Secure information exchange occurs in many recently emerging cooperative-based networks, such as 5G networks (especially those with a Device to Device architecture), the Internet of Things, and vehicular ad hoc networks. However, the existing information exchange protocols only focus on either pairwise information exchange or group information exchange, and none of these protocols enable private and public information exchange to occur simultaneously. Thus, a general secure information exchange protocol for a multiuser channel is desirable. With this motivation, this paper investigates simultaneous private and public information exchange in a Multiple Inputs Multiple Outputs (MIMO) multiuser relay channel. In an aim to achieve this goal, signal alignment is chosen as the core technique. With the designed precoding matrix for each user, private information is aligned with its exchange partner, and public information forms a coding chain at the relay. With the aligned private signal and public coding chain, neither an untrusted relay nor external eavesdroppers can recover the original individual information. Performance analyses of the proposed protocol are conducted. First, we conduct transmission performance analyses from the perspective of time slot cost. Second, we conduct a security analysis for private information exchange and public information exchange. Third, we conduct secrecy sum-rate analysis for three attack scenarios: an untrusted relay attack only, an eavesdropper attack only, and both an untrusted relay and eavesdropper attack. The simulations are conducted to demonstrate that the proposed protocol can enable simultaneous private and public information exchange while resisting attacks by an undesired receiver, an untrusted relay, and external eavesdroppers.
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