Forced withdrawals

force

One of the coolest features we’ve built into Superbridge is the ability to force asset withdrawal from rollups. Forced withdrawals act as an escape hatch for users and allows them to exit rollups without needing to trust rollup operators to include their transactions.

Before we jump into forced withdrawals it’s important to understand how withdrawals work in the context of OP rollups, then we can look at deposit transactions and how they’re special, before combining them together for forced withdrawals.

Withdrawals

A withdrawal operation in the context of an OP rollup refers to any transaction initiated on the rollup and completed on the L1. The most obvious use case for withdrawals is obviously token bridging, where tokens are locked/burned on the rollup and released/minted on the L1, but actually any valid transaction can be submitted as a withdrawal via the L2ToL1MessagePasser contract.

However, the user experience around withdrawals is a little rough - it’s not the case that after initiating a transaction on the L2 that the action happens on the L1 immediately. First, the state root of the of the epoch containing your transaction needs to be proposed to the L1. Most mainnet deployments of OP rollups have a submission period of every 1800 blocks, or roughly every hour.

After this state root has been proposed, proof needs to be submitted that your transaction is present in the state the root represents. This is done via the OptimismPortal contract, and a merkle trie is used to validate that the withdrawal hash you are proving exists in the proposed state root.

After proof has been submitted and accepted, the withdrawal enters its 7 day challenge period. Witnesses are able to challenge the integrity of submitted state roots and have the ability to discard state roots that are invalid.

It’s important to note that the 7 day challenge period applies to both the withdrawal and the state root proposal separately. So if you’re looking for the withdrawal to take as little time as possible it’s important to do the prove operation as soon as possible.

Finally, after the 7 day challenge period is over the withdrawal can be finalised (executed) via a final call to the OptimismPortal contract.

For more information, please refer to the withdrawal specs in the OP Stack documentation.

Deposit transactions

A deposit operation in the OP Stack actually refers to any transaction initiated on the L1 and completed on the rollup. For our purposes, deposits are lot simpler to explain than withdrawals but many of the same properties hold. As with withdrawals, deposits aren’t solely used for bridging assets, any transaction can be submitted as a deposit transaction. That means you could submit a transaction on the L1 that simply transfers some of your ETH on the rollup to your buddy. You might not want to, because of the L1 gas costs involved, but doing this is censorship resistant and ensures your transaction will land - the key to understanding why forced withdrawals are useful.

Deposit transactions are initiated by calling depositTransaction function on the L1 OptimismPortal contract. They emit a TransactionDeposited event encoding the L2 transaction data which rollup nodes are constantly listening for. Once rollup nodes find these events they queue them up to be executed first thing in the new L2 block. And here comes the magic - if the sequencer doesn’t include one of these deposit transactions, which can be checked by anyone running their own rollup nodes, their bond can be slashed and forfeited to the watchful witness. This core premise that L1 initiated deposit transactions need to be included in the L2 is what allows users to transact, if they wish, with the same level of censorship resistance they would find on Ethereum.

A really cool example of using these deposit transactions in non standard ways, that we believe Zora pioneered, is their “bridge & mint” flow. When bridging via the Zora frontend they actually encode a contract call to be executed on the L2, calling mint() on an NFT contract. So by simply bridging ETH over to Zora you get an NFT along with your ETH.

For a more comprehensive understanding of deposits, please refer to the Optimism specs document on deposits.