  Side-channel attacks on practical quantum key distribution systems compromise
its security. Although some of these attacks can be taken into account, the
general recipe of how to eliminate all side-channel flaws is still missing. In
this work, we propose a method for estimation of the total passive side-channel
information leakage from the Alice's light source. The method relies on
Hong-Ou-Mandel interference between different signals emitted by Alice, which
reveals their overall mode mismatch without the necessity to measure all
individual degrees of freedom independently. We include experimental values of
interference visibility in the security proof for the decoy-state BB84
protocol, and lower-bound the secure key rate for realistic light sources. The
obtained results provide a tool that can be used for certification of the
current QKD systems and pave the way towards the loophole-free design of the
future ones.
