A copyright protection system comprising a recording device for recording encrypted content on a recording medium and a decryption device, wherein the recording device includes: an encrypting unit (15,111,112) operable to generate a random number and to encrypt content; a generating unit operable to acquire, from an unrewritable area of a recording medium, a media number unique to the recording medium, and generate license information by performing a one way function using the media number; and a recording unit operable to record the license information and the random number on the recording medium; wherein the decryption device includes: a judging unit (35, 36) operable to acquire the media number from the unrewritable area of the recording medium, to acquire the license information and the random number from the rewritable area of the recording medium, and to judge whether the license information can be obtained if the one way function is performed using the media number; and a decrypting unit operable to decrypt data; characterized in that the recording device further includes a first storing unit (11,19) operable to store therein key information; wherein the encrypting unit (15,111,112) is further operable encrypt the content using both the random number and the key information to generate an encrypted content; the generating unit is operable to generate the license information by performing the one way function using the media number, the random number, and the key information so that the license information reflects the media number, the random number, and the key information; and the recording unit is further operable to record the encrypted content on the recording medium; wherein the decryption device further includes: a second storing unit (31,42) operable to store therein the key information; and a warning unit operable to issue a warning if the judgment by the judging unit is negative; wherein the judging unit (35, 36) is operable to judges whether the license information can be obtained if the one-way function is performed using the media number, the random number, and the key information; and the decrypting unit is operable to acquire the encrypted content from the rewritable area of the recording medium, and decrypt the encrypted content using both the random number and the key information only if the judgment by the judging unit (35, 36) is affirmative. an encrypting unit (15,111,112) operable to generate a random number and to encrypt content; a generating unit operable to acquire, from an unrewritable area of a recording medium, a media number unique to the recording medium, and generate license information by performing a one way function using the media number; and a recording unit operable to record the license information and the random number on the recording medium; a judging unit (35, 36) operable to acquire the media number from the unrewritable area of the recording medium, to acquire the license information and the random number from the rewritable area of the recording medium, and to judge whether the license information can be obtained if the one way function is performed using the media number; and a decrypting unit operable to decrypt data; the recording device further includes a first storing unit (11,19) operable to store therein key information; wherein the encrypting unit (15,111,112) is further operable encrypt the content using both the random number and the key information to generate an encrypted content; the generating unit is operable to generate the license information by performing the one way function using the media number, the random number, and the key information so that the license information reflects the media number, the random number, and the key information; and the recording unit is further operable to record the encrypted content on the recording medium; wherein a second storing unit (31,42) operable to store therein the key information; and a warning unit operable to issue a warning if the judgment by the judging unit is negative; wherein the judging unit (35, 36) is operable to judges whether the license information can be obtained if the one-way function is performed using the media number, the random number, and the key information; and the decrypting unit is operable to acquire the encrypted content from the rewritable area of the recording medium, and decrypt the encrypted content using both the random number and the key information only if the judgment by the judging unit (35, 36) is affirmative.