Cryptanalysis and Improvement on Authenticated Semi-quantum Direct Communication Protocol using Bell States

Luo and Hwang [Quantum Inf. Process (2016) 15:947–958] proposed the authenticated semi-quantum direct communication protocols for a quantum participant to be able to transmit secret messages to a classical participant without employing an authenticated classical channel. However, this study indicates that there is an information leakage problem in Luo and Hwang’s measure-resend protocol. The information leakage problem might allow an attacker to use an intercept-resend attack to obtain partial secret messages. Finally, an improved method is proposed to overcome the information leakage problem.