Bitcoin Clique: Channel-Free Off-Chain Payments Using Two-Shot Adaptor Signatures

Blockchains suffer from scalability limitations, both in terms of latency and throughput. Various approaches to alleviate this have been proposed, most prominent of which are payment and state channels, sidechains, commit-chains, rollups, and sharding. This work puts forth a novel commit-chain protocol, Bitcoin Clique. It is the first trustless commit-chain that is compatible with all major blockchains, including (an upcoming version of) Bitcoin. Clique enables a pool of users to pay each other off-chain, i.e., without interacting with the blockchain, thus sidestepping its bottlenecks. A user can directly send its coins to any other user in the Clique: In contrast to payment channels, its funds are not tied to a specific counterparty, avoiding the need for multi-hop payments. An untrusted operator facilitates payments by verifiably recording them. Furthermore, a novel technique of independent interest is used at the core of Bitcoin Clique. It builds on Adaptor Signatures and allows the extraction of the witness only after two signatures are published on the blockchain.