Security analysis of SKI protocol

A great number of authentication protocols used in several applications such as payment systems are inherently vulnerable against relay attacks. Distance bounding protocols provide practical solutions against such attacks; some of which are lightweight and implementable on cheap RFID tags. In recent years, numerous lightweight distance bounding protocols have been proposed; however, a few of them appeared to be resistant against the existing attacks up to the present. One of most well-known protocols in this field is the SKI protocol which has a security proof based on a non-standard strong assumption named Circular Keying. In this paper, we demonstrate that without this strong and non-standard assumption, the protocol is not secure against man-in-the-middle attacks. Subsequently, we propose a countermeasure solution to secure the SKI protocol without the circular keying assumption.