I. Introduction
Recently, leveraging relay to enhance wireless security has received considerable attentions [1], [2]. For these work, additional nodes are introduced into the network as relays who can provide extra degree of freedom in reinforcing the communication security. However, as mentioned in [3] and [4], all these work are based on an assumption that the relay has the same security clearance as the source and destination. In practical networks, there may exist relays with lower security clearance or untrustworthy relays that we need to take into consideration. Moreover, this issue becomes more serious if the untrusted relay is the only way for source and destination to establish connections between them.