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

Abstract

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 preferably 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 preferably encrypted by the head-end server prior to transmission, using a program key, K , 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 , used to encrypt transmitted programs is a linear combination of a defined set of k-bit master keys, m . . . m . The head-end server preferably generates a new set of master keys for the matrix, M, once per billing period. Since each program key, K , is a linear combination of the set of master keys, M, a customer desiring r programs, obtains access to the smallest linear subspace of programs, U, that contains those r programs. In addition, a package consists of (2 −1) program identifiers for some i less than or equal to n, which need not all be assigned to programs. An optional check matrix, C, allows the set-top terminal to determine, in advance, whether a received program is in the entitled subspace, U.