Most of the existing work on key generation from wireless fading channels requires a direct wireless link between legitimate users so that they can obtain correlated observations from the common wireless link. This paper studies the key generation problem in the two-way relay channel, in which there is no direct channel between the key generating terminals. We propose an effective key generation scheme that achieves a substantially larger key rate than that of a direct channel mimic approach. Unlike existing schemes, there is no need for the key generating terminals to obtain correlated observations in our scheme. We also investigate the effects of an active attacker on the proposed key generation protocol. We characterize the optimal attacker's strategy that minimizes the key rate of the proposed scheme. Furthermore, we establish the maximal attacker's power under which our scheme can still achieve a nonzero key rate.