Patent ID: 7810113

Claim:
A security apparatus of a conditional access system, the apparatus comprising: a key database for storing a user key assigned to a subscriber; and a decryptor for sequentially decrypting an update key encrypted with the user key, a service key encrypted with the update key, and a control word encrypted with the service key, and outputting a control word; wherein at least one of the update key, the service key and the control word are received from a head end of a conditional access system, and the user key is received from the key database; the update key is used for updating the service key and is updated in a predetermined sequence; the encrypted service key comprises a channel encryption key, and the encrypted control word is encrypted with the channel encryption key; and the channel encryption key is generated corresponding to a service channel in one-to-one correspondence and updated in a certain time, or is generated from a set of channel encryption keys used during a certain time and then sequentially updated.