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 ( \(\textsf{ECA}\) ). In an \(\textsf{ECA}\) scheme, writers can send transaction messages to readers via servers (e.g., blockchain servers). There are many challenges for \(\textsf{ECA}\) — decoupling writers and readers, message confidentiality, reliability of the system, and the tolerance of malicious behaviors. We define the syntax for the \(\textsf{ECA}\) system and formalize three security notions for \(\textsf{ECA}\) , namely token verifiability, t-robustness, and t-collusion resistant CPA security. We propose an \(\textsf{ECA}\) scheme and provide formal proofs of these notions for our proposed \(\textsf{ECA}\) scheme. Our \(\textsf{ECA}\) 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 \(\textsf{ECA}\) 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 \(\textsf{ECA}\) construction satisfying all these security notions.