  Classical communications are used in the post-processing procedure of quantum
key distribution. Since the security of quantum key distribution is based on
the principles of quantum mechanics, intuitively the secret key can only be
derived from the quantum states. We find that classical communications are
incorrectly used in the so-called quantum dialogue type protocols. In these
protocols, public communications are used to transmit secret messages. Our
calculations show that half of Alice's and Bob's secret message is leaked
through classical channel. By applying Holevo bound, we can see that the
quantum efficiency claimed in the quantum dialogue type of protocols is not
achievable.
