  The payment channel, which allows two parties to perform micropayments
without involving the blockchain, has become a promising proposal to improve
the scalability of decentralized ledgers such as Bitcoin and Ethereum. Payment
channels have been extended to the payment network, through which users can
utilize existing channels as intermediary links to route coins to others.
However, routing payments through multiple channels bears nontrivial overheads.
It requires every intermediary channel to lock a portion of its available
capacity until the payment is settled. This may lead to deadlock in a
concurrent situation. The intermediary nodes in a payment path may also charge
fees for routing a payment. The longer the routing path, the more serious the
above problems.
  In this paper, we design and develop a novel off-chain system to shorten the
routing path for the payment network. In particular, we propose the channel
hub, which is an extension of the payment hub, to allows transferring coins
directly from one payment channel to another within the same hub. That is, the
channel hub can be viewed as a shortcut device for the underlying payment
network. We design a new protocol named Boros to perform secure off-chain
cross-channel transfers through the channel hub. We not only present the
security definition of the Boros protocol formally but also prove its security
using the UC-framework. To demonstrate the feasibility of the Boros protocol,
we develop a proof-of-concept prototype running on the Ethereum. Our evaluation
shows that our system can effectively shorten the off-chain routing path.
