In 2006, Pomykala et al. proposed an elliptic curve based threshold proxy signature scheme as well as a proxy-protected version. They claimed their scheme had the properties of secrecy, unforgeability, non-repudiation and signer’s identification. In this study, we showed inter conspiracy attack on the basic scheme and gave another attack on the proxy-protected version. We pointed out that even if the central authority is engaged, the basic model is not a proxy-protected one. We also proposed an improved scheme that can resist the known attacks to achieve higher security, and gave the security analysis of the improved scheme in detail.
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