  We consider the security of a system of quantum key distribution (QKD) using
only practical devices. Currently, attenuated laser pulses are widely used and
considered to be the most practical light source. For the receiver of photons,
threshold (or on/off) photon detectors are almost the only choice. Combining
the decoy-state idea and the security argument based on the uncertainty
principle, we show that a QKD system composed of such practical devices can
achieve the unconditional security without any significant penalty in the key
rate and the distance limitation.
