Secure communication using an untrusted relay via sources and channels

Confidential communication aided by a relay without security clearance is studied. General strategies and outer bounds are derived for the problem of secret communication and secret key generation when correlated observations at all terminals are available. In a variation of the problem, it is assumed that the quality of the channel to the relay is known only to the relay. If the throughput-maximizing strategy is used according to the relay´s claimed channel quality, the question is: what should the relay claim about the channel in order to maximize its eavesdropping capabilities? We propose a strategy that Alice and Bob may agree on in order to suppress any leakage of confidential communication between the source and the receiver.