  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.
