Double C-NOT Attack on a Single-State Semi-Quantum Key Distribution Protocol and Its Improvement

Recently, Zhang et al. proposed a single-state semi-quantum key distribution protocol to help a quantum participant share a secret key with a classical participant. However, this study shows that an eavesdropper can use a double C-NOT attack to obtain parts of the final shared key without being detected by the participants. To avoid this problem, a modification is proposed here.