Optimism bridge is the canonical smart contract to move assets from Ethereum mainnet to Optimism Layer-2 (L2). Assets are locked on mainnet and minted on Optimism.
Optimism is a Layer-2 (L2) blockchain for the Ethereum mainnet that uses optimistic rollups as its scaling method. Rollups are a general method to scaling transactions above the means of the Layer-1 (L1) by essentially 'rolling up' all the transaction on Arbitrum into one transaction recorded on the L1 to save gas fees. Optimism is built on top of and secured by the Ethereum blockchain as it processes all transaction on its own sidechain and posts the updated chain state back to the Ethereum mainnet. The Optimism rollup is optimistic because the transactions are always assumed to be correct and validators are incentivized to submit only valid transactions. The validity of a transaction can be challenged for up to seven days.
The Optimism bridge allows users to transfer native (or canonical) assets directly from Ethereum to Optimism via its two-way messaging system. To use the chain, users execute a deposit transaction which transfers funds to the bridge contract on the L1 and credits the same funds to you on the Optimism L2 at the specified user address. Withdrawing back from Optimism to Ethereum involves burning the asset on L2 and subsequently releasing the locked funds on the L1.
Due to the inherent design of optimistic rollups, all withdrawals back to the Ethereum mainnet must wait the duration of the challenge period of seven days. The use of a centralized sequencer in the initial stages can influence transaction ordering, and if there are no honest validators, then a malicious actor can steal funds by posting invalid blocks and state commitments.