Cooperation with an Untrusted Relay: A Secrecy Perspective

TL;DR

This paper investigates whether cooperation with an untrusted relay can enhance secrecy rates in different relay network models, providing new achievable rates and bounds that sometimes surpass treating the relay as an eavesdropper.

Contribution

It introduces achievable secrecy rates for untrusted relay channels and derives new upper bounds, demonstrating scenarios where cooperation improves secrecy capacity.

Findings

Cooperation with an untrusted relay can increase secrecy rates in certain models.

The equivocation capacity region is negative for the first model, but positive for the second.

New upper bounds improve existing bounds for specific relay channel classes.

Abstract

We consider the communication scenario where a source-destination pair wishes to keep the information secret from a relay node despite wanting to enlist its help. For this scenario, an interesting question is whether the relay node should be deployed at all. That is, whether cooperation with an untrusted relay node can ever be beneficial. We first provide an achievable secrecy rate for the general untrusted relay channel, and proceed to investigate this question for two types of relay networks with orthogonal components. For the first model, there is an orthogonal link from the source to the relay. For the second model, there is an orthogonal link from the relay to the destination. For the first model, we find the equivocation capacity region and show that answer is negative. In contrast, for the second model, we find that the answer is positive. Specifically, we show by means of the achievable secrecy rate based on compress-and-forward, that, by asking the untrusted relay node to relay information, we can achieve a higher secrecy rate than just treating the relay as an eavesdropper. For a special class of the second model, where the relay is not interfering itself, we derive an upper bound for the secrecy rate using an argument whose net effect is to separate the eavesdropper from the relay. The merit of the new upper bound is demonstrated on two channels that belong to this special class. The Gaussian case of the second model mentioned above benefits from this approach in that the new upper bound improves the previously known bounds. For the Cover-Kim deterministic relay channel, the new upper bound finds the secrecy capacity when the source-destination link is not worse than the source-relay link, by matching with the achievable rate we present.