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Security of the quantum secure direct communication protocol (i.e.,the C-S QSDC protocol) recently proposed by Cao and Song [Chin.Phys.Lett.23 (2006) 290]is analyzed in the case of considerable quantum channel noise.The eavesdropping scheme is presented,which reveals that the C-S QSDC protocol is not secure if the quantum bit error rate (QBER) caused by quantum channel noise is higher than 4.17%.Our eavesdropping scheme induces about 4.17% QBER for those check qubits.However,such QBER can be hidden in the counterpart induced by the noisy quantum channel if the eavesdropper Eve replaces the original noisy channel by an ideal one.Furthermore,if the QBER induced by quantum channel noise is lower than 4.17%,then in the eavesdropping scheme Eve still can eavesdrop part of the secret messages by safely attacking a fraction of the transmitted qubits.Finally,an improvement on the C-S QSDC protocol is put forward.
Security of the quantum secure direct communication protocol (ie, the CS QSDC protocol) recently proposed by Cao and Song [Chin. Phys. Lett. 23 (2006) 290] is analyzed in the case of quantum quantum noise. The eavesdropping scheme is presented, which reveals that the CS QSDC protocol is not secure if the quantum bit error rate (QBER) caused by quantum channel noise is higher than 4.17% .Our eavesdropping scheme induces about 4.17% QBER for those check qubits. be hidden in the counterpart induced by the noisy quantum channel if the eavesdropper Eve replaces the original noisy channel by an ideal one. Morerther, if the QBER induced by quantum channel noise is lower than 4.17%, then in the eavesdropping scheme Eve still can eavesdrop part of the secret messages by safely attacking a fraction of the transmitted qubits. Finally, an improvement on the CS QSDC protocol is put forward.