A Special Eavesdropping on One-Sender Versus N-Receiver QSDC Protocol

We analyse the security of a quantum secure direct communication (QSDC)
protocol and find that an eavesdropper can utilize a special property of GHZ states to elicit all or part of the transmitted secrets without being detected. The particular attack strategy is presented in detail. We give an improved version of this protocol so that it can resist this attack.