Consensus for Crosschain Communications

Crosschain communications allows information to be communicated between blockchains. Consensus in the context of crosschain communications relates to how participants are able to agree on the state of one blockchain and communicate that information to another blockchain such that the information is trusted. This paper surveys crosschain consensus techniques and analyses them based on crosschain usage scenarios and desirable attributes of crosschain systems. The top level scenarios upon which all complex usages are derived are reviewed: value transfer and atomic swaps, reading, writing, and state pinning. The attributes that must be maintained by a crosschain consensus technique for permissionless and permissioned blockchains such that the properties of each blockchain are not compromised are described. Finally, this paper reviews the crosschain consensus techniques based on these scenarios and attributes: Hash Time Locked Contracts, block header relaying (BTC Relay, XCLAIM, Pegged Sidechains and Ion), relay chains (Polkadot and Cosmos), and function call commitment schemes (Atomic Crosschain Transactions for Ethereum Private Sidechains and General Purpose Atomic Crosschain Transactions).