  To mitigate the scalability problem of decentralized cryptocurrencies such as
Bitcoin and Ethereum, the payment channel, which allows two parties to perform
secure coin transfers without involving the blockchain, has been proposed. The
payment channel increases the transaction throughput of two parties to a level
that is only limited by their network bandwidth. Recent proposals focus on
extending the two-party payment channel to the N-party payment hub.
Unfortunately, none of them can achieve efficiency, flexibility in the absence
of a trusted third-party. In this paper, we propose Garou, a secure N-party
payment hub that allows multiple parties to perform secure off-chain coin
transfers. Except in the case of disputes, participants within the payment hub
can make concurrent and direct coin transfers with each other without the
involvement of the blockchain or any third-party intermediaries. This allows
Garou to achieve both high-performance and flexibility. Garou also guarantees
that an honest party always maintains its balance security against strong
adversarial capabilities. To demonstrate the feasibility of the Garou protocol,
we develop a proof of concept prototype for the Ethereum network. Our
evaluation results show that the maximum transaction throughput of Garou is 20
times higher than that of state-of-art payment hubs.
