominance values of at least one block of the pixels of the plain image.
4. Method according to any one of the above claims  wherein the transform is capable of converting the plain image stream into a converted image stream containing as many pieces of information on each image as does the plain image stream.
5. Method according to any one of the above claims  wherein the method comprises the modification  at predetermined time intervals or in response to a command  of the secret invertible transform.
6. Method according to claim 5  wherein the method comprises the sending (118) of a message comprising a cryptogram enabling the reception device to find an inverse transform that can be used to rebuild the plain image stream from the converted image stream.
7. Method according to claim 6  wherein the method comprises the insertion (110) of the message into the converted image stream before it is encoded so that the message is sent at the same time as the image stream.
8. Method according to any one of the above claims  wherein the method comprises the scrambling (114) of the compressed image stream before it is sent.
9. Method for displaying a stream of images sent according to a method according to any one of the above claims  wherein the method comprises:
the reception (120) of the stream of images  and
the decoding (126) of the received stream of images so as to obtain a decompressed stream of images 
characterized in that  after the decoding of the image stream  the method includes the conversion (136)  by means of the inverse of the secret transform used during sending  of the images of the stream of converted images to obtain the stream of images in unencrypted form  and then the display of the image stream thus obtained.
10. Information-recording medium characterized in that it comprises instructions to implement a method according to any one of the above claims when these instructions are executed by an electronic computer.
11. Device for sending a stream of images in plain form  this device comprising:
an encoder (16) of the image stream so as to obtain a compressed image stream  and
a sender of the compressed image stream to at least one reception device 
characterized in that the device comprises a conversion module (30) capable  by means of a secret invertible transform  of converting the images of the plain image stream to obtain a converted image stream that is encoded and sent instead of the plain image stream  this secret invertible transform converting each image into a succession of plain images:
in permutating  identically  the position of pixels of each image of the succession of plain images and/or
in modifying  identically  colors of pixels of each image of the succession of plain images 
so that the conversion made to each plain image is directly perceptible by a human being when the converted image is displayed on the screen without preliminarily having had the inverse transform applied to it.
12. Inverse conversion module (88) for the implementation of a method of display according to claim 9  characterized in that the module is capable  by means of the inverse of the secret transform used when sending  of converting the images of the converted image stream to obtain the plain image stream.