  The method of decoy-state quantum key distribution (QKD) requests different
intensities of light pulses. Existing theory has assumed exact control of
intensities. Here we propose a simple protocol which is secure and efficient
even there are errors in intensity control. In our protocol, decoy pulses and
signal pulses are generated from the same father pulses with a two-value
attenuation. Given the upper bound of fluctuation of the father pulses, our
protocol is secure provided that the two-value attenuation is done exactly. We
propose to use unbalanced beam-splitters for a stable attenuation. Given that
the intensity error is bounded by $\pm5%$, with the same key rate, our method
can achieve a secure distance only 1 km shorter than that of an ideal protocol
with exactly controlled source.
