  In the paper [Zhang, Li and Guo, Phys. Rev. A 64, 024302 (2001)], a quantum
key distribution protocol based on quantum encryption was proposed, in which
the quantum key can be reused. However, it is shown that, if Eve employs a
special strategy to attack, this protocol becomes insecure because of the
reused quantum key. That is, Eve can elicit partial information about the key
bits without being detected. Finally, a possible improvement of the
Zhang-Li-Guo protocol is proposed.
