Optimistic Bridge

An optimistic bridge is a type of blockchain bridge inspired by the concept of optimistic rollups, designed to enhance interoperability within a blockchain ecosystem. Operating on the principle of honesty, it assumes all bridge transactions are valid by default and only scrutinizes them at the Layer 1 level if suspicious activity is detected.

The operation typically involves three parties:

• Users: Participants who wish to transfer their tokens between networks.
• Relayers: Actors responsible for processing bridge transactions and facilitating user requests. They are incentivized by fees paid by users, often referred to as the relayer fee.
• Disputers: Parties that monitor relayers’ activities and report suspicious actions to ensure network integrity.

The honesty assumption and elimination of default verification at Layer 1 simplify computation processes, speeding up transactions and reducing costs. However, this also increases the theoretical risk of relayers executing erroneous or fraudulent transactions, which could undermine the network’s integrity.

To deter such actions, optimistic bridges mandate that each transaction executed by a relayer undergo a timeframe known as the challenge period. During this period, disputers inspect the transactions and flag any suspicious ones by submitting a fraud proof. This proof should contain detailed information about the transaction, including the Merkle proof for the transaction, the block hash, and other relevant details.

The submitted proof is then reviewed on the settlement layer, which ultimately determines the transaction’s validity based on the evidence presented. If the settlement layer deems the disputer’s claim valid, concluding that the relayer’s action was inappropriate, the relayer will be penalized by having their bond slashed. This mechanism, combined with the challenge period, is essential for maintaining the integrity of optimistic bridge networks.