The protection resulting from features, inherent in some crypto-equipment, that conceal the presence of valid messages on a communications circuit; normally achieved by causing the circuit to appear busy at all times. 2. Measures used to conceal the presence of valid messages in an on-line cryptosystem or secure communications system. Note: Encryption of sending and receiving addresses and causing the circuit to appear busy at all times by sending dummy traffic are two methods of traffic-flow security. A more common method is to send a continuous encrypted signal, whether or not traffic is being transmitted.