Anonymous Opinion Exchange over Untrusted Social Networks Mouna Kacimi Max Planck Institute for Informatics Saarbrucken, Germany mkacimi@mpi-inf.mpg.de Stefano Ortolani Vrije Universiteit Amsterdam, The Netherlands ortolani@few.vu.nl Bruno Crispo University of Trento Trento, Italy crispo@disi.unitn.it ABSTRACT Social networks are the fastest growing Internet applications. They offer the possibility to get in touch with current friends, discover where the old ones are, and make new ones. While these applications are a great enabler for our social life, they are also well known to fall short on privacy. The lack of ade- quate privacy enhancing technology is particularly important in these applications due to the nature of information they deal with, and the fact that many users are underage. This paper provides a contribution in this direction by present- ing a protocol, tailored for social network applications, that allows users to ask and/or submit personal opinions while preserving their anonymity. 1. INTRODUCTION Online social networks are one of the most successful stories in the life of Internet. For an increasing num- ber of people, a big part of their social life is now en- tirely built and developed online. The most popular platforms claim tens of millions of users, where a good portion of them spend several hours a day using such platforms. Users can publish their personal content and share it with other people in the network. More- over, they can create social relations with their friends (from real life), discover new people with common in- terests, express opinions and exchange comments about disparate subjects and topics. Examples of social net- work platforms include Facebook, MySpace, Flickr and Orkut. The impact such networks may have on users’ pri- vacy raises serious concerns [16, 12]. Most users share and exchange private information without a clear un- Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. SNS’09, March 31, 2009, Nuremberg, Germany. Copyright 2009 ACM ISBN 978-1-60558-463-8 ...$5.00.. derstanding of the possible consequences. The reason is that users are not in control of where information will be stored and how it is used by other users and by the platform. Thus, privacy enhancing technologies (PETs), specifically tailored for online social networks, are sorely needed. This paper presents a protocol that allows anony- mous opinion exchange among users. Such anonymity is still needed today to avoid serious consequences of what people could say. For example, in countries with oppressive regimes, citizens are not free to express dis- agreement or criticism about their government. Thus, anonymity in such environments is an important step towards freedom of speech. Furthermore, by imple- menting anonymous opinion exchange, the protocol ad- dresses the problem of the spiral of silence. 1 Existing solutions [5, 6, 3] providing anonymous in- formation sharing rely on a global knowledge of the net- work topology. This knowledge cannot be realistically assumed in online social network applications. Addi- tionally, they require the requestor to know the respon- der in advance . For these reasons, existing solutions cannot be adopted to solve the problem presented in this paper. In social networks, users do not know the connectivity graph. Thus, the proposed protocol re- lies only on local knowledge for transmitting informa- tion. Additionally, since the protocol aims to preserve anonymity, the requestor does not know who the re- sponder (if any) will be. This paper proposes a new friends-to-friends deliv- ery protocol allowing anonymous and private sharing of opinions between a large number of users. This mecha- nism is deployed (1) over an untrusted network and (2) in the presence of a central infrastructure, such as the servers of the online platform. To achieve anonymity of both requestors and responders, queries and answers are forwarded via trusted links (friends). Queries and an- swers travel hop-by-hop in the friendship graph. Each 1 As the spiral of silence theory[11] asserts, a person belong- ing to the minority is less likely to voice an opinion on a topic for fear of reprisal or isolation from the majority. 26