Cryptographic methods and - a device for limiting the access to the contents of transmitted programmes by widened head fields

摘要

A system for restricting access to transmitted programming content is disclosed, which transmits the encryption key used to encrypt the program to the customer 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 one or more package keys, S.sub.J, periodically from the service provider, each corresponding to a package 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.sub.P, which may be unique to the program. Header information is transmitted with the encrypted program to the customers, containing a package pair for each package to which the program belongs. A package pair preferably includes an identifier of the package, as well as the program key, K.sub.P, encrypted by the corresponding package key, S.sub.J. The broadcast of a given program, p, preferably consists of a header portion containing a package pair for each package that the program belongs to, and a program portion containing the program encrypted with the program key, K.sub.P. If a customer is entitled to a particular program, the set-top terminal will be able to decrypt the encrypted program key, K.sub.P, using an appropriate stored package key, S.sub.J, and thereafter use the program key, K.sub.P, to decrypt the encrypted program. The header information can be interleaved with the program portion or transmitted on a separate dedicated control channel.