The standard weak coherent state implementation of quantum cryptography has non-trivial security issues because an eavesdropper can, in principle, suppress single-photon signals relative to multi-photon signals. This leads to fundamental limits to the key generation rate [of order O (\eta2), where \eta is the transmission probability of the channel] and distance of uncondiitonally secure quantum key distribution [of order 20km for Telecom fibers]. Here, we show that the decoy state, first suggested by Hwang, can be used to prove unconditional security in the standard entanglement distillation approach. Our result shows that rather surprisingly, by using only current technology, unconditionally secure quantum key distribution can be achieved with a key generation rate of order O (\eta) and substantially longer distances (more than double). In our method, Alice simply turns the power of her laser up and down and prepares decoy states. The statistical properties of the transmitted decoy states are then analyzed to derive constraints on the signal states.