  Lattice coding over a Gaussian wiretap channel, where an eavesdropper listens
to transmissions between a transmitter and a legitimate receiver, is
considered. A new lattice invariant called the secrecy gain is used as a code
design criterion for wiretap lattice codes since it was shown to characterize
the confusion that a chosen lattice can cause at the eavesdropper: the higher
the secrecy gain of the lattice, the more confusion. In this paper, a formula
for the secrecy gain of unimodular lattices is derived. Secrecy gains of
extremal odd unimodular lattices as well as unimodular lattices in dimension n,
16 \leq n \leq 23 are computed, covering the 4 extremal odd unimodular lattices
and all the 111 nonextremal unimodular lattices (both odd and even) providing
thus a classification of the best wiretap lattice codes coming from unimodular
lattices in dimension n, 8 < n \leq 23. Finally, to permit lattice encoding via
Construction A, the corresponding error correction codes are determined.
