Security protocols are an important concept in teaching information security. Students need to understand both the sequence of passed information and computations, as well as the various attacks on them via eavesdroppers, forged messages, communication blocks, and message replays. A traditional approach to teaching protocols is to use a static diagram showing the transfer of messages between participants over time. This paper describes an interactive visualization tool that allows arbitrary protocols to be demonstrated visually in a user-controlled step-wise manner.