Decentralized Bridges: Trustless Cross-Chain Value Transfers

Janus governs the transfer of value within and across the Quai Network.

The Quai Network utilizes coincident blocks created from merged mining to transfer state across blockchains. A coincident block is created when a block is mined which has sufficient difficulty to satisfy the mining requirement in more than one blockchain that is being merged mined. A coincident block creates a block header which is valid in more than one block chain, hence the name coincident block. The valid header will have references to data in both of the blockchains, including the previous block hash, which then creates a hash link back to the previous coincident block and a provable reference to all of the state transitions that have occured between the current coincident block and the previous coincident block with the respective blockchain. This allows a blockchain which periodically experience coincidence blocks to have provable references to adjacent blockchains which can be used to cause internal state transitions in the local blockchain. These internal transitions can be driven by a protocol level understanding of the adjacent blockchain state or it can be driven by smart contract level state transitions which are triggered by playing valid external state transition proofs to the local blockchain. This creates a mechanism by which any merged mined blockchain can have decentralized atomic bridges between itself and the blockchains with which it is merged mined.

Merged Mined Headers

The heirarchy of blockchains in the Quai network are connected through merged mined blocks which are made possible through the creation of headers which have references to more than one blockchain. The Quai blockchain header is structered as an array of 3 Ethereum like headers. The set of headers contain a zone, region, and prime header which represents a slice of the hierarchy of blockchains in the Quai network. A location specification is added to the header which determines which zone blockchain the header pertains to, and the region and prime header's blockchains are then inferred as being the parent of the specified zone. Each element in the array references a state root which is valid for that header's corresponding blockchain if and only if the difficulty of the block that is found meets the required difficulty for it's respective blockchain. This means that very often only the zone header is valid and the region a prime header data is strictly surperfulous and creates no cononical record. However, when the difficulty of the found block becomes great enough to meet the region difficulty a valid region block is found and only the prime portion of the header is superfulous data. This is a region coincident block because by definition, if the difficulty is met for a region block it will also be met for the zone which means the both a zone and region block are found simultaneously.