Two attacks and counterattacks on the mutual semi-quantum key agreement protocol using Bell states

Abstract: Recently, a mutual semi-quantum key agreement protocol using Bell states is proposed by Yan et al. (Mod. Phys. Lett. A, 34, 1950294, 2019). The proposed protocol tries to help a quantum participant share a key with a classical participant who just has limited quantum capacities. Yan et al. claimed that both the participants have the same influence on the final shared key. However, this study points out that the classical participant can manipulate the final shared key by himself/herself without being detected. To solve this problem, an improved method is proposed here.