Comments on the "Efficient quantum multi-proxy signature"

Recently, a novel quantum multi-proxy signature scheme was proposed by Qin et al. It has flexible proxy delegation and high efficiency. Unfortunately, it is not secure against proxy attacks and forgery attack. In this paper, first, the proxy attacks for the quantum proxy signature scheme are introduced. We demonstrate that every proxy signer can delegate the proxy signing power to a malicious arbitrator, who can sign any message on behalf of all the proxy signers. At the same time, the original signer can also impersonate the proxy signer to generate a quantum proxy signature without being detected. The forgery attacks may also cause some other serious security drawbacks for the quantum proxy signature. Second, the forgery under chosen-signature attack is introduced. We demonstrate that Qin et al.'s quantum multi-proxy signature cannot resist against the signature receiver's forgery under chosen-signature attack. The proposed proxy attacks and the forgery under chosen-signature attack can be used as the security analysis tools, which are useful for analyzing and improving the security of the quantum proxy signatures. At last, an improved quantum multi-proxy signature scheme was proposed. The improved scheme is secure against the original signer's proxy attack and the forgery under chosen-signature attack. Its security against the proxy signer's proxy attack depends on the credibility of the arbitrator.