A key feature of the non-orthogonal multiple access
(NOMA) technique is that users with better channel conditions
have prior information about the messages of other users. The
technique to exploit the prior knowledge of strong users in
order to improve the performance of weak users is known as
cooperative NOMA. In this paper, we study the physical layer
security in such a cooperative NOMA system. In order to reduce
the complexity, the considered system in this paper has two users.
Through the cooperative NOMA concept, the performance of the
weak user is enhanced by the strong user. Given that there is
an eavesdropper in the system that can hear all transmissions,
we study the secrecy rate of the strong and the weak users.
More specifically, we make an attempt to derive the secrecy
outage probability (SOP) of both the users. Due to the intractable
nature of the exact analysis for the weak user, we provide the
closed form expression for the SOP of this user in high SNR
regime while keeping the exactness for the strong user. Through
numerical simulations, we verify the correctness of our analytical
derivations under different scenarios. Besides, we provide the
insights of achieving optimal secrecy performance in such a
system