Patent ID: 8478108

Claim:
An apparatus to play back content that is recorded on a recording medium and that is encrypted by a first cryptographic key, the first cryptographic key being encrypted by using a second cryptographic key and being recorded on the recording medium, the apparatus comprising: a key storing unit that stores the second cryptographic key and a public key; a key decryption unit that decrypts encrypted first cryptographic key by using the second cryptographic key; a playback unit that decrypts encrypted content by using the first cryptographic key and plays back decrypted content; a certificate verifying unit that verifies whether a content certificate exists in the recording medium, the content certificate including a certificate type registering a content type indicative of a record type of the content on the recording medium, a hash value based on information of a portion of the content, and a signature created for the certificate type and the hash value by using a private key that corresponds to the public key, wherein when the content certificate exists in the recording medium, the certificate verifying unit determines whether a combination of the certificate type of the content certificate recorded on the recording medium and a medium type of the recording medium is a proper combination; a hash-value verifying unit that verifies, when the combination of the certificate type and the medium type of the recording medium is the proper combination, whether the hash value included in the content certificate matches a hash value calculated from the content; and a digital-watermark verifying unit that verifies, when the content certificate does not exist in the recording medium, whether, at the time of playback of the content, a digital watermark is embedded in the content, wherein the playback unit continues to play back the content when the hash value included in the content certificate matches the hash value calculated from the content and the playback unit aborts playback of the content when the combination of the certificate type and the medium type of the recording medium is not the proper combination, when the hash value included in the content certificate does not match the hash value calculated from the content, or when the digital watermark is embedded in the content.