  Cooperative relaying is utilized as an efficient method for data
communication in wireless sensor networks and the Internet of Things (IoT).
However, sometimes due to the necessity of multi-hop relaying in such
communication networks, it is challenging to guarantee the secrecy of
cooperative transmissions when the relays may themselves be eavesdroppers,
i.e., we may face with the untrusted relaying scenario where the relays are
both necessary helpers and potential adversary. To obviate this issue, a new
cooperative jamming scheme is proposed in this paper, in which the data can be
confidentially communicated from the source to the destination through multiple
untrusted relays. In our proposed secure transmission scheme, all the
legitimate nodes contribute to providing secure communication by intelligently
injecting artificial noises to the network in different communication phases.
For the sake of analysis, we consider a multi-hop untrusted relaying network
with two successive intermediate nodes, i.e, a three-hop communications
network. Given this system model, a new closed-form expression is presented in
the high signal-to-noise ratio (SNR) region for the Ergodic secrecy rate (ESR).
Furthermore, we evaluate the high SNR slope and power offset of the ESR to gain
an insightful comparison of the proposed secure transmission scheme and the
state-of-arts. Our numerical results highlight that the proposed secure
transmission scheme provides better secrecy rate performance compared with the
two-hop untrusted relaying as well as the direct transmission schemes.
