As the concept of radio-frequency identification (RFID) relay attack has been successfully implemented and demonstrated, the research of RFID distance-bounding protocols to deter RFID relay attacks has drawn much attention from both the industry and academia. Conventionally, researchers adopted linear composition of secrets to resist terrorist fraud attacks. Recently, Peris-Lopez et al. studied the weaknesses of previous RFID distance-bounding protocols and proposed that non-linear composition of secrets and inclusion of more random nonce could help RFID resist key disclosure attack and terrorist fraud attack. In this paper, we will show that non-linear composition of secrets cannot help enhance the security actually. Copyright © 2012 John Wiley & Sons, Ltd.