Multi-proxy signature allows an original signer authorizing a proxy group as his proxy agent and only the cooperation of all proxy signers in the group can create a proxy signature on behalf of the original signer. Recently, Jin and Wen defined a formal model of certificateless multi-proxy signature and proposed a concrete scheme. They claimed that their scheme is provably secure in their security model. Unfortunately, by giving concrete attacks, we show that Jin-Wen's certificateless multi-proxy signature scheme is not secure according to their security model. Possible improvements of their scheme are also suggested to prevent these attacks.
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