Small, mobile devices or infrastructure devices without user interfaces,such as Bluetooth headsets, wireless LAN access points, or printers,often need to communicate securely over wireless networks. Activeattacks can only be prevented by authenticating wireless communication,which is problematic when devices do not have any a priori informationabout each other. In this article, we describe three different authenticationmethods for device-to-device authentication based on sensor datafrom various physical out-of-band channels: shaking devices together,authentication based on spatial reference, and transmission via visiblelaser.