Privacy Issues in Decentralized Online Social Networks and other Decentralized Systems

Abstract

Popular Online Social Networks (OSNs), such as Facebook or Twitter, are logically centralized systems. The massive information aggregation of sensitive personal data at the central providers of these services is an inherent threat to the privacy of the users. Leakages of these data collections happen regularly – both intentionally, for example by selling of user data to third parties and unintentionally, for example when outsiders successfully attack a provider.

Motivated by this insight, the concept of Decentralized Online Social Networks (DOSNs) has emerged. In these proposed systems, no single, central provider keeps a data collection of all users. Instead, the data is spread out across multiple servers or is distributed completely among user devices that form a peer-to-peer (P2P) network. Encryption is used to enforce access rights of shared content and communication partners ideally connect directly to each other. DOSNs solve one of the biggest privacy concerns of centralized OSNs in a quite forthright way – by getting rid of the central provider. Furthermore, these decentralized systems can be designed to be more immune to censorship than centralized services. But when decentralizing OSNs, two main challenges have to be met: to provide user privacy under a significantly different threat model, and to implement equal usability and functionality without centralized components.

In this work we analyze the general privacy-problems in DOSNs, especially those arising from the more exposed metadata in these systems. Furthermore, we suggest three privacy-preserving implementations of standard OSN features, i.e. user authentication via password-login, user search via a knowledge threshold and an event invitation system with fine-grained privacy-settings. These implementations do not rely on a trusted, central provider and are therefore applicable in a DOSN scenario but can be applied in other P2P or low-trust environments as well. Finally, we analyze a concrete attack on a specific decentralized system, the Tor anonymization network, and suggest improvements for mitigating the identified threats.