cqvip:Portable communication systems can provide mobile users with global roaming services. Recently, Youn and Lim proposed a delegation-based authentication protocol which achieves unlinkability for secure roaming services. This paper indicates that there are two drawbacks in Youn and Lim's protocol: 1) the synchronization problem will lead to a fail in on-line authentication; and 2) the exhaustive search puts a heavy burden on the off-line authentication process. Moreover, based on Youn and Lim's protocol, a remedy is proposed to address these problems. It is worthwhile to note that the proposed remedy not only keeps the original advantages but also enhances the security and performance.
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