Effect of Transmission Loss on The Fundamental Security of Quantum Key Distribution

It is shown that the effect of transmission loss has often not been properly taken into account in the security proofs on quantum key distribution. A class of general attacks to be called probabilistic re-sends attack is described that has not been accounted for, which is a generalization of the well-known unique state determination attack. In the case of the four-state single-photon BB84 protocol, it is shown in detail how such attacks are not accounted for in the known security proofs against the simplest individual attacks.