Watchtower Schemes For Blockchain Payment Channels And State Channels

The low throughput problem of public blockchain is a bottleneck limiting its development.Channels are one of the most promising technologies for solving the low throughput problem by transferring a large number of transactions off-chain.However,the most widely used channels,namely payment channels and state channels,both rely on the strict assumption that users continuously monitor the blockchain for fund security,which is unrealistic for users who cannot guarantee continuous network access.Watchtowers are introduced as an infrastructure for securing large-scale applications of channels,which are always online to monitor the blockchain for users and perform specific operations when necessary.However,existing payment channel watchtower schemes have problems in trust and linear growth of storage overhead.Existing state channel watchtower schemes can effectively solve the trust problem and storage overhead problem,but they cannot meet the user demand of assisting off-chain state update.To address above insufficiencies,we propose two payment channel watchtower schemes and one state channel watchtower scheme.To solve the trust problem of payment channel watchtowers,we propose an efficient and accountable payment channel watchtower scheme.Based on the analysis of the transaction relationship of payment channels,the scheme introduces new transaction types and transaction relationships,and realizes a collateral mechanism and a reward mechanism.Compared with the existing accountable watchtower scheme Cerberus,this scheme complements the binding of rewards and watchtower work status,and reduces the on-chain overhead for hiring watchtowers by 20%by utilizing transaction correlation and multiple signatures.To solve the trust problem and the linearly growing storage overhead problem of payment channel watchtowers,based on the analysis of transaction structure,we propose a low storage overhead payment channel watchtower scheme without modifying the existing transaction relationships of payment channels.The scheme extends a single watchtower to a set of watchtowers,and relaxes the trust assumption of watchtowers by using the threshold signature algorithm to ensure the security of channel users’ funds when some watchtowers are not working honestly.In addition,the use of secret sharing and key derivation makes it possible to replace every revocation transaction stored in the watchtowers with the data for constructing and signing revocation transactions,which reduces the storage overhead of the watchtower to a constant level.To solve the problem that state channel watchtowers cannot assist in off-chain state updates,we improve the generic state channel smart contract and propose a practical state channel watchtower scheme based on the improved state channel smart contract.The scheme first designs the state commitment method based on vector commitment,and then designs the watchtower smart contract,the mechanism to resolve disputes on the chain,and the mechanism to assist in off-chain state updates.Compared with the existing state channel watchtower scheme Pisa,this scheme reduces the on-chain state parameters and protects the off-chain state privacy.