  Decentralized Online Social Networks (DOSNs) have been proposed as an
alternative solution to the current centralized Online Social Networks (OSNs).
Online Social Networks are based on centralized architecture (e.g., Facebook,
Twitter, or Google+), while DOSNs do not have a service provider that acts as
central authority and users have more control over their information. Several
DOSNs have been proposed during the last years. However, the decentralization
of the OSN requires efficient solutions for protecting the privacy of users,
and to evaluate the trust between users. Blockchain represents a disruptive
technology which has been applied to several fields, among these also to Social
Networks. In this paper, we propose a manageable, user-driven and auditable
access control framework for DOSNs using blockchain technology. In the proposed
approach, the blockchain is used as a support for the definition of privacy
policies. The resource owner uses the public key of the subject to define
flexible role-based access control policies, while the private key associated
with the subject's Ethereum account is used to decrypt the private data once
access permission is validated on the blockchain. We evaluate our solution by
exploiting the Rinkeby Ethereum testnet to deploy the smart contract, and to
evaluate its performance. Experimental results show the feasibility of the
proposed scheme in achieving auditable and user-driven access control via smart
contract deployed on the Blockchain.
