Vinod M. Prabhakaran Krishnan Eswaran Kannan Ramchandran Secrecy via Sources and Channels -- A Secret Key - Secret Message Rate Trade-off Region http://arxiv.org/abs/0807.0942 2008 CoRR abs/0807.0942 db/journals/corr/corr0807.html#abs-0807-0942 informal publication