Reverse Edge Cut-Set Bounds for Secure Network Coding
  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. 
Wentao Huang and Tracey Ho and Michael Langberg and Joerg Kliewer
cs.IT math.IT