  Bitcoin's enormous success has inspired the development of alternative
blockchains, such as consortium chains. Several cross-chain protocols have been
proposed as ways of connecting these universes of individual blockchains in a
distributed and secure manner. In this paper, we present Niji, a new
cross-chain protocol that allows parties to perform virtual Bitcoin payment
securely on a consortium chain, without any trusted third-party or mediators.
Our work focuses on the issue that it is difficult for a consortium chain's
token to hold a stable market value, and Niji makes it possible for smart
contract services to acquire means of payment in the consortium chain. With the
Bitcoin payment channel built on the consortium chain, the process from payment
to service provision runs autonomously without any interaction between parties.
Niji introduces the concept of a transaction template to validate Bitcoin
payments efficiently on different blockchains, and it allows a service provider
to delegate all of its tasks for verifying state updates to a smart contract on
the consortium chain. We also propose a novel bi-directional payment channel
adapted for design of the Niji protocol, which can update payments
non-interactively between parties. We implemented a prototype of the Niji
protocol and conducted an experiment measuring the computational cost and
latency that demonstrates the protocol's feasibility on practical platforms.
