Revisiting quantum secure direct communication with W state

The quantum secure direct communication protocol recently proposed by Cao and Song [Chin, Phys. Lett. 23 (2006) 290] (i.e., the C-S QSDC protocol) is revisited. A security leak is pointed out. Taking advantage of this leak, an eavesdropper may adopt the intercept-measure-resend strategy to attack the quantum channel such that in the C-S QSDC protocol the secret message can be completely eavesdropped. To fix the leak, the original version of the C-S QSDC protocol is revised. As a consequence, the security is improved and assured at least in the ideal case of an ideal quantum channel.

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