Critical packet partial encryption

An encryption arrangement for multiple encryption of television programs. A system according to embodiments of the present invention multiple encrypts only a portion of the data required for full presentation of a television program to permit multiple manufacturer's set-top boxes (36 and 136) within a single system. In one embodiment, only critical packets such as those carrying a payload incorporating packetized elementary stream header information is encrypted. By only encrypting a portion of the program, dramatically less bandwidth is consumed that the alternative of multiple encryption of all program data, thus permitting a larger number of multiple conditional access systems in a single cable television system.