  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.
