  We consider the problem of secure communication over a network in the
presence of wiretappers. We give a new cut-set bound on secrecy capacity which
takes into account the contribution of both forward and backward edges crossing
the cut, and the connectivity between their endpoints in the rest of the
network. We show the bound is tight on a class of networks, which demonstrates
that it is not possible to find a tighter bound by considering only cut set
edges and their connectivity.
