Two-way secrecy schemes for the broadcast channel with internal eavesdroppers

The secrecy problem in a broadcast setting with one source and a number of users is considered, where the message to a given user must be kept secret from all of the other users. Multi-user diversity suggests an opportunistic approach that sends a message secretly to the user with the current best channel; however, the secrecy rate goes to zero in the limit of a large number of users. Here, channel reciprocity is exploited via a two-way secrecy scheme to provide a constant positive secrecy rate to the user with the best channel. Next, motivated by the desire to transmit to a given user (rather than opportunistically to the user with the best channel), a second scheme is developed that employs relaying from other users from whom the message is still kept secret. In this non-opportunistic case, a positive secrecy rate is again shown to be achievable in the limit of a large number of system users.