Patent ID: 7088822

Claim:
An information playback device for playing back information from a recording medium having encrypted content recorded thereon by a content recording entity, the information playback device comprising: a cryptosystem unit operable to determine the validity of a public key certificate of the content recording entity, to acquire a public key of the content recording entity from the public key certificate if the public key certificate is valid, and to decrypt the encrypted content if the validity of a digital signature of the content recording entity is verified based on the acquired public key; whereby the device corresponds to a leaf of a key-tree structure, said key-tree structure including a plurality of nodes and a plurality of leaves, said plurality of nodes including a root node, and each of said nodes and each of said leaves corresponding to a respective encryption key; and whereby decryption by a device of said encrypted content is selectively inhibited by changing all keys corresponding to nodes included in a node path between said leaf corresponding to an inhibited device and said root node to generate a plurality of changed keys, said changed keys being propagated through said key-tree structure by encrypting each changed key according to a lower-level changed key, wherein the lowest level node of the key-tree structure to which a changed key is assigned has its said assigned changed key encrypted by a device leaf key other than the inhibited device's leaf key.