  Incorporating the physical layer characteristics to secure communications has
received considerable attention in recent years. Moreover, cooperation with
some nodes of network can give benefits of multiple-antenna systems, increasing
the secrecy capacity of such channels. In this paper, we consider cooperative
wiretap channel with the help of an Amplify and Forward (AF) relay to transmit
confidential messages from source to legitimate receiver in the presence of an
eavesdropper. In this regard, the secrecy capacity of AF relying is derived,
assuming the relay is subject to a peak power constraint. To this end, an
achievable secrecy rate for Gaussian input is evaluated through solving a
non-convex optimization problem. Then, it is proved that any rates greater than
this secrecy rate is not achievable. To do this, the capacity of a genie-aided
channel as an upper bound for the secrecy capacity of the underlying channel is
derived, showing this upper bound is equal to the computed achievable secrecy
rate with Gaussian input. Accordingly, the corresponding secrecy capacity is
compared to the Decode and Forward (DF) strategy which is served as the
benchmark in the current work.
