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We propose a secure handoff for integration of UMTS and IEEE 802.11 WLANs. The handoff between 802.11 WLAN and UMTS has some drawbacks and could be hijacked in the middle of a communication session. An architecture built for a secure handoff scheme is proposed to fix that problem. When UMTS users handover to an 802.11 WLAN environment, the dynamic key exchange protocol (DKEP) is used to build a secure handoff environment. The mobile station (MS) and the access point (AP) compute their session key individually. The protocol includes three phases, and all the steps of the phases are protected by public-key encryption. Therefore, no information can be hijacked between MS and AP. From the security analysis we know that the handoff between WLAN and UMTS is guaranteed in various aspects. For example, user identity and new register can be protected, avoiding denial of service, key reuse, and so on.