Enhancing Permissioned Blockchains with Controlled Data Authorization

Blockchains provide data with availability and integrity. Many applications, however, also require strong confidentiality, a goal that conventional blockchains fail to achieve. This work enhances permissioned blockchains based on Byzantine fault tolerance (BFT) with the ability to manage confidential data in a controlled manner. For this goal, we formally define threshold encryption with controlled authorization (ECA). In an ECA scheme, writers can send transaction messages to readers via servers (e.g., blockchain servers). There are many challenges for ECA — decoupling writers and readers, message confidentiality, reliability of the system, and the tolerance of malicious behaviors. We define the syntax for the ECA system and formalize three security notions for ECA, namely token verifiability, t-robustness, and t-collusion resistant CPA security. We propose an ECA scheme and provide formal proofs of these notions for our proposed ECA scheme. Our ECA scheme decouples writers and readers with the help of servers. The token verifiability guarantees all the servers obtain the right tokens from writers. The t-robustness makes sure our ECA scheme runs well even if t malicious servers implement active attacks. The t-collusion resistant CPA security guarantees message privacy from servers, even if t servers collude with each other to implement active attacks. Finally, we offer an efficient ECA construction satisfying all these security notions.