Abstract:
The present invention controls copying of an information signal, comprises the steps of prior to recording and/or transmission, applying to the information signal a substantially imperceptible modification representing copy control data including a password securely encoded according to a predetermined algorithm; upon reproduction for copying by a user, deriving (S 1 , S 3 ) the copy control data from the modified information signal; comparing (S 8 , S 9 , S 11 , S 13 , S 15 ) the derived securely encoded password with a reference password securely encoded according to a predetermined algorithm; and enabling (S 5 ) copying of the information signal if the securely encoded password derived from the information signal and the securely encoded reference password have a predetermined relationship, otherwise disabling copying (S 7 ). The reference password is sent to the user via a channel, which is separate from a channel used to send the information signal to the user.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to embedding data in an information signal. Examples of the invention relate to: a method of and system for controlling copying of an information signal; an information signal; a data carrier on which an information signal is recorded; apparatus for modifying an information signal; a reproducing apparatus; and a computer program. 
   2. Description of the Prior Art 
   U.S. Pat. No. 5,161,210 (US Philips Corporation) discloses a system for inhibiting copying of audio signals. An audio signal is divided into frequency sub-bands and sub-band samples are quantized. The quantized samples are combined with samples of an auxiliary signal. The combined audio signal is recorded on a record carrier or transmitted. The auxiliary signal is inaudible in the combined audio signal. An audio signal reproducer having a recording unit also has a unit for detecting the auxiliary signal and generating a record control signal. The recording unit is constructed so that if a record control signal appears on its record control input the recording unit does not record the audio signal. 
   WO 00/51348 (Macrovision) discloses a method and apparatus for inhibiting copying of audio or video signals transmitted over a cable television or direct satellite broadcast or the Internet. The signal is protected from unwanted copying by the combination of a watermark embedded in the signal at the head end together with additional copy protection data inserted in the signal. The additional data is a ticket. If the consumer pays a fee, the signal is transmitted to the consumer. The consumer can record only if the watermark and a mathematical function of the ticket match. The function is for example a hash function. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention there is provided a method of controlling copying of an information signal in a system having a source of the information signal and a device for copying the information signal, the method comprising the steps of: 
   at an information signal modification source, prior to transmission of the information signal to the copying device, generating a copy control password and a related reference password, and applying to the information signal a substantially imperceptible modification representing copy control data including the copy control password securely encoded according to a predetermined algorithm; 
   delivering the modified information signal from the modification source to the copying device via a communications channel; 
   delivering the reference password from the modification source to the copying device via a separate communications channel independent of the modified information signal; 
   upon reception of the modified signal deriving the copy control data from the modified information signal; 
   comparing the derived securely encoded password with the separately provided reference password securely encoded according to a predetermined algorithm; and 
   enabling copying of the information signal if the securely encoded password derived from the information signal and the securely encoded reference password have a predetermined relationship. 
   Thus the present invention provides for conditional control of recording of an information signal instead of, or in addition to, simple denial of any recording and simple complete freedom to record or otherwise copy. Recording may be any form of storage including but not limited to storage on a linear data carrier, for example magnetic tape. 
   Furthermore, the reference password is not in the information signal or on any carrier of the information signal but is provided to the copying device independently of the information signal. 
   The information signal may represent any one of, or a combination of, image information (including still and moving images), audio, video, text, and data which may be executable or otherwise. 
   In examples of the invention the said copy control data includes other data indicating that copying is permitted subject to the provision by a user of a correct reference password. 
   The reference password is preferably securely encoded according to the same algorithm as the password derived from the information signal. The said predetermined algorithm may be an encryption algorithm. Preferably the said algorithm is a hash function in which case the reference password and the password derived from the information signal are the same. The reference password may be delivered to the copying device as a plain password or securely encoded, e.g. encrypted in which case it is decrypted at the copying device. 
   The reference password may be provided via a secure communications channel which is separate from the transmission channel of the information signal. 
   In an example of the invention, the reference password is provided to a user who wishes to copy the information signal. The password may be provided on a secure data carrier, e.g. a smart card or in some other, preferably secure, way. If provided on a smart card the reference password can be kept secure even from the user. The user provides the password to the copying device via an input device , e.g. a keyboard or a card reader. The user may be prompted to provide the password. Preferably that is done in response to the copy control data which indicates that copying is conditional upon the provision of the password. Thus the user is required to take positive action if they wish to copy an information signal for which copying is conditionally allowed. 
   A second aspect of the invention provides a method of applying copy control data to an information signal comprising the steps of: 
   determining whether copying of the information signal is allowed, not allowed or conditionally allowed; and 
   applying to the signal a substantially imperceptible modification representing copy control data, the copy control data comprising 
   a) first data if copying is allowed, 
   b) second data if copying is not allowed, and 
   c) third data if copying is conditionally allowed, 
   the third data including at least a password securely encoded according to a predetermined algorithm. 
   A third aspect of the invention provides a method of controlling the operation of a signal copying device having a recording unit controlled by a processor, the copying device being operable to record an information signal produced by the method of the second aspect, the method comprising the steps of: 
   using the processor to derive the copy control data from the information signal and to determine whether the control data is the first, second or third data and to 
   a) allow the recording unit to record if the first data is present in the information signal, 
   b) disable the record unit if the second data is present in the information signal; and 
   c) allow the recording unit to record if the third data is present in the information signal and a reference password is provided which when securely encoded by a predetermined algorithm has a predetermined relationship to the said securely encoded password of the third data. 
   The second and third aspects of the present invention provide copy control data which explicitly indicates, and allows, conditional control of recording of an information signal and, in addition, simple denial of any recording and simple complete freedom to record. 
   It is possible that the copying device receives an information signal which does not have an imperceptible modification representing copy control data. In that case the copying device may be arranged to allow copying (or in the alternative not allow copying). 
   The copy control data may be used to control the copying device to operate in a predetermined manner. For example it may control the form of any copies for example indicating the form of copy control data to be included in any copy. 
   These and other aspects of the invention are set out in the accompanying claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the invention will be apparent from the following detailed description of illustrative embodiments which is to be read in connection with the accompanying drawings, in which: 
       FIG. 1  is a schematic block diagram of an illustrative apparatus for applying a modification to an information signal in accordance with the invention; 
       FIG. 2  is a schematic block diagram of an illustrative signal reproducing and recording apparatus in accordance with the invention; 
       FIGS. 3A  and B are flow diagrams illustrating an operation performed by a processor of the apparatus of  FIG. 2 ; 
       FIG. 4  is a schematic block diagram of an illustrative system for controlling copying in accordance with the invention; 
       FIG. 5  is a schematic block diagram of an illustrative watermarking apparatus useful in the apparatus of  FIG. 1 ; and 
       FIG. 6  is a schematic block diagram of an illustrative apparatus useful in the apparatus of  FIG. 2  for detecting a watermark and extracting data therefrom. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , a signal modifying apparatus  30  comprises a source  2  which produces an information signal which may be for example an audio signal, a video signal, an audio/video signal, a data signal and/or an image signals. The source  2  may be any suitable source for example a signal reproducer which reproduces the signal from a record or an original source for example a camera in the case of video or microphone in the case of audio. 
   For ease of explanation the following description assumes the information signal is a video signal, but the invention is not limited to video. 
   A watermarking apparatus  6  receives the video signal from the source  2  and applies to it copy control data from a source  4 . In this example the watermarking apparatus  6  embeds the copy control data in the video in such a way that the embedded data is substantially imperceptible in the video. Watermarking techniques are known in the art of video and an example of such a technique is described below with reference to  FIG. 5  but any other suitable known watermarking technique may be used. 
   The watermarking apparatus may optionally embed as a watermark provenance data, metadata, or other data in addition to the copy control data. 
   The copy control data is embedded for the purpose of controlling copying of the video signal. In one example of the invention, the data comprises a code h which is a hash function H(p) of a password p: i.e. the code h=H(p). In another example of the invention, the control data h is associated with further data, for example 01 or 10, which indicates the presence of the code h. 
   In an example of the invention, the copy control data comprises a selected one of the following copy status codes: 
   00 which indicates no copying is allowed; 
   11 which indicates copying is freely allowed; and 
   01 or 10 together with code h which indicates that copying is allowed provided a reference password is provided which when hashed by the hash function H matches the code h. 
   The codes 00, 11 and 10 or 01 are examples of codes for simplicity of explanation. More complex codes may be used, Preferably codes which are unlikely to occur by chance are used. 
   The following description describes the currently preferred example but the invention is not limited to that example. 
   The video into which the control data is embedded by the watermarking apparatus  6  is fed to a recorder or transmitter  8 . If fed to a recorder, the recorder  8  may record the watermarked video on a data carrier for example a disc or tape or semiconductor memory. If transmitted, the transmitter  8  may be for example a broadcast apparatus or a server which transmits the watermarked video to a distribution system. 
   Reference numeral  10  indicates a schematic representation of a distribution system which may be amongst other examples: an electronic communications network  15  for transmitting the video e.g. a broadcast network, a PSTN or the Internet; or a physical distribution network via which tapes  13  or discs  11  or other data carriers on which the video is recorded are distributed. 
   The code source  4  also provides a reference password which in this example is the password p. The reference password p is fed via an interface  5  to a secure data carrier, e.g. a smart card SC for delivery to a user of the video separately from the video. It will be appreciated that the reference password could be delivered by other means, e.g. another form of data carrier, on paper through the post or via the Internet or telephone system. 
     FIG. 2  illustrates a copying device which in this example is a reproducing and recording apparatus  32 . The apparatus  32  comprises a source  12  of video which may or may not be watermarked. The source  12  may reproduce the video from a data carrier or receive the video from a broadcast or other communications system as described above. The video is applied to a recording unit  22 . The recording unit  22  is controlled by a record control signal produced by a control processor  14 . 
   Assume the video is watermarked. 
   The control processor  14  comprises a watermark processor  16  which detects the watermark and derives the copy control data therefrom. Watermark processors capable of doing that are known and an example is described with reference to  FIG. 6  below. A processor  18  decodes the copy control data and applies the appropriate record control signal to the recording unit. The control processor also has an input device  20  for entering the reference password. The input device may be a keyboard, smart card reader, an interface with an electronic communications channel, amongst other examples. In addition a display  17  may be provided. The display is used in an example of the invention to prompt the user to enter the reference password via the input device, for example by inserting the smart card SC into the input device  20 . 
   Referring to  FIGS. 3A  and B, the control processor  14  operates as follows: 
   In step S 1  the watermark processor  16  detects the watermark and derives the copy control data. In step S 3  the processor  18  determines the value of the copy control data. 
   If the copy control data is: 
   00 then the record control signal is set S 7  to disable the recording unit; 
   11 then the record control signal is set S 5  to enable recording by the recording unit; and 
   01 or 10 together with a code of any value then the user is requested S 8  to provide a reference password p′. 
   Referring to  FIG. 3B , in step S 81 , the processor  18  detects whether the copy control data includes the code 01 or 10 indicating copying is allowed if the reference password is provided. If so, it causes a prompt to be displayed S 82  by the display  17  requiring the user to enter the password. 
   If a reference password p′ is entered S 9  via an input device  20  the processor performs S 11 A hash function H(p′) on the password and compares S 13  the hashed reference password with the hash value h derived from the watermarked video. If the hashed values are the same S 15  then copying is allowed S 5 . If the hashed values are not the same S 15  then the recording unit is disabled S 7 . 
   If recording is enabled because the reference password is correct or because the copy control data=11, then the watermarked video including the original copy control data is recorded to provide copy control of the copy. 
   The user of the reproducing apparatus  32  requires the reference password to copy video which has as copy control data the code 01 or 10 plus the hash value h. As will be described with reference to  FIG. 4 , the reference password is supplied to the apparatus  32  separately from and independently of the watermarked video. In this example the password is provided on the smart card SC which is read by a card reader  20  which is the input device  20 . This enables the password to be provided to a user and kept secret even from the user. 
   The smart card may be provided to a user on payment by the user of a fee. 
   The video form the source  12  may not have a copy control watermark. If the watermark processor  16  detects the absence of such a watermark, it indicates that to the processor  18 ; see step S 1  of  FIG. 3A ). The absence of the watermark may be regarded as either indicating copying is not allowed so that the processor outputs code 00 to processor  18 . Currently it is preferred that the absence of the watermark indicates copying is allowed. Thus the processor  16  outputs code  11  to the processor  18 . 
   The system of  FIGS. 1 and 2  is illustrated by way of example as a special purpose system but could be implemented using programmable data processors. 
   In the system of  FIG. 4 , the modifying apparatus  30  and the reproducing apparatus  32  are linked by communications interfaces I/F to a communications network  38  which is for example the Internet. There may be many reproducing apparatus  321  to  32   n  connected to the network  38 . For simplicity of description, it is assumed that the apparatus  30  is controlled by a seller and the user of the reproducing apparatus  32  is a buyer. Watermarked video is sent to buyers for example on a disc D via a distribution channel  10  for example a postal service or shop. If a buyer wishes to copy the watermarked video and it is watermarked with the copy control code 01 or 10 plus the hash value h, then the copy function in the reproducing apparatus  32  is disabled until the buyer pays a fee. The fee may be paid via the network  38  and a server  34 , which for that purpose is linked to a financial institution  36 , e.g. a credit card company. 
   The copy control hash values and/or the password may be unique to each user for increased security and traceability of unauthorised copies. 
   The payment of the fee is communicated to the seller via the network  38 . The seller then releases the reference password for example on a smart card SC or in a secure manner via the communications network  38 . Alternatively, the server may release the reference password on a smart card or via the network  38 . For that purpose the server may communicate with the seller to obtain the reference password. It will be appreciated that the reference password could be delivered by other means, e.g. on a data carrier, on paper through the post or via the Internet or telephone system. Preferably the reference password is encrypted before transmission to the copying device  32  in which case it is decrypted at the copying device. Any known encryption system may be used. 
   The present invention assumes that all reproducing and recording apparatus are, equipped with watermark detection and decoding apparatus which disables the recording unit of the apparatus. 
   Watermarking,  FIG. 5   
     FIG. 5  illustrates the watermarking apparatus denoted as embedder  120  in more detail. The watermark embedder  120  comprises pseudo-random sequence generator  220 , an error correction coding generator  200 , a wavelet transformer  210 , an inverse wavelet transformer  250 , a first combiner  230 , a data converter  225  and a second combiner  240 . The wavelet transformer  210  includes a frame store FS 1 . The inverse transformer  250  includes a frame store FS 2 . The frame store FS 1  stores a frame of unmodified coefficients Ci. The frame store FS 2  stores a frame of modified coefficients Ci′. 
   The error correction coding generator  200  receives the copy control data and outputs an error correction coded copy control data to the first combiner  230 . The pseudo-random sequence generator  220  outputs a pseudo-random binary sequence (PRBS) Pi, where i is the i th  bit of the sequence, to the first combiner  230 . The PRBS has a length L×J of bits where J is the number of bits in the error correction encoded copy control data. Each bit j of the error correction encoded copy control data then modulates a section of length L of the PRBS. The first combiner  230  logically combines the error correction encoded copy control data with the PRBS to produce a watermark having bits Ri. A bit Wj=0 of the error correction encoded copy control data inverts L bits of the PRBS. A bit Wj=1 of the error correction encoded copy control data does not invert the PRBS. Thus bits Wj of the error correction encoded copy control data are spread over L bits of the PRBS. The data converter  225  converts binary 1 to symbol +1 and binary 0 to symbol −1 to ensure that binary  0  bits contribute to a correlation value used in the decoder of  FIG. 5 . 
   The wavelet transformer  210  receives the video image I from the source  110  and outputs wavelet coefficients Ci to the second combiner  240 . 
   The second combiner  240  receives the watermark Ri, the wavelet coefficients Ci and watermark strength αi and outputs modified coefficients Ci′ where
 
 Ci′=Ci+αi Ri 
 
   The inverse wavelet transformer  250  receives the modified coefficients Ci′ and outputs a spatial domain watermarked image I′. 
   The embedder includes an ECC generator  200 . The use of error correction coding to produce an error correction coded copy control data is advantageous since it allows the copy control data  175  to be reconstructed more readily should some information be lost. This provides a degree of robustness to future processing or attacks against the watermark. The use of a pseudo-random sequence Pi to generate a spread spectrum signal for use as a watermark is advantageous since it allows the error correction coded copy control data  205  to be spread across a large number of bits. Also, it allows the watermark to be more effectively hidden and reduces the visibility of the watermark. Applying the watermark to a wavelet transform of the image is advantageous since this reduces the perceptibility of the watermark. Furthermore, the strength of the watermark is adjusted by αi to ensure that the watermark is not perceptible. 
   Detecting Copy Control Data in Watermarked Video,  FIG. 6   
   The operation of the watermark processor denoted as decoder  140  will now be explained in more detail with reference to  FIG. 6 . The watermark decoder  140  receives the watermarked image I′ and outputs the restored copy control data. The watermark decoder  140  comprises a wavelet transformer  310 , a reference pseudo-random sequence (PRBS) generator  320 , a correlator  330 , a selector  340  and a error correction coding decoder  350 . The PRBS generated by the generator  320  is identical to that generated by the PRBS generator  220  of  FIG. 2  and converted by a data converter (not shown) to values +1 and −1 as described above. 
   The wavelet transformer  310  receives the watermarked image I′ and, in known manner, outputs the modified wavelet coefficients Ci′. The correlator  330  receives the reference pseudo-random sequence PRBS having symbols Pi of values +1 and −1 from the pseudo-random sequence generator  320 , and the wavelet coefficients Ci′ and outputs a watermark image bit correlation sequence  335 . The watermarked image bit correlation sequence is determined in the following way. 
   The modified wavelet coefficients Ci′=Ci+α i R i  where R i  are bits of PRBS modulated by error-correction encoded bits Wj of copy control data. Each bit Wj modulates L bits of PRBS. There are JL bits in the modulated PRBS. For each error correction encoded bit Wj, the correlator  330  calculates a correlation value 
             S   j   ′     =       ∑     i   =     jL   +   1         jL   +   L       ⁢       Ci   ′     ·   Pi             
where j=0, 1, 2 . . . J−1, and J is the number of error correction encoded bits. A sequence  335  of correlation values S′ j  is produced.
 
   The correlation sequence  335  is received by the selector  340  which outputs an uncorrected copy control data  345 . The selector  340  outputs a bit value “1” for a value of S′ greater than 0 and a bit value “0” for S′ less than or equal to 0. The error correction code decoder  350  receives the uncorrected copy control data  345  and in known manner outputs the restored copy control data  145 . 
   The reference PRBS Pi is synchronised with the modulated PRBS in the watermarked image. For that purpose a synchroniser (not shown) is used. Such synchronisation is known in the art. 
   Modifications 
   Although  FIGS. 5 and 6  give an example of watermarking using Wavelet coefficients, the invention is not limited to Wavelets but can be implemented using other watermarking techniques including the use of DCT coefficients. 
   Although the invention has been illustrated by reference to  FIGS. 1 to 4  which show schematics of special purpose hardware, it is envisaged that the invention may be implemented in software on programmable machines. Thus the invention also encompasses software which when run on suitable data processing equipment implements the functions described herein. 
   The information signal whether stored on a data carrier or sent as a signal via a communications channel may include several parts representing different sections of content or different items of content. For example a disc or tape typically has several tracks. In an embodiment of the invention each section may have its own copy control data embedded as an imperceptible watermark each with its own copy status and/or its own password. For example if a disc has tracks one to four the copy control data may be as follows: 
                                               Track   Copy Status Data   Password                           Track 1   00   —           Track 2   01   nnnnnn           Track 3   01   mmmm           Track 4   11   —                        
Thus track  1  can be copied freely. Track  2  may be copied if the password nnnnn is provided. Track  3  may be copied if password mmmmm is provided, Copying of track five is not allowed. A user may be provided with only one of the passwords.
 
   In a further embodiment, the copy status codes define copying rights for a user in addition to copying allowed, not allowed and conditionally allowed. For example code 01(plus the password) may indicate copying is conditionally allowed but the copies retain the original copy control data signifying the conditional copying status whereas code  10  may indicate copying is conditionally allowed and the copy may be freely copied, the copy control data not being retained in the first copy. Code  10  may be used to protect content whilst in transit between a supplier and the user for example. 
   Other status codes with passwords forming the copy control data may indicate other copying status. 
   In embodiments in which the copy status data is changed, the watermark must be amendable. Watermarks which may be removed are described in for example co-pending European patent application 1215880. The processors  16  and  18  in the control processor  14  of  FIG. 2  are arranged to remove the original watermark and replace it with a new one. The new one may be permanent or removable. 
   Although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope and spirit of the invention as defined by the appended claims.