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.