Enhanced key management for cable TV service

Cable TV service providers must manage encryption/decryption keys to ensure that only authorized set-top boxes can decrypt pay-TV channels; ITU have recommended a key management scheme for this purpose. Nevertheless, some crackers crack and reproduce set-top boxes for profit, and many people use the pirated set-top boxes to decrypt and watch pay-TV channels without paying a subscription fee. We enhance the key management scheme to protect the service provider's revenue against piracy. Using the enhanced scheme, even if a cracker could crack and reproduce set-top boxes, the pirated set-top boxes are only applicable in a small geographical region (i.e., they have limited spatial applicability) and they cannot function within one charging period (i.e., they have limited temporal applicability).