In this paper, we consider a two-way relay network where two sources can communicate only through an unauthenticated intermediate relay node. We investigate the secure communication of this two-way relay scenario using physical layer security. Specifically, we treat the relay node as an eavesdropper from whom the information transmitted by the sources needs to be kept secret, despite the fact that its cooperation in relaying this information is essential. We first find that a non-zero secrecy rate is indeed achievable in this two-way relay network even without external jammers. Further still, with the help of friendly jammers that transmit the jamming signals to confuse the malicious relay, a positive gain of the secrecy rate can be realized. In order to obtain the maximum secrecy rate, we define and then analyze a source optimization problem. Finally, an optimal solution on the transmit power allocation of all the nodes is provided for the system without and with using friendly jammers.