  The role of the side channels in the continuous-variable quantum key
distribution is studied. It is shown how the information leakage through a side
channel from the trusted sender station increases the vulnerability of the
protocols to the eavesdropping in the main quantum communication channel.
Moreover, the untrusted noise infusion by an eavesdropper on the trusted
receiving side breaks the security even for a purely attenuating main quantum
channel. As a method to compensate for the effect of the side-channel leakage
on the sender side, we suggest several types of manipulations on the
side-channel input. It is shown that by applying the modulated coherent light
on the input of the side channel that is optimally correlated to the modulation
on the main signal and optionally, introducing additional squeezing in the case
of the squeezed-state protocol, the negative influence of the lossy side
channel on the sender side can be completely removed. For the trusted receiving
side, the method of optimal monitoring of the residual noise from the
side-channel noise infusion is suggested and shown to be able to completely
eliminate the presence of the noisy side channel. We therefore prove that the
side-channel effects can be completely removed using feasible operations if the
trusted parties access the respective parts of the side channels.

Quantum Physics