Paper 2018/642

NOCUST - A Securely Scalable Commit-Chain

Rami Khalil and Arthur Gervais and Guillaume Felley

Abstract

To scale permissionless blockchains, one avenue is to enact transactions outside, or off- the blockchain, but to secure them by a parent-chain with collateral. Payment channels are one such instantiation, but suffer from expensive channel setups, fragmented deposit costs and further challenges. We present NOCUST, a 2nd-layer commit-chain, secure against an adversary that attempts to double-spend assets while controlling a set of malicious users, the commit-chain operator, or both. NOCUST does not require an additional consensus mechanism and the smart contract of a parent-blockchain acts as a means of efficient dispute resolution, via either provable consistency through zkSNARKs or users auditing once in a time-window the commit-chain's integrity. NOCUST (i) does not require a parent-chain transaction for peers to join, (ii) achieves delayed transaction finality without additional collateral, (iii) achieves instant finality with less collateral than payment channels and (iv) does not require recipients of transactions to be online. Users do not need to be constantly online to monitor their state, minimizing the need for watchtowers. NOCUST tempers the danger of channel closing timeouts of payment channels by enforcing fair mass exits. We show how NOCUST scales in practice to over one billion users on a PoW blockchain.