  Several simple yet secure protocols to authenticate the quantum channel of
various QKD schemes, by coupling the photon sender's knowledge of a shared
secret and the QBER Bob observes, are presented. It is shown that Alice can
encrypt certain portions of the information needed for the QKD protocols, using
a sequence whose security is based on computational-complexity, without
compromising all of the sequence's entropy. It is then shown that after a
Man-in-the-Middle attack on the quantum and classical channels, there is still
enough entropy left in the sequence for Bob to detect the presence of Eve by
monitoring the QBER. Finally, it is shown that the principles presented can be
implemented to authenticate the quantum channel associated with any type of QKD
scheme, and they can also be used for Alice to authenticate Bob.
