Quantum secure direct communication (QSDC) makes two quantum users transmit secret message directly without first producing a shared secret key. Semi-quantum secure direct communication is a particular case of QSDC when one of the two users is a classical user. How to develop a similar protocol if both users are classical? In this paper, we proposed a mediated semi-quantum secure direct communication protocol where both classical users can transmit secret message with the help of a fully quantum third party. Classical users can generate and measure qubits in the computational basis, so they must rely on the third party to prepare alternative bases and perform alternative measurements. The security analysis shows that the protocol can effectively prevent secret message from eavesdropping even if the third party is untrusted. Moreover, the analysis shows that TP’s measurement operation can be associated with linear optical Bell measurements, where only two of the four Bell states would be measured. Finally, we perform an efficiency analysis that shows the protocol can reduce quantum resources at the expense of efficiency.

量子安全直接通信（QSDC）使两个量子用户直接传输秘密消息，而无需先产生共享的秘密密钥。当两个用户之一是经典用户时，半量子安全直接通信是QSDC的一种特殊情况。如果两个用户都是经典的，该如何开发类似的协议？在本文中，我们提出了一种介导的半量子安全直接通信协议，在该协议中，两个经典用户都可以借助完全量子的第三方来传输秘密消息。传统用户可以在计算基础上生成和测量量子位，因此他们必须依靠第三方来准备替代基准并执行替代测量。安全性分析表明，即使第三方不受信任，该协议也可以有效地防止秘密消息的窃听。此外，分析表明，TP的测量操作可以与线性光学贝尔测量相关联，其中只能测量四种贝尔状态中的两种。最后，我们进行了效率分析，表明该协议可以减少量子资源，但会降低效率。