As disclosed in Non-Patent Document 1, in order to prevent unauthorized copy of contents recorded in the recording medium such as a DVD (Digital Versatile Disk), conventionally there is known a technique in which an encryption process is performed to each of the plural title contents which are of contents of the program video or sound using the different title key, and the encrypted title contents are recorded in the DVD medium.
In the technique disclosed in Non-Patent Document 1, each of the plural title key is encrypted using a device key and a random number, and the encrypted title key and the random number are registered in the title key file and recorded in the DVD medium. The device key is imparted in each recording and reproducing apparatus such as a DVD recorder which rightly records and reproduces the contents. The random number is randomly generated. In the case where the title contents are reproduced, the encrypted title key registered in the title key file is decrypted using the random number and the device key of the recording and reproducing apparatus used for the reproduction, and the title contents are decrypted using the decrypted title key to reproduce the title contents.
In the case where a part of the title contents is deleted in a rewritable DVD medium, the title key file is updated. Specifically, the title key file is decrypted once, a random number is newly generated, the title key file in which the title key corresponding to the title contents deleted from the DVD medium is deleted using the device key is encrypted again using the new random number and the device key, and the title key file is recorded in the DVD medium. Therefore, it is possible to prevent attack in which the title key corresponding to the deleted title contents is previously copied and the deleted title contents are improperly reproduced using the copied title key.
Like this contents protection using the title key is effective technique for preventing unauthorized use of the contents in the DVD medium. However, in the case where the title key file cannot be read because a part of an area cannot be read by stains and flaws in the DVD medium, or in the case where a user deletes the title key file by mistake, the title contents cannot be decrypted, and the reproduction cannot be performed. Therefore, a copy of the title key file is recorded as a backup file in the DVD medium, and the backup file is used as the title key file when the title key file cannot be read due to breakage of the title key file or the like.    [Non-Patent Document 1: Advanced Access Content System (AACS) Recordable Video Book Revision 0.90