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In ACISP 2006, Paterson and Schuldt(PS) presented an efficient identity-based signature scheme in the standard model. Their scheme is obtained from a modification of Waters' identity-based encryption scheme. It employed two Waters Hash functions, one for identity, other for message, so the security reduction efficiency is very low. We slightly modify PS's scheme and change the parameters selection in security proof to obtain a tight security reduction identity- based signature scheme. The new scheme's security rests on computational Diffie-Hellman assumption.