  The design of lattice coset codes for wiretap channels is considered. Bounds
on the eavesdropper's correct decoding probability and information leakage are
first revisited. From these bounds, it is explicit that both the information
leakage and error probability are controlled by the average flatness factor of
the eavesdropper's lattice, which we further interpret geometrically. It is
concluded that the minimization of the (average) flatness factor of the
eavesdropper's lattice leads to the study of well-rounded lattices, which are
shown to be among the optimal in order to achieve these minima. Constructions
of some well-rounded lattices are also provided.
