  Recently proposed quantum key distribution protocols are shown to be
vulnerable to a classic man-in-the-middle attack using entangled pairs created
by Eve. It appears that the attack could be applied to any protocol that relies
on manipulation and return of entangled qubits to create a shared key. The
protocols that are cryptanalyzed in this paper were proven secure with respect
to some eavesdropping approaches, and results reported here do not invalidate
these proofs. Rather, they suggest that quantum cryptographic protocols, like
conventional protocols, may be vulnerable to methods of attack that were not
envisaged by their designers.
