Modification of B92 protocol and the proof of its unconditional security

Shor and Preskill's method with which thy proved the unconditional security of BB84 quantum key distribution protocol was analyzed first. We also indicated that their method could not be applied to prove the unconditional security of B92 scheme directly. In order to take advantage of Shor-Preskill's techniques in their proof of the unconditional security of BB84, we have introduced in this paper a transformation that can exchange between BB84 and B92. By proving that the transformed B92 protocol leaks no more information to eavesdropper, we demonstrate the unconditional security of B92. We also solve the problem how to prove the unconditional security of B92 with Shor-Preskill's method.