Securing interactions between devices that do not know each other a priori is an important and challenging task. We present Amigo, a technique to authenticate co-located devices using knowledge of their shared radio environment as proof of physical proximity. We present evaluation results that show that our technique is robust against a range of passive and active attacks. The key advantages of our technique are that it does not require any additional hardware to be present on the devices beyond the radios that are already used for communication, it does not require user involvement to verify the validity of the authentication process, and it is not vulnerable to eavesdropping.