A method of operation of an audio/video content receiver having a content decoder capable of concurrently decoding two or more audio/video programmes from a single packetized data stream by using data packets defining decryption information comprises the steps of : receiving encoded audio/video content as two or more packetized data streams, each data stream comprising one or more programmes having data packets identified by respective sets of one or more packet identifiers, each packetized data stream comprising identification data mapping programmes to respective sets of the packet identifiers and service data including security authorisation information for content receivers; generating a composite packetized data stream having programme data and the service data from two or more of the packetized data streams; applying a digital security indicator to at least the service data included in the composite packetized data stream; supplying the composite packetized data stream to the content decoder for decoding two or programmes from the composite packetized data stream according to the packet identifiers in the composite stream identification data; receiving the service data from the content decoder; detecting the validity of the digital security indicator applied to the service data; and in response tothe service data, detecting whether the content receiver is authorised to decode received programme data.