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In 2004, Das presented a dynamic ID-based remote user authentication scheme using smart cards. Then in 2005, Liao showed that Das et al.'s scheme has some attacks and proposed a slight modification to their scheme to improve the weaknesses of it. This paper, however, will demonstrate that Liao et al.'s scheme has also some security flaws. Therefore, we propose an improved scheme to conquer their weaknesses. The improved scheme can enhance the security of Liao et al.'s scheme and is also efficient.