Improving the security of secure direct communication based on the secret transmitting order of particles

We analyzed the security of the secure direct communication protocol based on the secret transmitting order of particles recently proposed by Zhu, Xia, Fan, and ZhangPhys. Rev. A 73, 022338 (2006) and found that this scheme is insecure if an eavesdropper, say Eve, wants to steal the secret message with Trojan horse attack strategies. The vital loophole in this scheme is that the two authorized users check the security of their quantum channel only once. Eve can insert another spy photon, an invisible photon, or a delay one in each photon which the sender Alice sends to the receiver Bob, and capture the spy photon when it returns from Bob to Alice. After the authorized users check the security, Eve can obtain the secret message according to the information about the transmitting order published by Bob. Finally, we present a possible improvement of this protocol.