We present a cryptographic framework to achieve access control, privacy of social relations, secrecy of resources, and anonymity of users in social networks. We illustrate our technique on a core API for social networking, which includes methods for establishing social relations and for sharing resources. The cryptographic protocols implementing these methods use pseudonyms to hide user identities, signatures on these pseudonyms to establish social relations, and zero-knowledge proofs of knowledge of such signatures to demonstrate the existence of social relations without sacrificing user anonymity. As we do not put any constraints on the underlying social network, our framework is generally applicable and, in particular, constitutes an ideal plug-in for decentralized social networks.

We analyzed the security of our protocols by developing formal definitions of the aforementioned security properties and by verifying them using ProVerif, an automated theorem prover for cryptographic protocols. Finally, we built a prototypical implementation and conducted an experimental evaluation to demonstrate the efficiency and the scalability of our framework.

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