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The GHZ-state-based quantum secure direct communication (QSDC) protocol [X.-R. Jin, et al., Phys. Lett. A 354 (2006) 67] and its improved version [Z. Man, Y. Xia, Chin. Phys. Lett. 24 (2007) 15] are analyzed from the aspect of security. It shows that much information of the transmitted secret message will be leaked out in both protocols.
Two misuses of one-time pad in improving the efficiency of quantum communication are pointed out. One happens when some message bits are used to encrypt others, the other exists because the key bits are not truly random. Both of them result in the decrease of security. Therefore, one-time pad should be used carefully in designing quantum communication protocols.
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