  We address side-channel leakage in a trusted preparation station of
continuous-variable quantum key distribution with coherent and squeezed states.
We consider two different scenaria: multimode Gaussian modulation, directly
accessible to an eavesdropper, or side-channel loss of the signal states prior
to the modulation stage. We show the negative impact of excessive modulation on
both the coherent- and squeezed-state protocols. The impact is more pronounced
for squeezed-state protocols and may require optimization of squeezing in the
case of noisy quantum channels. Further, we demonstrate that coherent-state
protocol is immune to side-channel signal state leakage prior to modulation,
while the squeezed-state protocol is vulnerable to such attacks, becoming more
sensitive to the noise in the channel. In the general case of noisy quantum
channels the signal squeezing can be optimized to provide best performance of
the protocol in the presence of side-channel leakage prior to modulation. Our
results demonstrate that leakage from the trusted source in continuous-variable
quantum key distribution should not be underestimated and squeezing
optimization is needed to overcome coherent state protocols.
