Blockchain-based crowdsourcing solutions have been widely used in the industry to solve insider threats and free-riding/false-reporting issues. However, the existing solutions suffer from three main limitations: 1) inability to concurrently achieve task privacy protection and distributed transparent storage, 2) limited transaction throughput caused by serial transaction processing, and 3) low fault-tolerance ability of their practical byzantine-fault-tolerance (PBFT) consensus protocols. In order to overcome these limitations, this paper proposes a novel hybrid-blockchain crowdsourcing platform named CHChain. The core of CHChain is a hybrid blockchain structure, which achieves distributed and transparent storage while guaranteeing task privacy by isolating the private information of each task (e.g., task response, evaluation, and feedback) into its own private task chain, and recording other common information of tasks (e.g., task ID, reward, and deadline) into a common chain. To improve transaction throughput, we then design a parallel-transaction-processing chaincode (PTPC), a chaincode running on the hybrid blockchain structure, to process transactions in parallel, which is also responsible for evaluating the reputations of platform users. For fault-tolerance enhancement, we finally design a Reputation-based PBFT (R-PBFT) consensus protocol based on the hybrid reputation values of platform users. Extensive experiment results showed that the transaction throughput of CHChain is nearly twice as much as those of the existing solutions. Moreover, the fault tolerance of R-PBFT can be approximately 98%, which is much higher than that of the PBFT, 1/3.

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