  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.
