  Passive light-source side channel in quantum key distribution (QKD) makes the
quantum signals more distinguishable thus provides additional information about
the quantum signal to an eavesdropper. The explicit eavesdropping strategies
aimed at the passive side channel known to date were limited to the separate
measurement of the passive side channel in addition to the operational degree
of freedom. Here we show how to account for the joint eavesdropping on both
operational degree of freedom and the passive side channel of the generic form.
In particular, we use the optimal phase-covariant cloning of the signal photon
state, which is the most effective attack on the BB84 protocol without side
channels, followed by a joint collective measurement of the side channel and
the operational degree of freedom. To estimate QKD security under this attack,
we develop an effective error method and show its applicability to the BB84
decoy-state protocol.
