  Proof-of-Work (PoW) consensus mechanism is popular among current blockchain
systems, which leads to an increasing concern about the tremendous waste of
energy due to massive meaningless computation. To address this issue, we
propose a novel and energy-efficient blockchain system, CrowdMine, which
exploits useful crowdsourcing computation to achieve decentralized consensus.
CrowdMine solves user-proposed computing tasks and utilizes the computation
committed to the task solving process to secure decentralized on-chain storage.
With our designed ``Proof of Crowdsourcing Work'' (PoCW) protocol, our system
provides an efficient paradigm for computation and storage in a trustless and
decentralized environment. We further show that the system can defend against
potential attacks on blockchain, including the short-term 51\% attack, the
problem-constructing attack, and the solution-stealing attack. We also
implement the system with 40 distributed nodes to demonstrate its performance
and robustness. To the best of our knowledge, this is the first system that
enables decentralized Proof of Useful Work (PoUW) with general user-proposed
tasks posted in a permissionless and trustless network.
