In this illustration, we see Alice on the left looking rightward and Bob on the right looking leftwards. Above Alice is a computer labeled "Secure sender," and above Bob is a computer labeled "Secure receiver." The secure sender and secure receiver are connected by a black line labeled "Channel." In the middle and below Alice and Bob is Trudy, looking malicious. A blue arrow labeled "Data" extends from the left side of the figure to the secure sender. A blue arrow labeled "Control, data messages" points back and forth between the secure sender and the secure receiver. A blue arrow labeled "Data" extends from the secure receiver to the right side of the figure. A blue arrow also points back and forth from the secure sender to Trudy, while another points back and forth from the secure receiver to Trudy.