Comment on "Quantum key distribution via quantum encryption"

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.