Reexamining the Security of Controlled Quantum Secure Direct Communication by Using Four Particle Cluster States

A controlled quantum secure direct communication protocol (Zhang et al. in Int. J. Theor. Phys. 48:2971-2976, 2009) by using four particle cluster states was proposed recently. Yang et al. presented an attack with fake entangled particles (FEP attack) and gave an improvement (Yang et al. in Int. J. Theor. Phys. 50:395-400, 2010). In this paper, we reexamine the protocol's security and discover that, Bob can also take a different attack, disentanglement attack, to obtain Alice's secret message without controller's permission. Moreover, our attack strategy also works for Yang's improvement.