High Efficiency of Two Efficient QSDC with Authentication Is at the Cost of Their Security

Two efficient protocols of quantum secure direct communication with authentication Chin. Phys. Lett. 25(2008) 2354] were recently proposed by Liu et al. to improve the efficiency of two protocols presented in Phys. Rev. A 75(2007)026301] by four Pauli operations. We show that the high efficiency of the two protocols is at the expense of their security. The authenticator Trent can reach half the secret by a particular attack strategy in the first protocol. In the second protocol, not only Trent but also an eavesdropper outside can elicit half-information about the secret from the public declaration.