Fine-grained cloaking of sensitive positions in location sharing applications Maria Luisa Damiani Universit` a di Milano, I Claudio Silvestri Universit` a di Venezia, I Elisa Bertino Purdue University, USA July 22, 2010 Abstract Geo-social networking applications magnify the concern for location privacy because the users’ positions are disclosed to several parties and not all of these parties are trusted. A challenging privacy requirement concerns the protection of sensitive positions against the untrusted mem- bers of a geo-social network. Consider the case of a user who does not want to reveal her friends the sensitive place in which she is located, e.g., hospital. Typically the user discloses a cloaked region in place of the exact position. However a malicious observer aware of the geography of the territory as well of the distribution of positions can easily defeat the protection mechanism and detect the place in which the user is located. In this paper we elaborate on this inference attack, named semantic lo- cation identification, and present a comprehensive framework supporting semantic location cloaking. 1 Introduction Geo-social networking applications, i.e., Google Latitude and Geo-Twitter, raise important challenges for the protection of the location privacy. These appli- cations use advanced positioning functionalities to seamlessly localize mobile customers in both indoor and outdoor spaces with increasing precision. For example, the Skyhook positioning system integrates GPS, Cell Tower triangu- lation and Wi-Fi Positioning and computes the position of mobile phones with an accuracy of 10-20 metres 1 . In such a setting, the user’s position is disclosed to several parties including the positioning service provider which gathers loca- tion information and conveys it upon request; the location-based service (LBS) provider; and the members of the geo-social network. It is often the case that some of these parties are untrustworthy. Therefore, the position itself or details 1 http://www.skyhookwireless.com/ 1