  In this paper, we study the problem of secure transmission with covert
requirement in untrusted relaying networks. Our considered system model
consists of one source, one destination, one untrusted relay, and one Willie.
The untrusted relay tries to extract the information signal, while the goal of
Willie is to detect the presence of the information signal transmitted by the
source, in the current time slot. To overcome these two attacks, we illustrate
that the destination and the source should inject jamming signal to the network
in phase I and phase II, respectively. Accordingly, the communication in our
proposed system model is accomplished in two phases. In the first phase, when
the source transmits its data to the untrusted relay the destination broadcasts
its jamming signal. In the second phase, when the relay retransmits the
received signal, the source transmits a jamming signal with one of its
antennas. For this system model, we propose a power allocation strategy to
maximize the instantaneous secrecy rate subject to satisfying the covert
requirements in both of the phases. Since the proposed optimization problem is
non-convex, we adopt the Successive Convex Approximation (SCA) approach to
convert it to a convex optimization problem. Next, we extend our system model
to a practical system model where there are multiple untrusted relays and
multiple Willies under two scenarios of noncolluding Willies and colluding
Willies. Our findings highlight that unlike the direct transmission scheme, the
achievable secrecy rate of the proposed secure transmission scheme improve as
the number of untrusted relays increases.
