Access Control in Decentralized Online Social Networks: Applying a Policy-Hiding Cryptographic Scheme and Evaluating Its Performance

Privacy concerns in online social networking services have prompted a number of proposals for decentralized online social networks (DOSN) that remove the central provider and aim at giving the users control over their data and who can access it. This is usually done by cryptographic means. Existing DOSNs use cryptographic primitives that hide the data but reveal the access policies. At the same time, there are privacy-preserving variants of these cryptographic primitives that do not reveal access policies. They are, however, not suitable for usage in the DOSN context because of performance or storage constraints. A DOSN needs to achieve both privacy and performance to be useful. We analyze predicate encryption (PE) and adapt it to the DOSN context. We propose a univariate polynomial construction for access policies in PE that drastically increases performance of the scheme but leaks some part of the access policy to users with access rights. We utilize Bloom filters as a means of decreasing decryption time and indicate objects that can be decrypted by a particular user. We evaluate the performance of the adapted scheme in the concrete scenario of a news feed. Our PE scheme is best suited for encrypting for groups or small sets of separate identities.