ble this type of secure communications environment. Wireless Security First it was the laptop, then it was the cellular phone, and now it is the personal digital assistant (PDA). Mobile users can connect to the world through a wireless carrier or through a wireless LAN (WLAN) or through a Wi-Fi hotspot on which they may be connected. They are then connected to rest of the global network through some gateway. The expanded capabilities of mobile computing have now blurred the boundaries between the types of functionality available. People can access the global network from almost anywhere. Who could believe 10 years ago that cellular phones would be embedded with a color display and the capability of playing real-time video? The issues of security originally encountered by computers have now become everyday issues for all communicating devices. Secure access to servers, to the Internet for browsing or for conducting transactions is a key requirement for mobile devices that communicate through wireless links. From a network-processing standpoint, three fundamental types of security issues appear in this environment: End devices perform session authentication and, in some cases, encrypt and decrypt the two-way air interface, such as the part of the link between the wireless device and the base transceiver or repeater of the WLAN. The rest of the link to the other party with whom one communicates essen- tially remains insecure unbeknownst to the average user. This type of security (when available or when activated) is relatively low in computational requirements. It is easily handled by the embed- ded logic or software in the end device (a handset, laptop, and SO on). Mobile devices that need to engage in secure browsing and electronic transactions are often based on the Wireless Application Protocol (WAP), which uses a security architecture called Wireless Transport Layer Security (WTLS) that is reminiscent of SSL. Figure 17.1 illustrates the structure. The WAP gateway between the mobile device and the server is need