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Recently, Yin et al. (Int. J. Quantum Inform. 10 (2012) 1250041) proposed a quantum proxy group signature scheme with χ-type entangled states. The scheme combines the properties of group signature and proxy signature. The study points out that the semi-honest Trent can give the forged signature under the assumption of this scheme. And, we find that even if the three parties honestly perform the scheme, the signature still cannot be realized with high efficiency.

We study the security of quantum secure direct communication without perfect quantum channel. Our analysis is focused on a general individual attack, i.e. entangle-ancilla attack. The sufficient and necessary condition for a successful eavesdropping operation is obtained, and a particular attack is presented, by which Eve can elicit the whole secret without being discovered. Finally we find the root of this loophole and give a possible improvement of this protocol accordingly.