In this paper, we propose a secure handoff scheme for data transmission in integration of 3G and 802.11 wireless local area networks (WLANs). The handoff between 802.11 WLAN and the 3G suffers from some drawbacks and has been hijacked through the middle of a communication session. We propose an architecture based on cloud computing to build our scheme to fix the problems in such heterogeneous wireless networks. Adaptive Key Exchange Protocol is proposed to protect data transmission as 3G users hand over to an 802.11 WLAN. The approach includes three phases, and all steps of each phase are protected by robust public-key encryption. Therefore, no information can be hijacked in such environment. The security analysis shows that data transmission between 802.11 WLAN and 3G is robust and secure in various aspects. Copyright © 2012 John Wiley & Sons, Ltd.