The demand for cheap broadband Internet for nomadic users has created a market for Internet sharing. Wi- Fi communities which allow their users to share their wired Internet connections have emerged and become increasingly popular. Organizations like FON promise to provide free wireless Internet access in many places. However, user authentication is the Achilles heel of these systems. A user that allows other community members to use its access point must expect to be held responsible for other users? actions. Moreover, these Wi-Fi sharing systems are often insecure which allows eavesdroppers to gather sensitive information on the wireless link. This work provides efficient, scalable, and secure access control for large Wi-Fi sharing systems. The Host Identity Protocol (HIP) is used as a building block for a solution which supports strong user authentication as well as mobility support for nomadic users. In our presentation, we show the feasibility and effectiveness of this approach by demonstrating the PISA authentication protocol in action.