In 2003, Hsu et al. presented an improved proxy signature scheme based on Sun et al.'s scheme to effectively defeat its drawback that the threshold value can be changed by proxy signers. However, security analysis on the improved scheme shows that both the improved scheme and the original one can't stand the insider conspiracy attack and the forge attack. Moreover, the signature based on them is not identifiable or nonrepudiable. Aiming at these weaknesses, an improved scheme is proposed in this paper. Security analysis on the new improved scheme shows that it effectively overcomes those disadvantages of Hsu et al.' s scheme.
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