The number of passenger cars equipped with a smart key system continues to increase due to the convenience of the system. A smart key system allows the driver to enter and start a car without using a mechanical key through a wireless authentication process between the car and the key fob. Even though a smart key system has its own security scheme, it is vulnerable to the so-called relay attacks. In a relay attack, attackers with signal relaying devices enter and start a car by relaying signals from the car to the owner’s fob. In this study, a method to detect a relay attack is proposed. The signal strength is used to determine whether the signal received is from the fob or the attacker’s relaying devices. Our results show that relay attacks can be avoided by using the proposed method.
 A. I. Alrabady and S. M. Mahmud, “Some Attacks against Vehicle’s Passive Entry Security Systems and Their Solutions,” IEEE Transactions on Vehicular Technology, Vol. 52, No. 2, March 2003. http://dx.doi.org/10.1109/TVT.2003.808759
 A. I. Alrabady and S. M. Mahmud, “Analysis of Attacks against the Security of Keyless-Entry Systems for Vehicles and Suggestions for Improved Designs,” IEEE Transactions on Vehicular Technology, Vol. 54, No. 1, January 2005. http://dx.doi.org/10.1109/TVT.2004.838829
 F. Stajano, F.-L. Wong and B. Christianson, “Multichannel Protocols to Prevent Relay Attack,” University of Cambridge Computer Laboratory, Cambridge, United Kingdom, DSO National Laboratories, Singapore, University of Hertfordshire, School of Computer Science, Hatfield, United Kingdom.