Cryptographic method and apparatus for restricting access to transmitted programming content using hash functions and program identifiers

A system for restricting access to transmitted programming content is disclosed, which transmits a program identifier with the encrypted programming content. A set-top terminal or similar mechanism restricts access to the transmitted multimedia information using stored decryption keys. The set-top terminal receives entitlement information periodically from the head-end, corresponding to one or more packages of programs that the customer is entitled to for a given period. Each program is encrypted by the head-end server prior to transmission, using a program key, Kp, which may be unique to the program. The set-top terminal uses the received program identifier, p, together with the stored entitlement information, to derive the decryption key necessary to decrypt the program. Each of the k-bit program keys, Kp, used to encrypt transmitted programs is obtained by applying one or more pseudo-random hash functions, H, such as a length-doubling hash function, H, to a master key, m.