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Blockchain technologies have recently received considerable attention, partly due to the success of cryptocurrency applications such as Bitcoin and Ethereum. As the adoption of blockchain technologies by various sectors increases, there is a demand for tools that enable regulatory enforcement, which include monitoring, examining and ensuring compliance of the data stored by the blockchain systems, all in a privacy preserving way. Current blockchain solutions store transactions in append-only and immutable fashion without any indexing, which contributes to limited and inefficient queries. Additionally, there is no support for privacy-preserving query processing. To address these issues, in this paper, we propose a system that can provide auditors, enforcing regulatory compliance, with efficient, scalable and richer blockchain query processing over Hadoop and synchronized Ethereum clients. The system …