Interdependent and Multi-Subject Privacy: Threats, Analysis and Protection

In Alan Westin's generally accepted definition of privacy, he describes it as an individual's right 'to control, edit, manage, and delete information about them[selves] and decide when, how, and to what extent information is communicated to others.' Therefore, privacy is an individual and independent human right. The great Mahatma Gandhi once said that '€œinterdependence is and ought to be as much the ideal of man as selfsufficiency. Man is a social being.' To ensure this independent right to inherently social beings, it will be difficult, if not impossible. This is especially true as today's world is highly interconnected, technology evolves rapidly, data sharing is increasingly abundant, and regulations do not provide sufficient guidance in the realm of interdependency.

In this thesis, we explore the topic of interdependent privacy from an adversarial point of view by exposing threats, as well as from an end-user point of view, by exploring awareness, preferences and privacy protection needs.

First, we quantify the effect of co-locations on location privacy, considering an adversary such as a social-network operator that has access to this information: Not only can a user be localized due to her reported locations and mobility patterns, but also due to those of her friends (and the friends of her friends and so on). We formalize this problem and propose effective inference algorithms that substantially reduce the complexity of localization attacks that make use of co-locations. Our results show that an adversary can effectively incorporate co-locations in attacks to substantially reduce users'€™ location privacy; this exposes a real and severe threat.

Second, we investigate the interplay between the privacy risks and the social benefits of users when sharing (co-)locations on OSNs. We propose a game-theoretic framework for analyzing users' strategic behaviors. We conduct a survey of Facebook users and quantify their benefits of sharing vs. viewing information and their preference for privacy vs. benefits. Our survey exposes deficits in users' awareness of privacy risks in OSNs. Our results further show how users' individual preferences influence, sometimes in a negative way, each other's decisions.

Third, we consider various types of interdependent and multi-subject data (photo, colocation, genome, etc.) that often have privacy implications for data subjects other than the uploader, yet can be shared without their consent or awareness. We propose a system for sharing such data in a consensual and privacy-preserving manner. We implement it in the case of photos, by relying on image-processing and cryptographic techniques, as well as on a two-tier architecture. We conduct a survey of Facebook users; it indicates that there is interest in such a system, and that users have increasing privacy concerns due to prejudice or discrimination that they have been or could still easily be exposed to.

In conclusion, this thesis provides new insights on users' privacy in the context of interdependence and constitutes a step towards the design of novel privacy-protection mechanisms. It should be seen as a warning message for service providers and regulatory institutions: Unless the interdependent aspects of privacy are considered, this fundamental human right can never be guaranteed.