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,
K.sub.p, 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, K.sub.p, 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.
