In this illustration, we see Alice looking rightward at Bob, who is looking leftward back at Alice. Both have a dotted line extending vertically downward, representing the passage of time. A blue arrow labeled "I am Alice" extends horizontally rightward from Alice to Bob. A blue arrow labeled "R" extends diagonally downward to the left from Bob to Alice. A blue arrow labeled "K_A-B(R)" extends horizontally rightward from Alice to Bob.