Secure and Usable Services in Opportunistic Networks

The last 10 years of Internet development have been dominated by the growing popularity of cloud computing and online social networks. These enable the building of new services based on social information and the reducing cost of building new services. Despite the benefits cloud computing and online social networks bring, the necessity of the cloud availability, ownership of data stored in the cloud, and security of data may be blocking factors for the development of some services.  In this dissertation, we examine the possibilities of creating inexpensive, secure and localized services that can operate independently of cloud services. The starting point for this work is opportunistic networks. In these networks, human-carried mobile devices communicate directly via a short-range wireless technology without any infrastructure support. Despite their presence for the last couple of years, opportunistic networks have not gained the expected popularity.  We argue that providing efficient privacy-preserving tools in opportunistic networks offers an advantage for some services over their cloud equivalents. Thus, we present an efficient privacy-preserving protocol for discovering common friends and length of social path between total strangers. We argue that the existence and strength of a social relationship is an important factor that can be used by opportunistic services to build trust and make access control decisions for nearby devices. Furthermore, we speculate that our protocols can be easily extended to discover any social attributes between users, which can enable building secure and socially-aware localized services.  The second part of this dissertation concerns mechanisms for increasing user engagement in opportunistic networks. One of the reasons behind the slow adoption of opportunistic networks is that some mobile operating systems do not permit running opportunistic software on their platforms. To address this problem, we enable the accessing of and generating content in opportunistic networks through web browsers. Furthermore, we propose usage of web storage capabilities to forward messages to increase the overall capacity of opportunistic networks. We also present a generic framework that enables running sophisticated web applications in the opportunistic environment.  In the end, we discuss how the contributions of this work can also be applied in other use cases in opportunistic networks, in particular for social-based routing, topology control, message forwarding strategies, trust establishment, and opportunistic computing.