Online social networks (OSNs) such as Facebook and Google+ have transformed the way our society communicates. However, this success has come at the cost of user privacy; in today's OSNs, users are not in control of their own data, and depend on OSN operators to enforce access control policies. A multitude of privacy breaches has spurred research into privacy-preserving alternatives for social networking, exploring a number of techniques for storing, disseminating, and controlling access to data in a decentralized fashion. In this paper, we argue that a combination of techniques is necessary to efficiently support the complex functionality requirements of OSNs.

A promising approach to mitigate the privacy risks in Online Social Networks (OSNs) is to shift access control enforcement from the OSN provider to the user by means of encryption. However, this creates the challenge of key management to support complex policies involved in OSNs and dynamic groups. To address this, we propose EASiER, an architecture that supports fine-grained access control policies and dynamic group membership by using attribute-based encryption. A key and novel feature of our architecture, however, is that it is possible to remove access from a user without issuing new keys to other users or re-encrypting existing ciphertexts. We achieve this by creating a proxy that participates in the decryption process and enforces revocation constraints. The proxy is minimally trusted and cannot decrypt ciphertexts or provide access to previously revoked users. We describe EASiER architecture and construction, provide performance evaluation, and prototype application of our approach on Facebook.

Attribute-based Access Control (ABAC) based on XACML can substantially improve the security and management of access rights on databases. However, existing implementations rely on high-level policy interpretation and are not as efficient as mechanisms natively supported by commodity databases. In this paper we explore advantages and challenges arising from compiling XACML policies for database access into Access Control Lists (ACLs) natively supported by the database. The main contributions are an architecture and algorithms for efficiently addressing incremental changes in attributes that could trigger changes to the ACLs. We consider this in a context of reflective database access control where attributes used in access decisions are stored in the database itself. Our implementation and experiments demonstrate a significant improvement in access decision times compared to the best available optimizations for general XACML access engines.