  A simple photon-number splitting attack is described which works on any lossy
quantum key distribution system with a multi-photon source independently of the
mean source photon number, and with no induced error rate. In particular, it
cannot be detected by decoy states. The quantitative loss of security is
similar when the user employs photon-number resolving detectors or threshold
detectors. Numerical values indicate that existing implementations of concrete
QKD systems are fundamentally insecure against this attack because a large
portion of leaked sifted key bits is not accounted for. The possibility of
other damaging photon-number splitting attacks is discussed. Some morals will
be drawn.
