Security Analysis and Improvement of a Blind Semi-quantum Signature

Recently, Xia et al. proposed a semi-quantum blind signature protocol based on five-particle GHZ state. Their protocol can reduce the computation burden of the communicants. However, their protocol is not secure against forgery attack. We prove that the message sender and the signature receiver may conspire to forge the signer’s signature, because they master all the private keys of the signer. Then, both the signer and the signature receiver can deny a valid signature. Then, based on the three-particle GHZ state, an improved semi-quantum blind signature protocol is proposed. In the improved protocol, the signer shares the signing key with the trusted arbitrator. Even the message sender and the signature receiver conspire, it is infeasible for them to forge the signer’s signature. The improved scheme is secure against the disavowal attack. It has the better efficiency and practicability than the old version as well.