  The decoy-state method in quantum key distribution (QKD) is a popular
technique to approximately achieve the performance of ideal single-photon
sources by means of simpler and practical laser sources. In high-speed
decoy-state QKD systems, however, intensity correlations between succeeding
pulses leak information about the users' intensity settings, thus invalidating
a key assumption of this approach. Here, we solve this pressing problem by
developing a general technique to incorporate arbitrary intensity correlations
to the security analysis of decoy-state QKD. This technique only requires to
experimentally quantify two main parameters: the correlation range and the
maximum relative deviation between the selected and the actually emitted
intensities. As a side contribution, we provide a non-standard derivation of
the asymptotic secret key rate formula from the non-asymptotic one, in so
revealing a necessary condition for the significance of the former.
