Traceability in Permissioned Blockchain In this paper, we have achieved privacy protection and high transparency in a permissioned blockchain. There is a sidechain that connects the permissionless blockchain and the permissioned blockchain. The behavior in the permissioned blockchain is almost a black box from the perspective of the permissionless blockchain. While this fact is useful for privacy protection, there is room for improvement in terms of transparency. To improve the transparency of the permissioned blockchain under privacy protection, we consider traceability in the permissioned blockchain consisting of the following three properties: trade privacy (who trades with whom and at what asset amount), preservation (the total amount inside the permissioned blockchain, including deposits and withdrawals to the permissionless blockchain, is immutable), and noninvolvement (some members in the permissioned blockchain are not involved in some trades, and it is possible to prove that specified members performed the transaction). To the best of our knowledge, we are the first to achieve both preservation and noninvolvement while protecting the privacy of transactions. Our approach is as follows. We model traceability based on the hidden Markov model. Because the proof of traceability requires the calculation of more than quadratic degrees, we encrypt this model by homomorphic encryption. The number of participants in the permissioned blockchain corresponds to the number of additions in the model. Then, we can construct the encrypted model by employing somewhat homomorphic encryption. The establishment of the original model is verifiable by applying the noninteractive zeroknowledge proof of the knowledge that the plaintext is equal to zero. This is an adaptation of Benhamouda  et al.  (Asiacrypt 2014).

Decentralized Access Control Encryption in Public Blockchain Since its invention, the public blockchain has attracted more attention from both the academia and industry because of its fully decentralization and persistency features. However, the privacy issue in public blockchain is still challengeable. While there exists privacy preservation mechanisms proposed for the public blockchain, almost all of them can only solve partial of the privacy issue, either user privacy or data privacy indeed, in it. In this work, we present a decentralized access control encryption scheme which ensures user and data privacy simultaneously in public blockchain. With our cryptographic solution, the validity of one specific transaction can be publicly verified, while its content can only be retrieved by its intended receivers. Moreover, the origin of this transaction cannot be identified by any participant except the receivers in the network. Our analysis shows that our solution is really suitable to deploy in public blockchain and is proven secure under mathematical assumptions.