Analysis and Improvement of an Efficient Controlled Quantum Secure Direct Communication and Authentication Protocol

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.