Two-Point Attack on the Two Nonorthogonal State QKD Protocol over a Fiber-Optic Channel

We suggest here a two-point eavesdropping strategy to two nonorthogonal states protocol of quantum key distribution over a fiber-optic channel. Suppose that the single-photon sources and detectors of Alice, Bob and Eves are ideal ones, the two-point attack can break the two nonorthogonal states protocol if the distance between Alice and Bob is sufficiently long. For single-photon type B92 protocol, the upper bound of distance is about 30 kilometers. It is interesting that Bennett's original multi-photon protocol is secure against both two-point attack and beam splitting attack, even if the protocol is realized with weak pulse source.