This paper points out that, due to a flaw in the sender's encoding, the receiver in Gao et al. 's controlled quantum secret direct communication (CQSDC) protocol [ Chin. Phys. 14 (2005), No. 5, p. 893 ] can reveal the whole secret message without permission from the controller. An improvement is proposed to avoid this flaw.
A security loophole exists in Gao et al.'s controlled quantum secure direct communication protocol. By employing the security loophole, the receiver can obtain the secret message sent by the sender without the permission of the controller in their protocol. In order to avoid this loophole, we present an improved protocol in this paper. In the improved protocol, entangled particles are prepared at random in two GHZ-like states, which ensure that the receiver is not able to recover the secret message without knowing the initially entangled state. Compared with the other improved version whose security depends on the perfect quantum channel, our improved protocol is secure in a noisy quantum channel. Therefore, our protocol is more practical.
Recently Xia and Song [Phys. Lett. A (In press)] have proposed a controlled quantum secure direct communication (CQSDC) protocol. They claimed that in their protocol only with the help of the controller Charlie, the receiver Alice can successfully extract the secret message from the sender Bob. In this letter, first we will show that within their protocol the controller Charlie's role can be excluded due to their unreasonable design. We then revise the Xia-Song CQSDC protocol such that its original advantages are retained and the CQSDC can be really realized.
Recently Xia and Song [Phys. Lett. A 364 (2007) 117] have proposed a controlled quantum secure direct communication (CQSDC) protocol. They claimed that in their protocol only with the help of the controller Charlie, the receiver Alice can successfully extract the secret message from the sender Bob. In this letter, first we will show that within their protocol the controller Charlie's role could have been excluded if it were not for their unreasonable design. We then revise the Xia–Song CQSDC protocol such that its original advantages are retained and the CQSDC can be really realized.
The controlled quantum secure direct communication (CQSDC) with authentication protocol based on four particle cluster states via quantum one-time pad and local unitary operations is cryptanalyzed. It is found that there are some serious security issues in this protocol. An eavesdropper (Eve) can eavesdrop on some information of the identity strings of the receiver and the controller without being detected by the selective-CNOT-operation (SCNO) attack. By the same attack, Eve can also steal some information of the secret message that the sender transmits. In addition, the receiver can take the same kind of attack to eavesdrop on some information of the secret message out of the control of the controller. This means that the requirements of CQSDC are not satisfied. At last, we improve the original CQSDC protocol to a secure one.