Hangout: A Privacy Preserving Location Based Social Networking Service. Murali Annavaram, Ming Hsieh Department of Electrical Engineering, University of Southern California, Los Angeles. annavara@usc.edu Quinn Jacobson Nokia Research Center, Palo Alto. quinn.jacobson@nokia.com Abstract As mobile devices enter a new era with high speed connectivity and increasing compute capabilities a new class of applications called social networking applications is being showcased as the next revolution in mobile computing. In this class of applications each user in a social group contributes their knowledge about their surrounding environments and the collective knowledge can then be exploited by the group members for a personal or social benefit. As the popularity of mobile social networks increases there is a growing realization that information collected about an individual user can compromise one's privacy and potentially security. It is in this context we developed HangOut a privacy preserving social networking application. Hangout protects user’s private information not only from other malicious users but even from system administrators who may have unrestricted access to the backend server that is providing the social networking service. Hangout uses location and time distortions, symmetric key encryption where the keys are exchanged in a peer-to-peer fashion and several client side controls to aggressively protect privacy with minimal degradation in user’s perceived service value. 1. Introduction to Device Variations The compute capability of today’s high end mobile devices rivals the desktop performance of the early 90s. Thanks to Moore’s law growth in transistor count these devices also integrate a rich set of environmental sensors such as GPS receivers, high resolution video cameras. As these feature rich devices ubiquitously connect us to the digital world they continuously collect and store information that pertains to the user. The information could be user location collected through GPS readings, pictures taken by the user from the cell phone camera. Some of the information stored will become sensitive information that relates to user’s movement history in the form of GPS track. Applications and service providers rely on accessing this sensitive information to provide useful services. One prominent application is location based services that use the GPS capability to provide information that is relevant to a user’s location. While location information is necessary for providing these services, these devices can potentially become tracking devices if the location information is continuously revealed to the application service providers. As the popularity of these applications grows information privacy is a cause for serious concern. In response to growing privacy concerns several mobile service providers have published their privacy policies. These policies are intended to restrict how the information collected from mobile users will be used. For instance, the policy guidelines include enforcing data access control where only a limited number of people or software modules are given access to the user location information. However, such policy guidelines alone have been shown to be ineffective either because the data integrity is compromised by hackers or because of inadvertent disclosure of information to those who don’t need access to such a data. The social expediency of preserving privacy in mobile environments can not be emphasized enough if mobile applications have to continue to flourish. This paper presents HangOut a new location based mobile service prototype developed at the Nokia Research Center in Palo Alto that focuses exclusively on preserving location privacy of the user while still providing location relevant information. More specifically, HangOut is a social networking application that allows its users to interact with each other without ever disclosing the precise location information of any single user. HangOut preserves participants’ privacy in two fundamental ways. It allows users to control their privacy while sharing their location with other participants. It also protects users against having their location tracked by anyone, including the service provider itself. HangOut ensures that there is no record of where users where that can be recreated from the system. The rest of this paper is organized as follows. Section 2 describes the HangOut prototype and the various usage models that are implemented in the prototype. Section Error! Reference source not found. presents