Location-based applications such as Foursquare, Glympse, or Waze attract millions of users by implementing points of interest finders, geosocial networking, trajectory sharing, or real-time traffic monitoring. An essential requirement for these applications is the knowledge of user location information, i.e., the user's position or his movement trajectory. Location-based applications typically act as clients to a location service, which manages mobile object location information in a scalable fashion and provides various clients with this information.

However, sharing location information raises user privacy concerns, especially if location service providers are not fully trustworthy and user location information can be exposed. For instance, an attacker successfully compromising a location service may misuse the revealed location information for stalking, mugging, or to derive personal user information like habits, preferences, or interests of the user. Driven by the increasing number of reported incidents where service providers did not succeed in protecting private user information adequately, user privacy concerns are further intensified.

Therefore, we present novel approaches protecting user location privacy when sharing location information without assuming location service providers to be fully trustworthy. To protect user position information, we present our position sharing concept. Position sharing allows to reveal only positions of decreased precision to different location services, while clients can query position shares from different location services to increase precision. To protect movement trajectories, we introduce our trajectory fragmentation approach and an approach protecting the speed information of movement trajectories.