Polygon's Proof of Stake (PoS) Bridge is the canonical smart contract to move assets to and from Polygon's PoS sidechain. Assets are locked on the source chain and minted on Polygon.
The Polygon PoS bridge supports the transfer of ERC-20 tokens and is the recommended option for most users. Deposits on the PoS bridge are completed almost instantly but withdrawals may take a while to confirm. When bridging tokens to Polygon, the tokens are locked on Ethereum with the same number of tokens minted on Polygon. When bridging back to Ethereum, the pegged tokens on Polygon are burned and the Ethereum assets are then unlocked.
The PoS bridge uses a Proof-of-Stake (PoS) consensus algorithm to secure its network. Polygon relies on a set of external validators who run full nodes, produce blocks, validate and participate in consensus. Validators must stake a certain amount of MATIC tokens which are at risk of getting slashed in case of any malicious actions like double signing. The bridge mechanism requires at least two-thirds of the validators to agree on the locked token event on Ethereum to mint the corresponding token amount on Polygon.
The Polygon PoS bridge requires greater trust assumptions as it is secured by a set of external validators who must post bond as collateral. The bridge contract also retains admin authority and is controlled by a 5/8 multisig wallet. Among the eight signers, four are Polygon co-founders, and the other four are key members from Polygon DeFi projects. Owners of the contract can upgrade and replace the contract at any time (without a timelock period), which means the multisig can withdraw all user assets in the contract.