Patent ID: 7617402

Claim:
A copyright protection system comprising: an encryption device and a decryption device, wherein cryptographic communication is performed between the encryption device and the decryption device using a contents encryption key and a contents decryption key, wherein the encryption device includes a contents storage section for storing contents, a first contents key generation section for generating the contents encryption key based on a second decryption limitation obtained by updating a first decryption limitation, and a first encryption section for encrypting the contents using the contents encryption key and outputting the encrypted contents, and wherein the decryption device includes a second contents key generation section for generating the contents decryption key from the second decryption limitation, the second decryption limitation obtained by updating the first decryption limitation in the decryption device, and a first decryption section for decrypting the encrypted contents transferred from the encryption device using the contents decryption key generated by the second contents key generation section, and wherein the encryption device further includes a third encryption section for encrypting the first decryption limitation using a time-varying key and outputting a second encrypted decryption limitation to the decryption device, and the decryption device further includes a third decryption section for decrypting the second encrypted decryption limitation transferred from the third encryption section using the time-varying key and outputting the first decryption limitation, and wherein the decryption device further includes a first decryption limitation updating section for updating the first decryption limitation to the second decryption limitation in accordance with a decryption limitation updating rule, and a second encryption section for encrypting the second decryption limitation using a time-varying key, and outputting the first encrypted decryption limitation, and the encryption device further includes a second decryption section for decrypting the first encrypted decryption limitation transferred from the second encryption section using the time-varying key to generate the second decryption limitation, wherein the time-varying key is not required to be transmitted to the decryption device; and wherein the first contents key generation section generates the contents encryption key based on the second decryption limitation generated by the second decryption section.