On Identity, Transaction, and Smart Contract Privacy on Permissioned and Permissionless Blockchain: A Comprehensive Survey

Blockchain is a decentralized distributed ledger that combines multiple technologies, including chain data structures, P2P networks, consensus algorithms, cryptography, and smart contracts. This gives the blockchain the characteristics of decentralization, immutability, and traceability. However, blockchain stores smart contracts and transactions in blocks publicly, which poses the risk of data leakage and misuse. For example, by mining and analyzing blockchain transaction information, attackers can correlate transactions with user information, resulting in the disclosure of user privacy. Many current reviews focus on the privacy of permissionless blockchains or cryptocurrencies, requiring more in-depth investigations and detailed categorical analysis. To fill this gap, this work comprehensively reviews the latest and traditional methods related to identity, transaction, and smart contract privacy within permissioned and permissionless blockchains. Additionally, we summarize the existing problems, threats, and challenges of data management in different blockchain architectures. Last, we discuss future research directions for blockchain privacy protection technology, which can offer feasible ideas for researchers to explore further.