  Recently, Zhang, Li, and Guo have proposed a particular eavesdropping attack
[Phys. Rev. A {\bf 63}, 036301 (2001), quant-ph/0009042] which shows that my
quantum key distribution protocol based on entanglement swapping [Phys. Rev. A
{\bf 61}, 052312 (2000), quant-ph/9911025] is insecure. However, security
against this attack can be attained with a simple modification. In addition, a
simpler version of the protocol using four qubits is introduced.
