Cryptanalysis and Improvement of Quantum Gate-Based Quantum Private Comparison

Recently, Lang put forward for the first time a quantum gate-based quantum private comparison (QPC) protocol (Int J Theor Phys, 59(2020)833) which uses the quantum controlled-not gates to implement comparison instead of classical exclusive-or computations. It was claimed that this quantum gate-based QPC protocol is secure against both the outside attack and the participant attack. However, in this paper, it is pointed out that this protocol cannot resist a special measurement attack from the third party (TP) and the disturbance attack from an outside attacker. The corresponding improvements are further suggested to remedy these security loopholes.