Abstract:
A copy prevention method and apparatus for a digital video system is disclosed including the steps of: (a) adding a header area of a header start code and key field to a reproduced bit stream: (b) decrypting and transmitting the bit stream to which the header area is added: (c) detecting a key field of the decrypted and transmitted bit stream and detecting copy prevention information; and (d) encrypting the bit stream according to information detected from stop (c) and recording it an tape.A copy protection method including receiving encrypted digital data to be recorded, key information which is required to decrypt the encrypted digital data, and first copy control information which indicates a copy permission status of the encrypted digital data; and recording the encrypted digital data, the key information, and second copy control information on a digital recording medium, based on at least a status of the first copy control information.

Description:
This is a continuation of application Ser. No.  09 / 053 , 288 , filed Apr.  1 ,  1998 , now U.S. Pat. No.  6 , 028 , 932 , which is a continuation of Ser. No.  08 / 562 , 042 , filed Nov.  22 ,  1995 , now U.S. Pat. No.  5 , 761 , 302 , issued Jun.  2 ,  1998 .This is a reissue application of U.S. Pat. No.  6 , 347 , 144 , which is a continuation of application Ser. No.  09 / 053 , 288 , filed Apr.  1 ,  1998 , now U.S. Pat. No.  6 , 028 , 932 , which is a continuation of Ser. No.  08 / 562 , 042 , filed Nov.  22 ,  1995 , now U.S. Pat. No.  5 , 761 , 302 . The contents of all these applications are incorporated by reference. The above-identified applications also claim priority to Korean Application No.  94 - 31373  filed on Nov.  26 ,  1994 . 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a copy prevention technology for a digital video system, and more particularly, to a copy prevention method and apparatus for a digital VCR to which encryption is introduced to display a picture only in a VCR internally containing a corresponding encryption code, thereby preventing tape from being copied. 
     General copy prevention methods for analog VCR are presented in U.S. Pat. Nos. 4,819,098, 4,571,642 and 4,577,216. 
     First, U.S. Pat. NO. 4,819,098 discloses a method in which an interference signal is inserted into a video waveform in an automatic gain control circuit (AGC) of a VCR. Here, the inserted signal does not affect the AGC of its monitor but has the AGC of the VCR record an accurate level of signal on a video tape. 
     In U.S. Pat. No.  4 , 571 , 642    4 , 577 , 216 , there is presented a method in which a phase noise or other corrected signal is inserted into the chrome  chroma burst of a video waveform. 
     However, all the conventional technologies insert a distributing signal to an analog signal using the difference between a circuit of a monitor and a corresponding circuit of a VCR. Some VCRs may perform copy normally despite of copy prevention. Some monitors cannot display images of the original video tape. A conventional copy prevention introduced to an analog VCR system is hard to be applied to digital storage media (DSM). 
     Specifically, in a satellite or high-definition TV decoder, as shown in  FIG. 2 , an MPEG bit stream received by a digital VCR is constructed to transmit a transport header, packetized elementary stream (PEG)  (PES) header and audio and video data respectively or simultaneously. 
     The PES header contains a PES header flag area of 14 bits which is a field for DSM such as digital VCR, and a PES header field having a variable length. The PES header flag area includes 1-bit copyright (CR) flag, 1-bit original-or-copy (OC) flag, 2-bit PD flag, 1-bit TM flag, and 1-bit AC flag. 
     The PES header field varies in length, and part thereof is set by the PC  PD, TM and AC flags. A PTS/DTS area is not present if the value of the PD flag is “00”. It is 40 bits if the value “10”. If the value is “11”, the area is 80 bits. A DSM trick mode field is not present if the TM flag is “0”. If the flag is “1”, the field is 8 bits. An additional copy information field is 8 bits if the AC flag is “1”. 
     When recording is carried out by the satellite receiver or high-definition TV decoder and compressed video data is encoded in encoder  101 , it is converted into a packet form in packet processing portion  122    102  as shown in FIG.  1 . If the compressed audio data is encoded in audio encoder  103 , it is converted into a packet form in packet processing portion  104 . 
     When the outputs of packet processing portions  102  and  104  are multiplexed in transmission multiplexer  105 , a fixed transmission stream shown in  FIG. 2  is output to a digital VCR. In this case, for copy prevention, a public-key encryption is applied which is suggested in U.S. Pat. No. 4,200,770. This solves disadvantages in key management or key distribution when a conventional block-cipher or stream cipher algorithm such as data encryption standard (DES) encrypts or decrypts only with a secret key. 
     This public-key encryption system has all users U hold unique encryption algorithm E PK   U  and description algorithm D PK   U . Here, encryption algorithm E PK   U  for the public-key is opened as a public-key to key supply portion  107 . Decryption algorithm D PK   U  for secret key is kept in secret. The characteristics of E PK   U  and D PK   U  are as follows. 
     First, with respect to all users U and message m transmitted, D PK   U (E PK   U (m))=m. 
     Second, encryption algorithm E PK   U  and decryption algorithm D PK   U  do not require complicated calculation. 
     Third, it is impossible to find D PK″   U  satisfying D PK″   U  (E PK   U (m))=m from encryption algorithm E PK   U . 
     In the encryption system having the above characteristics, as shown in  FIG. 3 , when user A transmits message m to user B, encryptor  106  receiving public-key algorithm E PK   U  for user B&#39;s public-key from key supply portion  107  encrypts message m (E PK   U (m))=c) and transmits the result to decrypter  109  via a public channel. Here, the public channel indicates a channel in which transmitted data is not kept in secret. 
     Key decrypter  108  receiving the key information from key supply portion  107  outputs an algorithm D PK   B , corresponding to encryption algorithm E PK   B , decryptor  109  decrypts (D PK   B (c))=m) the output of encrypter  106  with decryption algorithm D PK   B , and then transmits to user B. In other words, only user B can decrypt decryption algorithm D PK   B  corresponding to encryption algorithm E PK   B . 
     A concept developed from the public-key encryption is presented in U.S. Pat. No. 4,405,829. This public-key encryption system is called RSA system. A method in which the RSA public-key encryption is efficiently calculated via batch processing is presented in U.S. Pat. No. 4,964,164. 
     However, this public-key encryption is inappropriate for high-velocity encryption. A CA system is intended to present  prevent illegal view viewing. However, there is no method of protecting a program distributed through a digital storage medium such as a digital VCR. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to an illegal copy prevention method and apparatus for a digital video system in which, in copy tape, encrypted key information is transmitted and recorded so that a copied tape is reproducible only in a VCR having a corresponding encrypted key information, thereby prevented copy. 
     To accomplish the object of the present invention, there is provided a copy prevention method for a digital video system comprising the steps of: (a) adding a header area of a header start code and key field to a reproduced bit stream; (b) decrypting and transmitting the bit stream to which the header area is added; (c) detecting a key field of the decrypted and transmitted bit stream and detecting copy prevention information; and (d) encrypting the bit stream according to information detected from step (c) and recording it on tape. 
     For the object of the present invention, there is provided a copy prevention apparatus for a digital video system comprising: a reproduction block for adding key information to a reproduced bit stream, and decrypting and transmitting it; and a recording block for searching key information of the bit stream transmitted from the reproduction block is extract copy prevention information, and encrypting and recording the bit stream according to the extracted copy prevention information. 
     The reproduction block comprises: reproduction means for reproducing data recorded on tape; key insertion means for adding key information to the bit stream of the reproduction means; and decryption means for decrypting the output of the key insertion means and transmitting it to a recording-side VCR. 
     The recording block comprises: key detecting/correcting means for detecting key information from the transmitted bit stream of a reproducing-side VCR: copy prevention information detecting means for searching the key information detected from the key detecting/correcting means to detect copy prevention information; encrypting means for encrypting the bit stream according the copy prevention information of the copy prevention information detecting means; and recording means for recording the bit stream encrypted in the encrypting means. 
     The copy prevention information detecting means comprises: a PES header detecting portion for detecting a PES header from parallel data output from the key detecting/correcting means; and a copy prevention information extractor enabled by a PES header detection signal of the PES header detecting portion to detect an additional copy information field. 
     To accomplish the objects of the present invention, there is provided a copy protection method comprising receiving encrypted digital data to be recorded, key information which is required to decrypt the encrypted digital data, and first copy control information which indicates a copy permission status of the encrypted digital data; and recording the encrypted digital data, the key information, and second copy control information on a digital recording medium, based on at least a status of the first copy control information. 
     According to another object there is provided a copy protection method comprising ( a )  providing digital content, key information for controlling at least decryption of the digital content, and copy control information for controlling copying of the digital content; and (b) recording the key information, the copy control information and the digital content on a digital recording medium.    
     According to another object there is provided a copy protection method comprising receiving encrypted digital content, first key information for decrypting the encrypted digital content, and copy control information which indicates whether or not a copy of the encrypted digital content is permitted; and controlling a recording or a reproducing of the encrypted digital content on or from a recording medium based on the first key information and/or the copy control information. 
     According to another object there is provided a copy protection apparatus comprising a receiving unit configured to receive encrypted digital data to be recorded, key information which is required to decrypt the encrypted digital data, and first copy control information which indicates a copy permission status of the encrypted digital data; and a recording unit configured to record the encrypted digital data, the key information, and second copy control information on a digital recording medium, based on at least a status of the first copy control information. 
     According to another object there is provided a copy protection apparatus comprising a providing unit configured to provide digital content, key information for controlling at least decryption of the digital content, and copy control information for controlling copying of the digital content; and a recording unit configured to record the key information, the copy control information and the digital content on a digital recording medium. 
     
       According to another object there is provided a copy protection apparatus comprising a receiving unit configured to receive encrypted digital content, first key information for decrypting the encrypted digital content, and copy control information which indicates whether or not a copy of the encrypted digital content is permitted; and a controlling unit configured to control a recording or a reproducing of the encrypted digital content on or from a recording medium based on the first key information and/or the copy control information. 
      
     
    
    
     
       BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS 
         FIG. 1  is a block diagram of a conventional packet processing apparatus; 
         FIG. 2  shows an example of a general transmission stream; 
         FIG. 3  is a block diagram of a conventional public-key encryption system; 
         FIG. 4  shows connections of systems of the present invention; 
         FIG. 5  is a block diagram of a copy prevention apparatus for a digital video system of the present invention; 
         FIG. 6  is a block diagram of the copy prevention information detector of  FIG. 5 ; 
         FIG. 7  is a circuit diagram of the PES header detector of  FIG. 6 ; 
         FIGS. 8A-8F  are waveform diagrams of input/output at the respectrespective portions of  FIG. 7 ; 
         FIG. 9  is a circuit diagram of the copy prevention information extractor of  FIG. 4 ; 
         FIGS. 10A-10G  are waveform diagrams of input/output at the respective portions of  FIG. 9 ; and 
         FIGS. 11A-11D  show examples of a bit stream of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, a preferred embodiment of the present invention will be described below with reference to the attached drawings. 
     Referring to  FIG. 5 , a copy prevention apparatus of the present invention comprises a reproducing portion  1  for reproducing data recorded on tape, a key inserting portion  2  for adding a tape header start code and key field at the front end of a bit stream of reproducing portion  1 , a decrypting portion  3  for decrypting the output of key inserting portion  2  and transmitting it as parallel data, a key detecting/correcting portion  4  for detecting a key field from the parallel data transmitted from decrypting portion  3 , a copy prevention information detecting portion  5  for detecting a PES header from the key field detected and extracting copy prevention information, a copy prevention information correcting portion  6  for correcting the output of copy prevention information detecting portion  5  if necessary, an encrypting portion  7  for encrypting the output of copy prevention information correcting portion  6 , and a recording portion  8  for recording the output of encrypting portion  7  on tape. 
     As shown in  FIG. 6 , copy prevention information detecting portion  6    5  comprises a PES header detecting portion  10  for searching the parallel data in synchronization with a clock clk to detect the PES header, and a copy prevention information extractor  20  enabled by the PES header signal of PES header detecting portion  10  to detect the copy prevention information field. 
     Referring to  FIG. 7 , PES header detecting portion  10  comprises first and second flipflops  11  and  12  for sequentially delaying the parallel data according to clock elk  clk, a packet start code detector  13  for searching the parallel data and the output of first and second flipflops  11  and  12  to detect the packet start code of the PES header, a stream ID detector  14  for searching the output of second flipflop  12  to detect the stream ID of the PES header, a delay  15  for sequentially delaying the output is-pscp of packet start code detector  13  according to clock clk, and a detection signal generator  16  for logically multiplying the outputs of delay  15  and stream ID detector  14  and outputting a PES header detection signal is-PES-header. 
     As shown in  FIG. 9 , a copy prevention information extractor  20  comprises a D-flipflop  21  for holding the parallel data output from PES header detector  10 , a D-flipflop  22  for holding PES header detection signal is-PES-header of PES header detector  10 , a D-flipflop  23  cleared by the output of D-flipflop  22  and holding voltage (+5V) by a CR signal of the output of D-flipflop  21  and outputting a signal LCR, a D-flipflop  24  cleared by the output of D-flipflop  22  and holding voltage (+5V) by an OC signal of the output of D-flipflop  21  and outputting a signal LOR  LOC, a copy prevention information position operator  25  for searching the parallel data of PES header detector  10  and calculating the position of an additional copy information field, a counter  26  for counting the output of copy information position operator  25 , and a D-flipflop  27  for holding the additional copy information field of the output of D-flipflop  21 . 
     The operation and effect of the present invention will be explained below. Generally, in case of reproducing or copy recording data on tapa  tape, connections between systems are made as shown in FIG.  4 . 
     With those connections, an MPEG bit stream reproduced from VCR A is input to a satellite receiver or high-definition TV so that it cannot be recognized whether the stream is displayed on a screen or input to VCR B and recorded on another video tape. 
     For this reason, according to the present invention, in case that the bit stream reproduced from VCR A is copied from VCR B, information on copy prevention is transmitted to VCR B from VCR A. VCR B analyzes this information which is recorded with the bit stream. 
     Here, the insertion position of the copy prevention information contained in a GA bit stream is very limited because it must not affect decoding of the decoder of the satellite receiver or high-definition TV so that an image is displayed normally on a monitor. The copy prevention information may be inserted into the front end of the MPEG bit stream or inside the PES header. 
     When the MPEG bit stream is decoded in units or  of group of picture (GOP), the respective GOPs are classified by their cop  GOP start codes. This is useful in transmitting initialization data to a recording-side VCR because decoding is never affected even when a slight amount of data is added to the front end of the MPEG bit stream. 
     The case of inserting the copy prevention information into the PES header is useful in repeated transmission of information because copy prevention of a recording medium such as DSM is decided using CR and OC flaps of the PES header and additional copy information field. In this case, there are a variety of copy preventing methods. 
     First, when a mode of “No Copy” is detected from the additional copy information field of the PES header, VCR B is not able to enter its recording mode. 
     Second, when a mode of “Copy Permitted” is detected in order to implement a copy prevention such as DAT mode, VCR B records but “No Copy” mode is recorded in the additional copy information field to interrupt recopying from a copying tape. This means that a secondary source tape can be made, but a third source tape cannot. 
     Third, for “Back-up Copy”, tape B copied from VCR B is reproducible normally only in VCR A. According to this method, reproducing-side VCR A encrypts the bit stream with its own inherent key and records it on tape so that only reproducing-side VCR A decrypts the MPEG bit stream recording on the tape. For every VCR set, a unique key is provided, encrypted by VCR&#39;s key and recorded on tape B. However, the VCR set for recording tape B is VCR B and tape B is encrypted by VCR A&#39;s key so that VCR A&#39;s key needs to be transmitted to VCR B with GA bit stream. 
     Accordingly, when the key information of VCR A is transmitted as a header in advance prior to the bit stream in the “Back-up Copy”, it is recorded at the front end of tape B, which satisfies the insertion position of the copy prevention information mentioned before. 
     Here, as shown in  FIG. 2 , the position of the additional copy information field is varied within the PES header according to whether presentation time stamp (PTS)/Decoding time stamp (DTS) and DSM trick mode field are present or not. This varied position must be compensated. Here, information transmitted through the additional copy information is a copy prevention method to be performed by recording-side VCR B. 
     In case of recording the bit stream shown in  FIG. 11A  in the method of “Back-up Copy”, the format of the bit stream recorded on tape is determined as shown in FIG.  11 B. 
     Here, a header area added to the front of the MPEG bit stream is formed with a tape header start code, that is, the header identifier code, and a key field for storing key information. In case of encrypting the MPEG bit stream in units of GOP, encryption blocks are classified by the packet start code prefix and stream ID of the PES header. The encryption block is a basic unit of encryption and can change whether encryption is performed in units of the encryption block, and encryption algorithm and key selection. Here, the encryption blocks must not be encrypted until the additional copy information field of the PES header. Encryption is performed until the end of the encryption block after the additional copy information field. The first ‘transmission header’ is not encrypted. 
     The operation of performing the “back-up Copy” mode by adding the header will be described below. 
     First, in copying, when recording data of tape A is encrypted, reproducing-side VCR A decrypts it using the key information of the key field so as to make message m. Its key information is added to the header and transmitted in the format of FIG.  11 C. 
     Recording-side VCR B records the key information transmitted from reproducing-side VCR A on the header of copying tape B and then records the encrypted bit stream. Here, when the key information is transmitted from reproducing side to recording side, for security, a public-key encryption may be employed to the system because the information may be exposed to a pirate. 
     Such public-key encryption system ensures the secret of data even though the public-key is exposed but cannot be processed in real-time due to a great amount of calculation. Therefore, this system is not improper when the MPEG bit stream is encrypted directly. The “Back-up Copy” can be implemented when the MPEG bit stream is encrypted using a block-cipher algorithm or stream-cipher algorithm such as DES and a key used is encrypted in the public-key encryption. 
     In this case, every VCR u incorporates encryption algorithm E PK   U  corresponding to the public-key and decryption algorithm D PK   U  corresponding to the secret key. Encryption algorithm E PK   U  takes a power key of VCR u, and decryption algorithm D PK   U  an internal key of VCR u. 
     Here, the internal key may be opened to the public. Reproducing-side VCR A transmits the internal key on the key field of the header because another VCR encrypts using the internal key. Recording-side VCR B randomly selects a key Y used in the block-cipher algorithm such as DES and encrypts it with the public-key encryption system using an external key E PK   A . The result is recorded on the key field of copying tape B. 
     Sequentially, the data is divided into encryption blocks and encrypted and recorded in the block-cipher algorithm using key Y. In this method, the bit stream of  FIG. 11D  is recorded on copying tape B. 
     When copying tape B is reproduced in reproducing-side VCR A, key Y can be restored by decryption D PK   A [E PK   A  (Y)] in which data is decrypted properly. In other VCRs, key Y cannot be found, which disables the decryption of the bit stream. 
     AsAn embodiment of the present invention, shown in  FIG. 5 , for performing such an operation will be described below. 
     When playback starts for tape copying, reproducing portion  1  detects data recorded on tape as shown in  FIG. 11A , and amplifies it by a predetermined level. As shown in  FIG. 11B , key inserting portion  2  adds a header having a tape header start code and key field to the GA bit stream of reproducing portion  1  shown in FIG.  11 A. Copy prevention information is loaded on the additional copy information field of the PES header to form a format shown in FIG.  11 C. Here, decrypting portion  3  decrypts the bit stream formed in key inserting portion  2  and transmits it as parallel data to the recording-side VCR via an interface. 
     When the bit stream of  FIG. 11C  is transmitted to the recording-side VCR via the interface, key detecting/correcting portion  4  detects the key field added to the bit stream and corrects the key field if necessary. 
     Copying prevention information detecting portion  5  searches the PES header area to detect the additional copy information field. Here, though a slight amount of information is recorded in the additional copy information field, redundancy is provided in several areas of the bit stream to increase reliability of information transmitted. 
     Copy prevention information detecting portion  5  extracts the value of AC flag from the PES header flag in order to calculate the position of the additional copy information field because it varies within the PES header. Here, when copy prevention information correcting portion  6  corrects the output of copy prevention information detecting portion  5 , encrypting portion  7  performs encryption using the block-cipher algorithm such as DES. Here, copy prevention information correcting portion  6  performs correction while the input data is stored in a RAM. Accordingly, encrypting portion  7  records the encrypted bit stream on tape in recording portion  8 . Because the key information of the reproducing-side VCR is added on the copying tape, only a VCR having this key information can reproduce tape normally. 
     As shown in  FIG. 6 , in copy prevention information detecting portion  5 , PES header detecting portion  10  searches the output of key detecting/correcting portion  4  and outputs a header detection signal is-PES-header. After header detection signal is-PES-header is input, copy prevention information extractor  20  detects the additional copy information field and OC and CR flags. 
     PES header detector  10  for detecting the PES header is formed as shown in FIG.  7 . When bit stream data_in is input as shown in  FIG. 8A , first flipflop  11  synchronized to clock clk is delayed for a predetermined time to output the bit stream delayed as shown in FIG.  8 B. Second flipflop  12  delays the output of first flipflop  11  by are predetermined time and outputs the bit stream delayed as shown in FIG.  8 C. 
     Here, packet start code detecting portion  13  searches the bit stream shown in FIG.  8 A and the output of first and second flipflops  11  and  12  shown in  FIGS. 8B and 8C  in order to detect the packet start code of the PES header. When detection signal is-pscp is output as shown in  FIG. 8D , delay  15  in which flipflops are coupled at multi-stages delays it sequentially according to clock clk. 
     Meanwhile, stream ID code detector  14  searches the output of second flipflop  12  and detects the stream ID area of the PES header. Then, detection signal is-sid shown in  FIG. 8E  is output to detection signal generator  16 . Detection signal generator  16  logically multiplies the output of delay  15  and stream ID code detector  14 , and the flipflops hold the output of the AND gate according to clock clk so that PES header detection signal is-PES-header is output to copy prevention information extractor  20 , as shown in FIG.  8 P   8 F. 
     Here, copy prevention information extractor  20  for detecting the copy prevention information is formed as shown in FIG.  9 . When the parallel data output from PES header detector  10  and shown in  FIG. 10A  is held and output as shown in  FIG. 10B , D-flipflop  22  synchronized to PES header detection signal is-PES-header of PES header detector  10  shown in  FIG. 8F  holds voltage +5V so that a HIGH signal is output to the clear ports of D-flipflops  23 ,  24  and  27  to release the clear states. 
     D-flipflop  23  is synchronized to the CR flag or the output of D-flipflop  21  shown in  FIG. 10B  to hold voltage Vcc so that a HIGH signal LCR is output as shown in FIG.  10 C. D-flipflop  24  is synchronized to the OC flag of the output of D-flipflop  21  to hold voltage Vcc so that a HIGH signal LOC is output as shown in FIG.  10 D. 
     Copy prevention position detector  25  searches the PD, TM and AC flags of the parallel data of PES header detector  10  shown in  FIG. 10A  to calculate the position of the additional copy information field, which is output to counter  26  as shown in FIG.  10 E. Counter  26  receiving the 4-bit value performs counting so that a HIGH signal is output as shown in  FIG. 10F  at a predetermined counting value. 
     D-flipflop  27  synchronized to HIGH output rco of counter  26  holds the additional copy information field from the parallel data of D-flipflop  21  shown in FIG.  10 B. The field is output as shown in FIG.  10 C. 
     As described above, in the copy prevention method and apparatus for a digital video system of the present invention, a key information is recorded with a bit stream so that a VCR having the key information reproduces tape normally, thereby preventing illegal copy of tape. In addition, for key information transmission, the public-key encryption is introduced to disable a pirate to release the copy prevention, increasing reliability of copy prevention.