We define a new class of quantum quantum key distribution (QKD) protocols that are robust against the most general photon number splitting attacks. We analyze one of these protocols, which differs from the BB84 only in the classical sifting procedure. These results imply that a secret key can be extracted from the data of recent long-distance QKD experiments performed with weak laser pulses [D. Stucki et al., New J. Phys. {\bf 4}, 41 (2002)].