  We consider lattice coding for the Gaussian wiretap channel, where the
challenge is to ensure reliable communication between two authorized parties
while preventing an eavesdropper from learning the transmitted messages.
Recently, a measure called the secrecy function of a lattice coding scheme was
proposed as a design criterion to characterize the eavesdropper's probability
of correct decision. In this paper, the family of formally unimodular lattices
is presented and shown to possess the same secrecy function behavior as
unimodular and isodual lattices. Based on Construction A, we provide a
universal approach to determine the secrecy gain, i.e., the maximum value of
the secrecy function, for formally unimodular lattices obtained from formally
self-dual codes. Furthermore, we show that formally unimodular lattices can
achieve higher secrecy gain than the best-known unimodular lattices from the
literature.
