On the basis of YTH threshold proxy signature scheme, a nonrepudiable scheme with known signers was constructed by Hu et al. in 2009. They claimed that the weaknesses of frame attack, collusion attack and public-key substitute attack can be overcome in their scheme. Whereas this article will demonstrate two security leaks of frame attack and collusion attack inherent in Hu et al.'s threshold proxy signature scheme, which is not as same as they claimed, previously. That is, the original signer can forge a valid threshold proxy signature while the proxy group doesn't know it, and any t or more proxy signers could cooperatively reconstruct the secret polynomial function f(x) to fake a proxy signature. In this article, based on Hu et al.'s scheme, we propose a new and efficient scheme to resist those attacks.
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