  This paper studies the problem of information theoretic secure communication
when a source has private messages to transmit to $m$ destinations, in the
presence of a passive adversary who eavesdrops an unknown set of $k$ edges. The
information theoretic secure capacity is derived over unit-edge capacity
separable networks, for the cases when $k=1$ and $m$ is arbitrary, or $m=3$ and
$k$ is arbitrary. This is achieved by first showing that there exists a secure
polynomial-time code construction that matches an outer bound over two-layer
networks, followed by a deterministic mapping between two-layer and arbitrary
separable networks.
