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In 2004, Ari Juels  proposed a Yoking-Proofs protocol for RFID systems. The goal is to permit tags to generate a proof which is verifiable off-line by a trusted entity even when the readers are potentially untrusted. But the protocol not only doesnÂ¿t possess the anonymity property but also suffers from both the replay and off-line attacks. In 2005, Wong et al.  proposed an authentication scheme on RFID passive tags, attempting to as a standard for apparel products. However, the protocol suffers from the known-plain text attack. Recently Chou et al. proposed a scheme to improve both of the above schemes and claimed that their scheme is anonymous. In this paper, we point out that Chou et al.Â¿s scheme is vulnerable to the replay attack, the reflection attack and the server spoofing attack. A modification of Chou et al.Â¿s scheme to enhance their security is proposed. Our scheme is suitable for applications with high security requirement.