introDuction Motivated by the requirements for higher data rate, richer multimedia services, and broader radio range wireless mobile networks are currently in the stage evolving from the second-generation (2G), for example , global system for mobile communications (GSM), into the era of third-generation (3G) or beyond 3G or fourth-generation (4G). Universal mobile telecommunications system (UMTS) is the natural successor of the current popular GSM (http://www.3gpp.org) code division multiple access 2000 (CDMA2000) is the next generation version for the CDMA-95, which is predominantly deployed in North America and North Korea. Time division-sychrononous CDMA (TD-SCDMA) is in the framework of 3rd generation partnership project 2 (3GPP2) and is expected to be one of the principle wireless technologies employed in China in the future (http://www.3gpp.org; 3G TS 35.206). It is envisioned that each of three standards in the framework of international mobile telecommunications-2000 (IMT-2000) will play a significant role in the future due to the backward compatibility, investment , maintenance cost, and even politics. In all of the potential standards, access security is one of the primary demands as well as challenges to resolve the deficiency existing in the second generation wireless mobile networks such as GSM, in which only one-way authentication is performed for the 1438 Access Security in UMTS and IMS core network part to verify the user equipment (UE) (3G TS 24.008). Such access security may lead to the " man-in-middle " problem, which is a type of attack that can take place when two clients are communicating remotely and exchange public keys in order to initialize secure communications. If both of the two public keys are intercepted in the route by someone, he/she can act as a conduit and send in the messages with his/her own faked public key. As a result, the secure communication is eavesdropped by a third party. Multimedia service provisioning is one of the primary demands and motivations for the next generation wireless networks. To achieve this goal, the IP multimedia subsystem (IMS) is added as the core network in UMTS providing the multimedia service, for example, voice te-lephony, video conference, real-time streaming media, interactive game, voice over IP, picture, HTTP, and instant messaging (3G TS 33.203). The multimedia session management, initialization, and termination are specified and implemented in the session initiation protocol (SIP) (3G TS 29.228; Zhang & Fang, 2005). To ensure the secure communication in a multimedia session, an efficient access security mechanism …