Quantum Secure Direct Communication Based on Authentication

We propose two schemes of quantum secure direct communication (QSDC) combined ideas of user authentication [Phys. Rev. A 73(2006)042305] and direct communication with dense coding [Phys. Rev. A. 68(2003)042317]. In these protocols, the privacy of authentication keys and the properties of the EPR pairs not only ensure the realization of identity authentication but also further improve the security of communication, and no secret messages are leaked even if the messages were broken.