Merge pull request #4056 from ethereum-optimism/clabby/specs/two-step-withdrawals

specs: Update `withdrawals` spec

specs/withdrawals.md

 # Withdrawals
 
 <!-- All glossary references in this file. -->
+
 [g-deposits]: glossary.md#deposits
 [g-deposited]: glossary.md#deposited-transaction
 [deposit-tx-type]: glossary.md#deposited-transaction-type
-
 [g-withdrawal]: glossary.md#withdrawal
 [g-mpt]: glossary.md#merkle-patricia-trie
 [g-relayer]: glossary.md#withdrawals
 [g-execution-engine]: glossary.md#execution-engine
 
 [Withdrawals][g-withdrawal] are cross domain transactions which are initiated on L2, and finalized by a transaction
-executed on L1. Notably, withdrawals may be used by and L2 account to call an L1 contract, or to transfer ETH from
+executed on L1. Notably, withdrawals may be used by an L2 account to call an L1 contract, or to transfer ETH from
 an L2 account to an L1 account.
 
-**Vocabulary note**: *withdrawal* can refer to the transaction at various stages of the process, but we introduce
+**Vocabulary note**: _withdrawal_ can refer to the transaction at various stages of the process, but we introduce
 more specific terms to differentiate:
 
-- A *withdrawal initiating transaction* refers specifically to a transaction on L2 sent to the Withdrawals predeploy.
-- A *withdrawal finalizing transaction* refers specifically to an L1 transaction which finalizes and relays the
+- A _withdrawal initiating transaction_ refers specifically to a transaction on L2 sent to the Withdrawals predeploy.
+- A _withdrawal finalizing transaction_ refers specifically to an L1 transaction which finalizes and relays the
   withdrawal.
 
 Withdrawals are initiated on L2 via a call to the Message Passer predeploy contract, which records the important
@@ -55,18 +55,27 @@ We first describe the end to end flow of initiating and finalizing a withdrawal:
 ### On L2
 
 An L2 account sends a withdrawal message (and possibly also ETH) to the `L2ToL1MessagePasser` predeploy contract.
-   This is a very simple contract that stores the a hash of the withdrawal data.
+This is a very simple contract that stores the a hash of the withdrawal data.
 
 ### On L1
 
 1. A [relayer][g-relayer] submits the required inputs to the `OptimismPortal` contract. The relayer need
    not be the same entity which initiated the withdrawal on L2.
-   These inputs include the withdrawal transaction data, inclusion proofs, and a timestamp. The timestamp
+   These inputs include the withdrawal transaction data, inclusion proofs, and a block number. The block number
    must be one for which an L2 output root exists, which commits to the withdrawal as registered on L2.
-2. The `OptimismPortal` contract retrieves the output root for the given timestamp from the `OutputOracle`'s
-   `l2Outputs()` function, and performs the remainder of the verification process internally.
-3. If verification fails, the call reverts. Otherwise the call is forwarded, and the hash is recorded to prevent it from
-   from being replayed.
+1. The `OptimismPortal` contract retrieves the output root for the given block number from the `L2OutputOracle`'s
+   `getL2OutputAfter()` function, and performs the remainder of the verification process internally.
+1. If proof verification fails, the call reverts. Otherwise the call is forwarded, and the hash is recorded to
+   prevent it from being re-proven. Note that the withdrawal can be proven more than once if the corresponding
+   output root changes.
+1. After the withdrawal is proven, it enters a 7 day challenge period, allowing time for other network participants
+   to challenge its integrity.
+1. Once the challenge period has passed, a relayer submits the withdrawal transaction once again to the
+   `OptimismPortal` contract. Again, the relayer need not be the same entity which initiated the withdrawal on L2.
+1. The `OptimismPortal` contract receives the withdrawal transaction data and verifies that the withdrawal has
+   both been proven and passed the challenge period.
+1. If the requirements are not met, the call reverts. Otherwise the call is forwarded, and the hash is recorded to
+   prevent it from being replayed.
 
 ## The L2ToL1MessagePasser Contract
 
@@ -113,7 +122,7 @@ of withdrawals, which do not modify the sender's address. The difference is that
 
 - on L2, the deposit sender's address is returned by the `CALLER` opcode, meaning a contract cannot easily tell if the
   call originated on L1 or L2, whereas
-- on L1, the withdrawal sender's address is accessed by calling the `l2Sender`() function on the `OptimismPortal`
+- on L1, the withdrawal sender's address is accessed by calling the `l2Sender()` function on the `OptimismPortal`
   contract.
 
 Calling `l2Sender()` removes any ambiguity about which domain the call originated from. Still, developers will need to
@@ -125,6 +134,9 @@ The Optimism Portal serves as both the entry and exit point to the Optimism L2.
 the [DepositFeed](./deposits.md#deposit-contract) contract, and in addition provides the following interface for
 withdrawals:
 
+- [WithdrawalTransaction type](https://github.com/ethereum-optimism/optimism/blob/develop/packages/contracts-bedrock/contracts/libraries/Types.sol#L46-L56)
+- [OutputRootProof type](https://github.com/ethereum-optimism/optimism/blob/develop/packages/contracts-bedrock/contracts/libraries/Types.sol#L20-L29)
+
 ```js
 interface OptimismPortal {
 
@@ -132,23 +144,22 @@ interface OptimismPortal {
 
     function l2Sender() returns(address) external;
 
+    function proveWithdrawalTransaction(
+        Types.WithdrawalTransaction memory _tx,
+        uint256 _l2BlockNumber,
+        Types.OutputRootProof calldata _outputRootProof,
+        bytes[] calldata _withdrawalProof
+    ) external;
+
     function finalizeWithdrawalTransaction(
-        uint256 _nonce,
-        address _sender,
-        address _target,
-        uint256 _value,
-        uint256 _gasLimit,
-        bytes calldata _data,
-        uint256 _timestamp,
-        WithdrawalVerifier.OutputRootProof calldata _outputRootProof,
-        bytes calldata _withdrawalProof
-    )
+        Types.WithdrawalTransaction memory _tx
+    ) external;
 }
 ```
 
 ## Withdrawal Verification and Finalization
 
-The following inputs are required to verify and finalize a withdrawal:
+The following inputs are required to prove and finalize a withdrawal:
 
 - Withdrawal transaction data:
   - `nonce`: Nonce for the provided message.
@@ -158,15 +169,15 @@ The following inputs are required to verify and finalize a withdrawal:
   - `data`: Data to send to the target.
   - `gasLimit`: Gas to be forwarded to the target.
 - Proof and verification data:
-  - `timestamp`: The L2 timestamp corresponding with the output root.
+  - `l2BlockNumber`: The L2 block number that corresponds to the output root.
   - `outputRootProof`: Four `bytes32` values which are used to derive the output root.
   - `withdrawalProof`: An inclusion proof for the given withdrawal in the L2ToL1MessagePasser contract.
 
 These inputs must satisfy the following conditions:
 
-1. The `timestamp` is at least `FINALIZATION_PERIOD` seconds old.
-1. `OutputOracle.l2Outputs(timestamp)` returns a non-zero value `l2Output`.
-1. The keccak256 hash of the `outputRootProof` values is equal to the `l2Output`.
+1. The `l2BlockNumber` must be the block number that corresponds to the `OutputProposal` being proven.
+1. `L2OutputOracle.getL2OutputAfter(l2BlockNumber)` returns a non-zero `OutputProposal`.
+1. The keccak256 hash of the `outputRootProof` values is equal to the `outputRoot`.
 1. The `withdrawalProof` is a valid inclusion proof demonstrating that a hash of the Withdrawal transaction data
    is contained in the storage of the L2ToL1MessagePasser contract on L2.
 
@@ -181,6 +192,8 @@ These inputs must satisfy the following conditions:
    [polygon-dbl-spend]: https://gerhard-wagner.medium.com/double-spending-bug-in-polygons-plasma-bridge-2e0954ccadf1
 
 1. For each withdrawal initiated on L2 (ie. with a unique `nonce`), the following properties must hold:
+   1. It should only be possible to prove the withdrawal once, unless the outputRoot for the withdrawal
+      has changed.
    1. It should only be possible to finalize the withdrawal once.
    1. It should not be possible to relay the message with any of its fields modified, ie.
       1. Modifying the `sender` field would enable a 'spoofing' attack.