Reexamining the security of fair quantum blind signature schemes

In this paper, we reexamined the security of fair quantum blind signature protocols and found there exists a secure leakage caused by the reuse of signing key. To illustrate it, a new attack strategy is proposed based on unambiguous set discrimination of quantum state. In this attack, the attacker can forge a valid signature for any message when he owns three signatures. Finally, a feasible improved method is proposed to solve this problem.