  Unconditional security proofs of the Bennett-Brassard protocol of quantum key
distribution have been obtained recently. These proofs cover also practical
implementations that utilize weak coherent pulses in the four signal
polarizations. Proven secure rates leave open the possibility that new proofs
or new public discussion protocols obtain larger rates over increased distance.
In this paper we investigate limits to error rate and signal losses that can be
tolerated by future protocols and proofs.
