Patent ID: 7962745

Claim:
A user system which decrypts encrypted content provided by a content providing system, the user system comprising: a receiving device which receives, from the content providing system, encrypted content capable of being decrypted by use of a session key and at least a portion of header information that includes a session key encrypted in such a manner that the session key can be obtained by use of a decryption key assigned to each of one or more user systems and one or more values based on the user identification information about at least one of the one or more user systems which are permitted to obtain the session key; a session key decrypting device configured to decrypt an encrypted session key from the received header information on the basis of the decryption key and which uses as the decryption key a value obtained by substituting the user identification information of the user system into a key generation polynomial assigned to a sub-group to which the user identification information of the user system belongs; and a content decrypting device configured to decrypt the encrypted content on the basis of the decrypted session key.