A system that assures a user that two specific devices have exchanged information correctly over an insecure communication channel. In a typical application, the channel would be a wireless network connection and the information would consist of the public keys of the two devices. The exchanged public keys would subsequently be used to secure any further communication between the two devices. The system requires that both devices have an output modality interpretable by a human. One of the devices must have an input modality that can be operated by the user. The input and output modalities can be very simple e.g., they can consist of a button and LEDs. The users would press the button multiple times. On each press, the LEDs on both devices display a color and the user compares the colors and that they are synchronous. A noticeable delay or color difference signifies a security threat.