Abstract: A system for enciphering information bearing RF signals in digital form utilizes a first key common to all receivers. There are means to change the first key which means include, at the transmitter, means to send individual messages to each receiver, each message being itself enciphered in a second key which is peculiar to only one receiver or small group of receivers and which message includes a change of the first enciphering key. At each receiver there are means responsive to the correct address in a key change message which will thereafter permit the deciphering of the key change message in the key peculiar to the particular receiver or small group of receivers. The new first key is then used at the receiver to decipher subsequent information bearing messages.

Abstract: A method of controlling the simultaneous broadcast of enciphered digital information signals, for example in a radio or television broadcast environment, to a plurality of subscribers provides several levels of enciphering keys. The broadcast digital information signal is in a broadcast common service enciphering key and communication between the transmitter and subscribers may take place in a box key or in a group enciphering key common to a group of subscribers having a common interest in the reception of broadcast signals of a particular type. Each receiver will decipher the broadcast digital information in a specific service key which is common to that broadcast. The service key may be changed at one or more subscribers by communicating the change in the service key to the subscribers by means of the group enciphering key.