Abstract:
The invention is related to a method for burning digital data representative of a multimedia content on a secure disc. The multimedia content is intended to be downloaded from a remote server to a client device carrying out the burning on the secure disc. The secure disc contains a first encryption key. The method comprises the following steps: (a) retrieving the first encryption key from the secure disc; (b) transmitting to the remote server the first encryption key and an identifier of the multimedia content; (c) receiving from the remote server at least a second encryption key encrypted with the first encryption key and the multimedia content scrambled with the second encryption key; and (d) burning the second encryption key and the scrambled multimedia content on the secure disc containing the encrypted first encryption key, by the client device.

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001]     The invention relates generally to a method for burning digital data representative of a multimedia content on a secure digital video disc, the digital data being downloaded from a remote server.  
       BACKGROUND OF THE INVENTION  
       [0002]     Copyright multimedia contents can be stored on a secure digital video disc to prevent their illegal copy.  
         [0003]     Copyright multimedia contents can be requested, paid and downloaded from a content provider through Internet. In this case, the copyright multimedia contents are scrambled by the content provider and transmitted through Internet to the purchaser&#39;s computer in a scrambled form along with encryption keys. These encryption keys contain an identifier of the purchaser&#39;s computer so that the multimedia content can only be shown, listened or presented from this computer.  
         [0004]     After downloading on a purchaser&#39;s computer, these copyright multimedia contents can be copied on a writable support. However, a computer different from the purchaser&#39;s computer cannot descramble the multimedia content stored on this writable support since its identifier is different from the identifier used to build the encryption keys which are employed to scramble the multimedia content.  
         [0005]     Besides, known devices are adapted to download a multimedia content from a content provider through Internet, to scramble it and to burn it on a secure digital video disc.  
         [0006]     However, these devices are expensive because they require a great number of high level security systems to exclude any possibility of piracy of their scrambler module.  
         [0007]     In this context, there is a need to develop a cheaper method and device for burning on a secure digital video disc multimedia contents purchased and downloaded from a content provider and in which the resulting burnt multimedia content could be rendered on any legacy player, but a physical copy of the burnt secure digital video disc would not be rendered on any legacy player.  
       SUMMARY OF THE INVENTION  
       [0008]     In a first aspect, the invention relates to a method for burning digital data representative of a multimedia content on a secure disc containing a first encryption key. The multimedia content is intended to be downloaded from a remote server through a distribution network to a client device carrying out the burning on the secure disc. The method comprises the following steps performed by the client device: 
        retrieving the first encryption key from the secure disc;     transmitting to the remote server the first encryption key and an identifier of the multimedia content intended to be downloaded;     receiving from the remote server at least one second encryption key encrypted with the first encryption key and the multimedia content scrambled with the or each second encryption key; and     burning the or each second encryption key and the scrambled multimedia content received from the remote server on the secure disc containing the encrypted first encryption key.        
 
         [0013]     In a first preferred embodiment, the method further comprises a step of establishing a secure authenticated channel between the client device and the remote server previous to the step of transmitting the first encryption key and the identifier of the multimedia content. Preferably, the first and the second encryption keys, the identifier of the multimedia content and the multimedia content are transmitted only through this established secure authenticated channel.  
         [0014]     In a second preferred embodiment, the secure disc contains an encrypted first encryption key pre-burnt on the secure disc and the step of retrieving the first encryption key comprises: 
        reading the encrypted first encryption key on the secure disc; and     decrypting the encrypted first encryption key with the help of a master key stored in a memory of the client device to retrieve the first encryption key of the secure disc.        
 
         [0017]     In another particular embodiment of the invention, the first encryption key has a low probability of being common to two different secure discs.  
         [0018]     In yet another embodiment, the first encryption key is a unique number which is only pre-burnt on one unique specific secure disc.  
         [0019]     In a second aspect, the invention relates to a method for distributing digital data representative of a multimedia content able to be downloaded from a remote server to a client device through a distribution network. The multimedia content is intended to be burnt by the client device on a secure disc containing a first encryption key. The method comprises the following steps performed by the remote server: 
        receiving from the client device the first encryption key retrieved from the secure disc and an identifier of the multimedia content intended to be burnt;     generating at least one second encryption key;     scrambling the multimedia content intended to be burnt with the or each second encryption key;     encrypting the or each second encryption key with the first encryption key; and     transmitting to the client device the scrambled multimedia content and the or each encrypted second encryption key for burning them on the secure disc.        
 
         [0025]     In a third aspect, the invention relates to a remote server for distributing digital data representative of a multimedia content to a client device through a distribution network, the multimedia content being intended to be burnt on a secure disc containing a first encryption key. The server preferably comprises: 
        a network interface adapted to receive from the client device the first encryption key retrieved from the secure disc and an identifier of the multimedia content intended to be burnt;     a random number generator adapted to generate at least one second encryption key;     a scrambler module for scrambling the multimedia content intended to be burnt with the or each second encryption key; and     an encryption module for encrypting the or each second encryption key with the first encryption key.        
 
         [0030]     The network interface is adapted to transmit to the client device the scrambled multimedia content and the or each encrypted second encryption key through the distribution network for burning them on the secure disc.  
         [0031]     In a preferred embodiment of the remote server, the network interface is able to build up a secure authenticated channel with a client device to receive the identifier of the multimedia content intended to be burnt and the first encryption key and to transmit the scrambled multimedia content intended to be burnt and the or each encrypted second encryption key.  
         [0032]     In a fourth aspect, the invention relates to a client device for burning digital data representative of a multimedia content on a secure disc, the disc containing a first encryption key and the multimedia content being intended to be downloaded from a remote server through a distribution network. The client device preferably comprises: 
        a reader/burner device for retrieving the first encryption key from the secure disc and     a network interface for transmitting to the remote server the first encryption key and an identifier of the multimedia content intended to be downloaded.        
 
         [0035]     The network interface is adapted to receive at least a second encryption key encrypted with the first encryption key and the multimedia content downloaded from the remote server, the multimedia content being scrambled with the or each second encryption key; and the reader/burner device is adapted to burn the or each encrypted second encryption key and the scrambled multimedia content received from the remote server on the secure disc containing the first encryption key.  
         [0036]     In a preferred embodiment of the client device, the secure disc contains an encrypted first encryption key pre-burnt on the secure disc, and the reader/burner device is adapted to read the encrypted first encryption key on the secure disc; the client device comprising a memory storing a master key and a decryption module to decrypt the encrypted first encryption key with the help of the master key to retrieve the first encryption key of the secure disc.  
         [0037]     These and other aspects of the method will be apparent from the following description, drawings and from the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0038]      FIG. 1  is a schematic representation of a system adapted to generate secure CSS blank DVD used in the method according to the invention.  
         [0039]      FIG. 2  is a schematic representation of a system used for implementing the method according to the invention.  
         [0040]      FIG. 3  is a schematic representation showing the exchanges between a remote server, a client device and a content provider belonging to the system illustrated in  FIG. 2 . 
     
    
     DETAILED DESCRIPTION  
       [0041]     Referring to  FIG. 1 , a system  2  adapted to generate secure blank DVD, is illustrated.  
         [0042]     This system  2  comprises a manufacturer  4  of Digital Video Disc DVD) adapted to securely exchange data with a DVD Copy Control Authority (CCA) representative  6  to produce secure blank DVD  8 .  
         [0043]     In the illustrative embodiment hereunder described, the produced blank DVD  8  is protected according to a known protection system named “Content Scrambling System” (CSS).  
         [0044]     The DVD Manufacturer  4  is thus a CSS DVD manufacturer. It comprises a random number generator  9  for generating encryption keys DK, a burner  10  adapted to burn data on a DVD  8  and a network interface  11  for exchanging data with the CCA representative  6 .  
         [0045]     The produced blank DVD  8  comprises a blank data area  12  on which multimedia content MC can be burnt by a user&#39;s burner and a lead-in area  13  on which a secure disc key set SDKs is burnt by the DVD manufacturer&#39;s burner  10  before the delivery of the blank DVD  8 . This secure disc key set can be seen as an encrypted representation of a disc key DK, as explained later.  
         [0046]     According to the invention, each disc key DK is unique and is specific to the particular DVD  8  on which the corresponding secured disc key set is burnt. So, one disc key DK is associated with only one DVD.  
         [0047]     In variant, a certain number of prerecorded DVD with the same pre-recorded secure disc key set SDKs, i.e. associated with the same disk key DK, can be accepted if the probability that a same user acquires two identical pre-recorded DVDs during a time period is low and the probability that two users in the same geographical zone acquire identical prerecorded DVDs is also low. For; example, a probability of less than 1% may be considered low. Allowing low probability reduces the production cost for the DVD manufacturer while maintaining fair security for the overall system.  
         [0048]     The network interface  11  is adapted to send the disc key DK generated by the generator  9  to the CCA representative  6  and to receive in response the secure disc key set SDKs corresponding to the disc key DK.  
         [0049]     The CSS DVD CCA representative  6  includes a memory  17  for securely storing a set of master keys MK 1 , MK 2 , MK 3 , an encryption module  18  connected to the memory  17  and to a network interface  19 . Master keys may also be referred to as “manufacturer keys” in literature.  
         [0050]     The encryption module  18  is able to encrypt the disc key DK received from the DVD manufacturer  4  with each master key of the set of master keys MK 1 , MK 2 , MK 3  etc. The set of encrypted disc keys SDKs obtained from the encryption of one disc key DK with all master keys MK 1 , MK 2 , MK 3  is usually called a secure disc key set SDKs. This is the information written in the lead-in area  13  of blank discs by the DVD manufacturer  4 .  
         [0051]     The network interface  11  of the DVD Manufacturer and the network interface  19  of the representative  6  are able to establish a Secure Authenticated Channel SAC through a distribution network  20 .  
         [0052]     The secure authenticated channel SAC can be established through either a standard protocol like for example the Secure Socket Layer SSL protocol with authentication of both peers or a proprietary protocol like the SmartRight™ SAC protocol disclosed in international patent application No. PCT/EP04/052722. It can be based on standard Public Key Infrastructure (PKI) or on other techniques.  
         [0053]     Referring to  FIG. 2 , the system  21  for implementing the method according to the invention comprises a client device  22 , a remote server  23  and a multimedia content provider  24 , each adapted to exchange data through a distribution network  25 , for example the Internet network.  
         [0054]     The client device  22  is for example composed of a computer  26  connected to a particular reader/burner device  28 .  
         [0055]     The computer  26  contains a network interface  30  and a user interface  32  such as keyboard, screen and mouse to request and to receive multimedia contents MC from the remote server  23 , through the distribution network  25 .  
         [0056]     The computer  26  comprises further a decryption module  34  connected to a memory  36  storing at least one master key. One valid master key MK 1  is used to decrypt the secure disc key set SDKs burnt on the lead-in area  13  of the secure DVD  8 .  
         [0057]     The memory  36  is secured with state of the art protection system. Examples of such systems are currently implemented in legacy DVD players.  
         [0058]     The reader/burner device  28  is able to read the lead-in area  13  of the DVD  8  and to burn data on its data area  12 .  
         [0059]     The remote server  23  comprises a random number generator  38  linked to an encryption module  40  and to a scrambler module  42 . The scrambler module  42  is adapted to scramble the requested multimedia content MC before transmission to the client device  22 .  
         [0060]     The remote server  23  also contains a network interface  43  and a database  44  for temporally storing the requested multimedia content MC.  
         [0061]     The network interface  30  of the client device and the network interface  43  of the remote server are adapted to establish a secure authenticated channel SAC.  
         [0062]     The establishment of secure authenticated channel SAC requires the mutual authentication of the remote server  23  and of the client device  22 . The remote server  23  is authenticated to be sure that the multimedia content MC comes from an authorized source. The secure authenticated channel. SAC thus prevents from burning illegal content.  
         [0063]     The client device  22  is authenticated to prove his compliance to the remote server  23  so that the remote server knows that the client device  22  will not redistribute the multimedia content MC in an unauthorised manner.  
         [0064]     The content provider  24  contains a database  46  which stores multimedia contents proposed for burning, a processor  48  and a network interface  50 .  
         [0065]     The network interface  43  of the remote server and the network interface  50  of the content provider are adapted to establish a secure authenticated channel SAC.  
         [0066]     Referring to  FIG. 3 , at an initial step  100 , a user wishing to buy a multimedia content MC to be burnt on a CSS secure blank DVD  8 , connects the client device  22  to the content provider  24 , through the distribution network  25  and the remote server  23 . Then, he selects for example a multimedia content MC among a set of multimedia contents proposed for burning and stored in the database  46  of the content provider.  
         [0067]     At step  102 , the user fills up a content request indicating the identifier CID of the chosen multimedia content with the help of the user interface  32 . The user inserts a CSS secure blank DVD  8  in the reader/burner device  28  of the client device  22 .  
         [0068]     At step  104 , the reader/burner device  28  reads the secure disc key set SDKs pre-burnt on the lead-in area  13  of the DVD  8 .  
         [0069]     At step  106 , the decryption module  34  decrypts the secure disc key set SDKs read by the reader/burner device  28  with the help of the master key MK 1  stored in the memory  36  to retrieve the disc key DK associated to the DVD  8  bought by the user.  
         [0070]     In the same time, the decryption module  34  checks whether the blank DVD  8  is compliant. When the disc key DK of the blank DVD  8  cannot be decrypted then it means that the blank DVD  8  is not compliant and cannot be burnt. The client device  22  informs the user of this compliance problem through the user interface  32 .  
         [0071]     At step  108 , the remote server  23  and the client device  22  establish a Secure Authenticated. Channel SAC through which they can securely exchange data. Once authenticated, the remote server  23  and the client device  22  exchange data protected in integrity and/or in confidentiality.  
         [0072]     At step  110 , the client device  22  transmits the disc key DK specific of the bought blank DVD  8  and the filled up content request (containing the identifier CID) to the remote server  23 , through the secure authenticated channel SAC.  
         [0073]     If an identical disc key DK has already been received by the remote server  23 , the remote server transmits an error message to the client device  22  and the method ends.  
         [0074]     At step  112 , the identifier CID of the requested multimedia content MC is transmitted from the remote server  23  to the content provider  24 , through the distribution network  25 .  
         [0075]     At step  114 , the processor  48  retrieves the digital data representative of the requested multimedia content MC in the database  46  from its identifier CID.  
         [0076]     At step  116 , the content provider  24  establishes a secure authenticated channel SAC with the remote server  23 .  
         [0077]     At step  118 , the content provider  24  transmits the multimedia content MC to the remote server  23 . The multimedia content MC is then temporally stored in the database  44 .  
         [0078]     At step  120 , the generator  38  produces a random number to constitute a Title key TK and sends it to the scrambler module  42  and to the encryption module  40 .  
         [0079]     At step  122 , the multimedia content MC is transmitted to the scrambler module  42  which scrambles it using the Title key TK generated at step  120 .  
         [0080]     At step  124 , the encryption module  40  encrypts the title key TK with the help of the disc key DK received from the client device  22  and corresponding to the secure blank DVD  8  introduced into the reader/burner device  28 .  
         [0081]     Then, at step  126 , the scrambled multimedia content E TK (MC) and the encrypted title key E DK (TK) are transferred through the secure authenticated channel SAC to the client device  22 .  
         [0082]     Finally, at step  128 , the scrambled multimedia content E TK (MC) and the encrypted title key E DK (TK) are burnt on the data area  12  of the secure blank DVD  8  by the reader/burner device  28  of the client device.  
         [0083]     Steps  120  to  128  can be repeated for several pieces of multimedia content MC, a new title key TK being generated for each piece of content MC (e.g. in case of film burning, one TK can be generated for the movie itself and one TK can be generated for the making-of).  
         [0084]     According to the invention, multimedia content and encryption keys should be exchanged securely between the client device  22 , the remote server  23  and the content provider  24 , for example through a secure authenticated channel SAC.  
         [0085]     Since the client device stores only one or a few master key(s) MK 1  and does not comprise a scrambler module or a title key generator, the client device comprises only a protection system of a memory which is easier to build and is cheaper than a protection system of a processor.  
         [0086]     In the here above description, the Content Scrambling System (CSS) is used to protect the multimedia content burnt on DVD. In variant, other copy protection systems dedicated to DVD or to other storage media can also be used such as for example the Content Protection for Prerecorded Media (CPPM), the Content Protection for Recordable Media (CPRM), the Blue-ray Disc Copy Protection System (BD-CPS) or the Vidi System for DVD+R/+RW discs.  
         [0087]     In variant, the multimedia content MC flowing from the remote server  23  to the client device  22  is super-scrambled over the initial scrambling, e.g. CSS, using a possibly different scrambling algorithm, e.g. AES (for Advanced Encryption Standard). In this case, the client device  22  removes the super-scrambling before burning the initially scrambled multimedia content MC on the data area of the DVD  8 .  
         [0088]     In variant, the remote server  23  comprises the database  46  storing digital data representative of multimedia contents proposed for burning and a processor for retrieving the multimedia content from the database with the help of the identifier CID of the multimedia content. In this case, the remote server does not exchange data with a content provider.  
         [0089]     Advantageously, the multimedia content is delivered securely.  
         [0090]     Advantageously, the client device does not require a more secure environment than legacy DVD players.  
         [0091]     Advantageously, the content is scrambled within the server environment which is usually more secure than the client environment.  
         [0092]     Advantageously, the client device does not need to scramble the content and therefore does not require much processing capabilities.  
         [0093]     Advantageously, the prerecording of a disc key DK (in a secure disc key set) unique to each DVD (or a disc key having a low probability of being common for two different DVD) prevents from attack using bit-to-bit copy. This attack consists in literal copy of the data area (protected by a first disc key DK 1 ) from a first burnt DVD comprising a secure disc key set SDKs 1  onto a second DVD. As the second DVD contains a different secure disc key set SDKs 2 , then a legacy player cannot recover the disc key DK 1  and therefore cannot play back the copy.  
         [0094]     Advantageously, a different title key TK can be used for each burnt DVD and content. This may facilitate content tracking.