  Symmetry plays a fundamental role in the security analysis of quantum key
distribution (QKD). Here we review how symmetry is exploited in
continuous-variable (CV) QKD to prove the optimality of Gaussian attacks in the
finite-size regime. We then apply these results to improve the efficiency, and
thus the key rate, of these protocols. First we show how to improve the
efficiency of the energy test, which is one important routine in a CV QKD
protocol aimed at establishing an upper bound on the effective dimensions of
the otherwise infinite-dimensional Hilbert space of CV systems. Second, we show
how the routine of parameter estimation can be made resource efficient in
measurement-device independent (MDI) QKD. These results show that all the raw
data can be used both for key extraction and for the routines of energy test
and parameter estimation.
