transmitter makes new program keys and saves the new program keys in the memory 66 as the next program keys the encryption encoder 8 encrypts the new program keys and assembles another program key message segment pkms containing the new program keys and waits for an opportunity event 3 such as a null packet to transmit this program key message segment pkms while the encryption encoder 8 of the transmitter makes new program keys saves the new program keys and assembles the next program key message segment pkms the receiver decrypts the modification key message segment mkms that it has just received and saves the modifier message mm and the modification keys contained in this message during segments in which the encryption encoder 8 is not transmitting program key message segments pkms and modification key message segments mkms the encryption encoder 8 is using the active program keys to encrypt program data and is transmitting the encrypted program data to the receiver during segments in which the receiver is not receiving program key message segments pkms and modification key message segments mkms the receiver is using the active program keys to decrypt program data in an embodiment where message transmission and key use is synchronized to the occurrence of null packets there may be occasions when null packets are occurring with an undesirably high frequency for example during periods where there is little action in the video many null packets can occur during a single frame therefore it may.