Cryptanalysis and Improvement of Two GHZ-State-Based QSDC Protocols

In a recent paperJ. Korean. Phys. Soc. 49(2006)459, two GHZ-state-based quantum secure direct communication protocols were presented. Here we point out that an eavesdropper can utilize a special property of GHZ states, i.e. "correlation-elicitable" to obtain half information of the transmitted secrets without being detected in both protocols. The particular attack strategy is demonstrated in detail. Furthermore, a possible improvement is proposed, which makes the protocols secure against this kind of attack.