Transparent Registration-Based Encryption through Blockchain

Garg et al. (TCC 2018) defined the notion of registration-based encryption (RBE) where the private key generator (PKG) is decoupled from key management and replaced by a key curator (KC). KC does not possess any cryptographic secrets and only plays the role of aggregating the public keys of all the registered users and updating the public parameters whenever a new user joins the system, which solves the key escrow issue. Notwithstanding, RBE still places a significant amount of trust in KC, whose actions are not accountable, e.g., it could secretly register multiple keys for already registered users. In this article, we propose a blockchain-based RBE framework, which provides total transparency and decentralization of KC by leveraging smart contracts. Our framework transfers the right of key management from KC to individual participants and keeps publicly upgradable parameters on-chain. We provide a basic construction that calculates the public parameter on-chain and an extended construction with better efficiency, which merely calculates the roots of trees on-chain. Our basic version is theoretically feasible, while the extended version is practically feasible. In particular, the enhanced scheme reduces computing complexity to a constant level. Our prototype implementation and evaluation results demonstrate that our extended construction is satisfactorily efficient.