Patent Publication Number: US-2006018468-A1

Title: Data transmission method and data transmission apparatus

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a technique that prevents a malicious receiving side from making it possible to reproduce source contents without obtaining encrypted source contents from a transmission path by cracking the encryption applied first to the contents based on obtained large numbers of pairs of decrypted contents and encrypted source contents. The technique is used in a transmission system that reencrypts source contents supplied as the contents that have been encrypted according to a first encryption scheme using a second encryption scheme that is different from the first encryption scheme.  
      This application claims priority of Japanese Patent Application No. 2003-360024, filed on Oct. 20, 2003, the entirety of which is incorporated by reference herein.  
      2. Description of the Related Art  
      Video contents or audio contents (e.g., music) have been generally and widely distributed in the form of a recording medium that has stored the contents as digital data. In the case where a compression scheme that can obtain relatively high quality data, data to be distributed is encrypted so as to protect copyright of audio/video to be recorded or reproduced by users.  
      Various media including video or audio are now handled as digital contents on an information device, such as a computer, with the current progress of information technology. Further, advance of information-communication technology allows the contents to be distributed through satellite or terrestrial broadcasting, or through a wide-area network such as the Internet.  
      The distribution of video or audio contents has already been partly come into operation. Thanks to this contents distribution technology, distribution channels of the product or physical medium that has been conventionally used can be omitted, and even users at remote locations can easily obtain desired video/audio software. Further, in view of contents producer/provider side, quick and effective contents sale brings major profits to increase the willingness to create contents, leading to industry-wide advancement.  
      For example, a server/storage type broadcasting system that assumes that a television incorporates a large-capacity hard disk encrypts contents such as movie in a broadcasting station or other contents provider so as to distribute the contents, and charges purchasers, that is, viewers for the contents when delivering a cipher key to them, thereby ensuring a profit.  
      When video or audio contents provided as encrypted digital data is distributed, the following procedures are carried out: the sending side firstly decrypts the encrypted data, and then applies, at transmission time, encryption effective in a transmission system through which the contents data is transmitted to clear text data that has been decrypted so as to send the data to the receiving end; the receiving end applies decryption effective only at the transmission time to the data to obtain the original, clear text data safely.  
      As the above encryption processing, various schemes have been proposed to increase encryption strength, the schemes including, for example, an encryption scheme that transmits a file with a dummy file added to the file so that the receiving side can identify the dummy file (refer to, e.g., Jpn. Pat. Appln. Laid-Open Publication Nos. 2002-328603, and 2001-142396), one that inserts other data into data to be transmitted so as to encrypt it and removes the inserted data after applying decryption processing to the data on the receiving side (refer to e.g., Jpn. Pat. Appln. Laid-Open Publication No. 2001-305954), and one that allows a key to retain information related to a trick applied to data to be transmitted at the encryption time so as to remove dummy data added by the trick at the decryption time (refer to e.g., Jpn. Pat. Appln. Laid-Open Publication No. Hei.10-49048).  
      As shown in, for example,  FIG. 1 , in a normal data transmission method that has been carried out in a conventional data transmission system  100 , original data C 11  generated in the form of clear text is encrypted using one or more encryption techniques at distribution time for safe access and then supplied to a transmitter  110  as data C 11 •K 11  that has been encrypted using, for example, a key K 11  (stage S 11 ).  
      The data C 11 •K 11  that has been encrypted in the stage S 11  is then safely decrypted within the transmitter  110  to become clear text data C 11  for reproduction (stage S 12 ).  
      For safe transmission, the clear text data C 11  for reproduction generated within the transmitter  110  is reencrypted using the same encryption technique that can be decrypted both by the transmitter  110  and a receiver  120  at data transmission time to become data C 11 •K 12  for transmission that has been reencrypted using, for example, a key K 12  (stage S 13 ). The reencrypted data C 11 •K 12  for transmission is sent to the receiver  120 .  
      Since the data C 11 •K 12  for transmission is encrypted at the transmission time as described above, safety of the data can be ensured even when the data is output to an external device.  
      The receiver  120  decrypts the received data C 11 •K 12  for transmission using the encryption scheme employed at the transmission time, that is, using the key  12  (stage S 14 ). As a result, the clear text data C 11  for reproduction can be obtained within the receiver  120 .  
      Thus, it becomes possible to reproduce the clear text data C 11  for reproduction obtained as described above on the receiver  120  side (stage S 15 ). In this manner, data transmission has been completed with the safety of data ensured.  
      As shown in  FIG. 2 , clear text data C 11  for reproduction (corresponding to original data C 11 ) obtained by decrypting the encrypted data C 11 •K 11  on the transmitter  110  side is assumed to be [ABCD00EFGHI00JK].  
      The transmitter  110  side then applies the decrypted clear text data C 11  for reproduction, that is, [ABCD00EFGHI00JK] to reencryption using the key K 12  to generate data C 11 •K 12  for transmission, that is, for example, [ZYXWVUTSRQPONML], and sends the generated data to the receiver  120 .  
      The receiver  120  side decrypts the data C 11 •K 12  for transmission, that is, [ZYXWVUTSRQPONML] to obtain the clear text data C 11  for reproduction, that is, [ABCD00EFGHI00JK].  
      However, it has been indicated that the following problem will occur in the aforementioned data transmission system  100  that decrypts encrypted data, then applies, at transmission time, encryption effective in a transmission system to clear text data that has been decrypted so as to send the data to the receiving end, and the receiving end applies decryption effective only at the transmission time to the transmitted data to obtain the original, clear text data safely. That is, in the normal data transmission method that has been conventionally carried out, a malicious receiver  120  can determine the cipher key  11  used for the encryption applied in the stage  11  by obtaining, in large amounts, the clear text data C 11  for reproduction that is obtained in the stage S 14  and the data C 11 •K 11  that is delivered in the stage  11  and that corresponds to the data C 11  for reproduction. Therefore, there is possibility that strength of an encryption technique in the stage S 11  will be decreased.  
     SUMMARY OF THE INVENTION  
      According to a first aspect of the present invention, there is provided a data transmission method that transmits an input source data that has been encrypted using a first encryption scheme into the data form that a receiver can decrypt, comprising the steps of: decrypting an encryption according to the first encryption scheme applied to the input source data; applying predetermined process to data in a predetermined position in the decrypted source data; encrypting the source data in which the predetermined process has been applied to the data in a predetermined position into the data form that the receiver can decrypt using a second encryption scheme different from the first encryption scheme; and transmitting the data that has been encrypted according to the second encryption scheme to the receiver.  
      According to a second aspect of the present invention, there is provided a data transmission apparatus that transmits an input source data that has been encrypted using a first encryption scheme into the data form that a receiver can decrypt, comprising: a decryption means for decrypting an encryption according to the first encryption scheme applied to the input source data; a data process means for applying predetermined process to data in a predetermined position in the decrypted source data; an encryption means for encrypting the source data in which the predetermined process has been applied to the data in a predetermined position into the data form that the receiver can decrypt using a second encryption scheme different from the first encryption scheme; and a transmission means for transmitting the data that has been encrypted according to the second encryption scheme to the receiver.  
      According to a third aspect of the present invention, there is provided a data receiving apparatus that receives the transmitted encrypted reproduction data and reproduces the data, the data transmitted to the receiver having been obtained by decrypting an encryption according to the first encryption scheme applied to the input source data that has been encrypted, applying predetermined process to data in a predetermined position in the decrypted source data, encrypting the source data in which the predetermined process has been applied to the data in a predetermined position into the data form that the receiver can decrypt using a second encryption scheme different from the first encryption scheme. The receiver comprises: a reception means for receiving the encrypted reproduction data; a reproduction means for reproducing the source data; a decryption means for decrypting the encrypted reproduction data received by the reception means; a detection means for detecting predetermined process applied to a part of the decrypted reproduction data; and a reproduction data supplement means for generating reproduction data that the reproduction means can reproduce, which is substantially the same as a part of the source data that has been processed, and adding the reproduction data to the detected processed portion so as to supply the reproduction means with the supplemented data. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  schematically shows data processing operation and its state in each stage in a data transmission method carried out in a conventional data transmission system;  
       FIG. 2  schematically shows an example of data processing operation in each stage in the conventional data transmission method;  
       FIG. 3  is a block diagram showing a configuration of a data transmission system according to the present invention;  
       FIG. 4  schematically shows data processing operation and its state in each stage in a data transmission method carried out in the data transmission system according to the present invention, and  
       FIG. 5  schematically shows an example of data processing operation in each stage in the data transmission method according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      An embodiment of the present invention will be described below in detail with reference to the accompanying drawings.  
      The present invention is applied to, for example, a data transmission system  50  having the configuration as shown in  FIG. 3 .  
      The data transmission system  50  is constructed as a system that distributes, for example, video or audio contents that have been provided as encrypted digital data. The data transmission system  50  includes: a transmitter  10  that decrypts encrypted data and again applies, at transmission time, encryption effective in the transmission system through which the data is transmitted to the data so as to transmit it to the receiving side; and a receiver  20  that receives the data transmitted from the transmitter  10  and applies decryption effective only at the transmission time to the data to obtain the original, clear text data.  
      The transmitter  10  includes: a decryption section  11  that applies decryption to encrypted original data to generate clear text data for reproduction; and a reencryption section  12  that processes a part of the clear text data for reproduction that has been generated by the decryption section  11  and encrypts the processed clear text data for reproduction using one or more encryption techniques.  
      The receiver  20  includes: a decryption section  21  that applies decryption to the data transmitted from the transmitter  10  to generate clear text data for reproduction; and a reproduction section  22  that reproduces the clear text data for reproduction that has been generated by the decryption section  21 .  
      The transmitter  10  of the data transmission system  50  allows the decryption section  11  to decrypt encrypted original data and to generate decrypted clear text data for reproduction, and allows the reencryption section  12  to process a part of the clear text data for reproduction and to encrypt the processed clear text data for reproduction using one or more encryption techniques. After that, the transmitter  10  transmits the reencrypted data to the receiver  20  side.  
      The receiver  20  allows the decryption section  21  to decrypt the transmitted data that has been reencrypted by the transmitter  10  to generate processed clear text data for reproduction, and then allows the reproduction section  22  to reproduce the processed clear text data for reproduction.  
      The data transmission system  50  decrypts the data that has been obtained by encrypting, using one or more encryption techniques, data conforming to a format having an unallocated region such as a reserved region, applies, at transmission time, encryption effective in the transmission system through which the data is transmitted to the data so as to transmit it to the receiving side, and allows the receiving side to apply decryption effective only at the transmission time to the received data to obtain the original, clear text data safely.  
      Next, a description will be given of data processing operation and its state in each stage in the data transmission system  50  with reference to  FIG. 4 .  
      In the data transmission system  50 , original data C 1  that has been generated in the form of clear text is encrypted using one or more encryption technique at distribution time for safe access and supplied as data C 1 •K 1  that has been encrypted using, for example, a key K 1  to the transmitter  10  (stage S 1 ).  
      The data C 1 •K 1  that has been encrypted in the stage S 1  is then safely decrypted within the transmitter  10  by the decryption section  11  to become clear text data C 1  for reproduction (stage S 2 ).  
      On the transmitter  10  side, for safe transmission, a part of the clear text data C 1  for reproduction that has been decrypted in the stage S 2  is processed by the reencryption section  12  before reencryption processing to become processed clear text data C 1 ′ for reproduction (stage S 3 ).  
      The processed clear text data C 1 ′ for reproduction is reencrypted by the reencryption section  12  using the same encryption technique that can be decrypted both by the transmitter  10  and the receiver  20  to become data C 1 ′•K 2  for transmission that has been reencrypted using, for example, a key K 2  (stage S 4 ).  
      More specifically, in the stage S 3 , of the data for reproduction conforming to a format, the data in an unallocated region such as a reserved region is changed or deleted, and reencryption is performed for the transmission in the stage S 4 . The reencrypted data C 1 ′•K 2  for transmission is transmitted to the receiver  20 .  
      Since the data C 1 ′•K 2  for transmission is encrypted at the transmission time as described above, safety of the data can be ensured even when the data is output to an external device.  
      The receiver  20  decrypts the received data C 1 ′•K 2  for transmission by the decryption section  21  using the encryption technique employed at the transmission time, that is, using the key K 2  (stage S 5 ). As a result, the processed clear text data C 1 ′ for reproduction can be obtained within the receiver  20 .  
      The receiver  20  side is configured to be capable of reproducing the clear text data C 1 ′ for reproduction obtained as described above (stage S 6 ).  
      That is, in the data transmission system  50 , data transmission has been completed with the safety of data ensured.  
      The decryption section  21  on the receiver  20  side applies decryption effective at the transmission time to the transmitted data C 1 ′•K 2  for transmission to obtain clear text data C 1 ′ for reproduction. A part of the data that has not been transmitted is supplemented, as needed, with an appropriate value. The data obtained here does not entirely correspond to the original clear text data. However, since the data conforms to a format, it is possible to reproduce the data without problems.  
      In the data transmission system  50 , even if a malicious receiver  20  has obtained the original encrypted data C 1 ′K 1  in some way, the receiver  20  can obtain only the processed clear text data C 1 ′ for reproduction from the data C 1 ′•K 2  for transmission, that is, it cannot obtain a pair of the complete cipher text C 1 •K 1  and clear text C 1 . It is impossible to analyze the encryption technique using the key K 1  that has been applied to the original data C 1  in the stage S 1  with the processed clear text data C 1 ′ for reproduction. This prevents strength of the encryption technique to the original data C 1  in the stage S 1  from being decreased.  
      A concrete example of data processing operation in each stage in the data transmission system  50  will be described with reference to  FIG. 5 .  
      In the data transmission system  50 , clear text data C 1  for reproduction (corresponding to original data C 1 ) obtained by decrypting the encrypted data C 1 •K 1  on the transmitter  10  side is assumed to be [ABCD00EFGHI00JK]. In this format of the clear text data C 1  for reproduction, data [00] corresponding to a reserved region exists between data [ABCD] and data [EFGHI], and between data [EFGHI] and data [JK].  
      The data transmission system  50  to which the present invention is applied allows the reencryption section  12  on the transmitter  10  side to process the decrypted clear text data C 1  for reproduction, that is, [ABCD00EFGHI00JK] to randomly change each data [0] of the reserved region that exists between data [ABCD] and data EFGHI], and between data [EFGHI] and data [JK] into data [1], thereby generating processed clear text data C 1 ′ for reproduction, that is, for example, [ABCD10EFGHI11JK]. Naturally, this process needs to be performed unbeknownst to the receiver  20 . The reencryption section  12  then reencrypts the processed clear text data C 1 ′ for reproduction, that is, [ABCD10EFGHI11JK] using the key K 2  to generate data C 1 ′•K 2  for transmission, that is, for example, [KJHGFEDCBAZYXW], and transmits it to the receiver  20 .  
      The decryption section  21  on the receiver  20  side decrypts the transmitted data C 1 ′•K 2  for transmission, that is, [KJHGFEDCBAZYXW] to obtain the processed clear text data C 1 ′ for reproduction, that is, [ABCD10EFGHI11JK]. Naturally, the processed clear text data C 1 ′ for reproduction, that is, [ABCD10EFGHI11JK] does not entirely correspond to the original data C 1  to be transmitted under normal circumstances, that is, [ABCD00EFGHI00JK]. However, since the part different from the original data is a reserved region that is not used for process conforming to a format, it is possible for the reproduction section  22  of the receiver  20  to reproduce the data without problems.  
      As another embodiment, the transmitter  10  can delete a part of the reserved region on a predetermined format that does not influence contents reproduction so as to transmit data to the receiver  20 . In this case, the decryption section  21  of the receiver  20  detects the deleted reserved region, then supplements it with dummy data, and inputs the supplemented data into the reproduction section  22 , thereby reproducing its contents.  
      As described above, the data C 1 ′ for reproduction obtained within the receiver  20  is made different from the original data C 1  to be input to the transmitter  10  after being encrypted using the key K 1 . This makes it difficult for a malicious receiver  20  to derive the encryption process K 1  used for generating the encrypted original data from the data C 1 ′ for reproduction and the encrypted original data C 1 •K 1  even if the malicious receiver  20  has obtained large numbers of pairs of data C 1 ′ for reproduction and the encrypted original data C 1 •K 1  that corresponds to the data C 1 ′ for reproduction. As a result, the encryption technique to the original data C 1  can be protected.  
      In the data transmission system  50 , the method that randomly changes a reserved region is employed. Alternatively, however, a method that deletes data of the reserved region at transmission time and adds an appropriate data on the receiver  20  side can be employed. Also in this case, the same advantage as above can be obtained.  
      It is possible to process a data part within the allowable range depending on the type of transmission data.  
      In the case of voice data, if a data part is processed or deleted, the processed portion may noticeably influence reproduction output of the data. On the other hand, in the case of video data, even if a data part thereof is processed, the processed portion is less represented in the reproduction image thereof in many cases. Even if spatial or temporal correlated image is processed, for example, to delete one frame for each consecutive frames, the processed portion is less represented in the reproduction image. It is also relatively easy to supplement the data whose image data has been partly deleted with image data positioned before and after the deleted image data.  
      In a format independently having a region in which random data that does not influence data reproduction exists, it is possible to rewrite data in the region in encrypting original data. As a result, even if the original data is in the open for some reason, no dependency relation with the encryption technique applied to the original data exists.  
      In current encryption techniques, a very slight difference produces entirely different results. The above process eliminates dependency relation with the encryption technique applied to the original data, thereby safely performing data transmission with the strength of mutual encryption techniques maintained.