In this paper, we consider network assisted device-to-device (D2D) communication between two user equipments (UEs) in a cellular network. For the scenario, when both the UEs are in network coverage, we propose novel key exchange protocols for D2D communication. The proposed protocols are based on the standard Diffie-Hellman (DH) based key exchange and other cryptographic functions. We present detailed threat analysis for the proposed protocols and show their resilience against man-in-the-middle attack. Further, through numerical results, we show that the proposed protocols have low computational time and low communication overhead in comparison with existing key exchange protocols for D2D communication. The proposed protocols are practically implementable and highly suitable for D2D communications in 5G cellular networks. © 2016 IEEE.