Optimism standard bridge contract walkthrough
Optimism(opens in a new tab) is an Optimistic Rollup. Optimistic rollups can process transactions for a much lower price than Ethereum Mainnet (also known as layer 1 or L1) because transactions are only processed by a few nodes, instead of every node on the network. At the same time, the data is all written to L1 so everything can be proved and reconstructed with all the integrity and availability guarantees of Mainnet.
To use L1 assets on Optimism (or any other L2), the assets need to be bridged. One way to achieve this is for users to lock assets (ETH and ERC-20 tokens are the most common ones) on L1, and receive equivalent assets to use on L2. Eventually, whoever ends up with them might want to bridge them back to L1. When doing this, the assets are burned on L2 and then released back to the user on L1.
This is the way the Optimism standard bridge(opens in a new tab) works. In this article we go over the source code for that bridge to see how it works and study it as an example of well written Solidity code.
Control flows
The bridge has two main flows:
- Deposit (from L1 to L2)
- Withdrawal (from L2 to L1)
Deposit flow
Layer 1
- If depositing an ERC-20, the depositor gives the bridge an allowance to spend the amount being deposited
- The depositor calls the L1 bridge (
depositERC20
,depositERC20To
,depositETH
, ordepositETHTo
) - The L1 bridge takes possession of the bridged asset
- ETH: The asset is transferred by the depositor as part of the call
- ERC-20: The asset is transferred by the bridge to itself using the allowance provided by the depositor
- The L1 bridge uses the cross-domain message mechanism to call
finalizeDeposit
on the L2 bridge
Layer 2
- The L2 bridge verifies the call to
finalizeDeposit
is legitimate:- Came from the cross domain message contract
- Was originally from the bridge on L1
- The L2 bridge checks if the ERC-20 token contract on L2 is the correct one:
- The L2 contract reports that its L1 counterpart is the same as the one the tokens came from on L1
- The L2 contract reports that it supports the correct interface (using ERC-165(opens in a new tab)).
- If the L2 contract is the correct one, call it to mint the appropriate number of tokens to the appropriate address. If not, start a withdrawal process to allow the user to claim the tokens on L1.
Withdrawal flow
Layer 2
- The withdrawer calls the L2 bridge (
withdraw
orwithdrawTo
) - The L2 bridge burns the appropriate number of tokens belonging to
msg.sender
- The L2 bridge uses the cross-domain message mechanism to call
finalizeETHWithdrawal
orfinalizeERC20Withdrawal
on the L1 bridge
Layer 1
- The L1 bridge verifies the call to
finalizeETHWithdrawal
orfinalizeERC20Withdrawal
is legitimate:- Came from the cross domain message mechanism
- Was originally from the bridge on L2
- The L1 bridge transfers the appropriate asset (ETH or ERC-20) to the appropriate address
Layer 1 code
This is the code that runs on L1, the Ethereum Mainnet.
IL1ERC20Bridge
This interface is defined here(opens in a new tab). It includes functions and definitions required for bridging ERC-20 tokens.
1// SPDX-License-Identifier: MITKopiraj
Most of Optimism's code is released under the MIT license(opens in a new tab).
1pragma solidity >0.5.0 <0.9.0;Kopiraj
At writing the latest version of Solidity is 0.8.12. Until version 0.9.0 is released, we don't know if this code is compatible with it or not.
1/**2 * @title IL1ERC20Bridge3 */4interface IL1ERC20Bridge {5 /**********6 * Events *7 **********/89 event ERC20DepositInitiated(Prikaži sveKopiraj
In Optimism bridge terminology deposit means transfer from L1 to L2, and withdrawal means a transfer from L2 to L1.
1 address indexed _l1Token,2 address indexed _l2Token,Kopiraj
In most cases the address of an ERC-20 on L1 is not the same the address of the equivalent ERC-20 on L2.
You can see the list of token addresses here(opens in a new tab).
The address with chainId
1 is on L1 (Mainnet) and the address with chainId
10 is on L2 (Optimism).
The other two chainId
values are for the Kovan test network (42) and the Optimistic Kovan test network (69).
1 address indexed _from,2 address _to,3 uint256 _amount,4 bytes _data5 );Kopiraj
It is possible to add notes to transfers, in which case they are added to the events that report them.
1 event ERC20WithdrawalFinalized(2 address indexed _l1Token,3 address indexed _l2Token,4 address indexed _from,5 address _to,6 uint256 _amount,7 bytes _data8 );Kopiraj
The same bridge contract handles transfers in both directions. In the case of the L1 bridge, this means initialization of deposits and finalization of withdrawals.
12 /********************3 * Public Functions *4 ********************/56 /**7 * @dev get the address of the corresponding L2 bridge contract.8 * @return Address of the corresponding L2 bridge contract.9 */10 function l2TokenBridge() external returns (address);Prikaži sveKopiraj
This function is not really needed, because on L2 it is a predeployed contract, so it is always at address 0x4200000000000000000000000000000000000010
.
It is here for symmetry with the L2 bridge, because the address of the L1 bridge is not trivial to know.
1 /**2 * @dev deposit an amount of the ERC20 to the caller's balance on L2.3 * @param _l1Token Address of the L1 ERC20 we are depositing4 * @param _l2Token Address of the L1 respective L2 ERC205 * @param _amount Amount of the ERC20 to deposit6 * @param _l2Gas Gas limit required to complete the deposit on L2.7 * @param _data Optional data to forward to L2. This data is provided8 * solely as a convenience for external contracts. Aside from enforcing a maximum9 * length, these contracts provide no guarantees about its content.10 */11 function depositERC20(12 address _l1Token,13 address _l2Token,14 uint256 _amount,15 uint32 _l2Gas,16 bytes calldata _data17 ) external;Prikaži sveKopiraj
The _l2Gas
parameter is the amount of L2 gas the transaction is allowed to spend.
Up to a certain (high) limit, this is free(opens in a new tab), so unless the ERC-20 contract does something really strange when minting, it should not be an issue.
This function takes care of the common scenario, where a user bridges assets to the same address on a different blockchain.
1 /**2 * @dev deposit an amount of ERC20 to a recipient's balance on L2.3 * @param _l1Token Address of the L1 ERC20 we are depositing4 * @param _l2Token Address of the L1 respective L2 ERC205 * @param _to L2 address to credit the withdrawal to.6 * @param _amount Amount of the ERC20 to deposit.7 * @param _l2Gas Gas limit required to complete the deposit on L2.8 * @param _data Optional data to forward to L2. This data is provided9 * solely as a convenience for external contracts. Aside from enforcing a maximum10 * length, these contracts provide no guarantees about its content.11 */12 function depositERC20To(13 address _l1Token,14 address _l2Token,15 address _to,16 uint256 _amount,17 uint32 _l2Gas,18 bytes calldata _data19 ) external;Prikaži sveKopiraj
This function is almost identical to depositERC20
, but it lets you send the ERC-20 to a different address.
1 /*************************2 * Cross-chain Functions *3 *************************/45 /**6 * @dev Complete a withdrawal from L2 to L1, and credit funds to the recipient's balance of the7 * L1 ERC20 token.8 * This call will fail if the initialized withdrawal from L2 has not been finalized.9 *10 * @param _l1Token Address of L1 token to finalizeWithdrawal for.11 * @param _l2Token Address of L2 token where withdrawal was initiated.12 * @param _from L2 address initiating the transfer.13 * @param _to L1 address to credit the withdrawal to.14 * @param _amount Amount of the ERC20 to deposit.15 * @param _data Data provided by the sender on L2. This data is provided16 * solely as a convenience for external contracts. Aside from enforcing a maximum17 * length, these contracts provide no guarantees about its content.18 */19 function finalizeERC20Withdrawal(20 address _l1Token,21 address _l2Token,22 address _from,23 address _to,24 uint256 _amount,25 bytes calldata _data26 ) external;27}Prikaži sveKopiraj
Withdrawals (and other messages from L2 to L1) in Optimism are a two step process:
- An initiating transaction on L2.
- A finalizing or claiming transaction on L1. This transaction needs to happen after the fault challenge period(opens in a new tab) for the L2 transaction ends.
IL1StandardBridge
This interface is defined here(opens in a new tab).
This file contains event and function definitions for ETH.
These definitions are very similar to those defined in IL1ERC20Bridge
above for ERC-20.
The bridge interface is divided between two files because some ERC-20 tokens require custom processing and cannot be handled by the standard bridge.
This way the custom bridge that handles such a token can implement IL1ERC20Bridge
and not have to also bridge ETH.
1// SPDX-License-Identifier: MIT2pragma solidity >0.5.0 <0.9.0;34import "./IL1ERC20Bridge.sol";56/**7 * @title IL1StandardBridge8 */9interface IL1StandardBridge is IL1ERC20Bridge {10 /**********11 * Events *12 **********/13 event ETHDepositInitiated(14 address indexed _from,15 address indexed _to,16 uint256 _amount,17 bytes _data18 );Prikaži sveKopiraj
This event is nearly identical to the ERC-20 version (ERC20DepositInitiated
), except without the L1 and L2 token addresses.
The same is true for the other events and the functions.
1 event ETHWithdrawalFinalized(2 .3 .4 .5 );67 /********************8 * Public Functions *9 ********************/1011 /**12 * @dev Deposit an amount of the ETH to the caller's balance on L2.13 .14 .15 .16 */17 function depositETH(uint32 _l2Gas, bytes calldata _data) external payable;1819 /**20 * @dev Deposit an amount of ETH to a recipient's balance on L2.21 .22 .23 .24 */25 function depositETHTo(26 address _to,27 uint32 _l2Gas,28 bytes calldata _data29 ) external payable;3031 /*************************32 * Cross-chain Functions *33 *************************/3435 /**36 * @dev Complete a withdrawal from L2 to L1, and credit funds to the recipient's balance of the37 * L1 ETH token. Since only the xDomainMessenger can call this function, it will never be called38 * before the withdrawal is finalized.39 .40 .41 .42 */43 function finalizeETHWithdrawal(44 address _from,45 address _to,46 uint256 _amount,47 bytes calldata _data48 ) external;49}Prikaži sveKopiraj
CrossDomainEnabled
This contract(opens in a new tab) is inherited by both bridges (L1 and L2) to send messages to the other layer.
1// SPDX-License-Identifier: MIT2pragma solidity >0.5.0 <0.9.0;34/* Interface Imports */5import { ICrossDomainMessenger } from "./ICrossDomainMessenger.sol";Kopiraj
This interface(opens in a new tab) tells the contract how to send messages to the other layer, using the cross domain messenger. This cross domain messenger is a whole other system, and deserves its own article, which I hope to write in the future.
1/**2 * @title CrossDomainEnabled3 * @dev Helper contract for contracts performing cross-domain communications4 *5 * Compiler used: defined by inheriting contract6 */7contract CrossDomainEnabled {8 /*************9 * Variables *10 *************/1112 // Messenger contract used to send and receive messages from the other domain.13 address public messenger;1415 /***************16 * Constructor *17 ***************/1819 /**20 * @param _messenger Address of the CrossDomainMessenger on the current layer.21 */22 constructor(address _messenger) {23 messenger = _messenger;24 }Prikaži sveKopiraj
The one parameter that the contract needs to know, the address of the cross domain messenger on this layer. This parameter is set once, in the constructor, and never changes.
12 /**********************3 * Function Modifiers *4 **********************/56 /**7 * Enforces that the modified function is only callable by a specific cross-domain account.8 * @param _sourceDomainAccount The only account on the originating domain which is9 * authenticated to call this function.10 */11 modifier onlyFromCrossDomainAccount(address _sourceDomainAccount) {Prikaži sveKopiraj
The cross domain messaging is accessible by any contract on the blockchain where it is running (either Ethereum mainnet or Optimism). But we need the bridge on each side to only trust certain messages if they come from the bridge on the other side.
1 require(2 msg.sender == address(getCrossDomainMessenger()),3 "OVM_XCHAIN: messenger contract unauthenticated"4 );Kopiraj
Only messages from the appropriate cross domain messenger (messenger
, as you see below) can be trusted.
12 require(3 getCrossDomainMessenger().xDomainMessageSender() == _sourceDomainAccount,4 "OVM_XCHAIN: wrong sender of cross-domain message"5 );Kopiraj
The way the cross domain messenger provides the address that sent a message with the other layer is the .xDomainMessageSender()
function(opens in a new tab).
As long as it is called in the transaction that was initiated by the message it can provide this information.
We need to make sure that the message we received came from the other bridge.
12 _;3 }45 /**********************6 * Internal Functions *7 **********************/89 /**10 * Gets the messenger, usually from storage. This function is exposed in case a child contract11 * needs to override.12 * @return The address of the cross-domain messenger contract which should be used.13 */14 function getCrossDomainMessenger() internal virtual returns (ICrossDomainMessenger) {15 return ICrossDomainMessenger(messenger);16 }Prikaži sveKopiraj
This function returns the cross domain messenger.
We use a function rather than the variable messenger
to allow contracts that inherit from this one to use an algorithm to specify which cross domain messenger to use.
12 /**3 * Sends a message to an account on another domain4 * @param _crossDomainTarget The intended recipient on the destination domain5 * @param _message The data to send to the target (usually calldata to a function with6 * `onlyFromCrossDomainAccount()`)7 * @param _gasLimit The gasLimit for the receipt of the message on the target domain.8 */9 function sendCrossDomainMessage(10 address _crossDomainTarget,11 uint32 _gasLimit,12 bytes memory _messagePrikaži sveKopiraj
Finally, the function that sends a message to the other layer.
1 ) internal {2 // slither-disable-next-line reentrancy-events, reentrancy-benignKopiraj
Slither(opens in a new tab) is a static analyzer Optimism runs on every contract to look for vulnerabilities and other potential problems. In this case, the following line triggers two vulnerabilities:
1 getCrossDomainMessenger().sendMessage(_crossDomainTarget, _message, _gasLimit);2 }3}Kopiraj
In this case we are not worried about reentrancy we know getCrossDomainMessenger()
returns a trustworthy address, even if Slither has no way to know that.
The L1 bridge contract
The source code for this contract is here(opens in a new tab).
1// SPDX-License-Identifier: MIT2pragma solidity ^0.8.9;Kopiraj
The interfaces can be part of other contracts, so they have to support a wide range of Solidity versions. But the bridge itself is our contract, and we can be strict about what Solidity version it uses.
1/* Interface Imports */2import { IL1StandardBridge } from "./IL1StandardBridge.sol";3import { IL1ERC20Bridge } from "./IL1ERC20Bridge.sol";Kopiraj
IL1ERC20Bridge and IL1StandardBridge are explained above.
1import { IL2ERC20Bridge } from "../../L2/messaging/IL2ERC20Bridge.sol";Kopiraj
This interface(opens in a new tab) lets us create messages to control the standard bridge on L2.
1import { IERC20 } from "@openzeppelin/contracts/token/ERC20/IERC20.sol";Kopiraj
This interface(opens in a new tab) lets us control ERC-20 contracts. You can read more about it here.
1/* Library Imports */2import { CrossDomainEnabled } from "../../libraries/bridge/CrossDomainEnabled.sol";Kopiraj
As explained above, this contract is used for interlayer messaging.
1import { Lib_PredeployAddresses } from "../../libraries/constants/Lib_PredeployAddresses.sol";Kopiraj
Lib_PredeployAddresses
(opens in a new tab) has the addresses for the L2 contracts that always have the same address. This includes the standard bridge on L2.
1import { Address } from "@openzeppelin/contracts/utils/Address.sol";Kopiraj
OpenZeppelin's Address utilities(opens in a new tab). It is used to distinguish between contract addresses and those belonging to externally owned accounts (EOA).
Note that this isn't a perfect solution, because there is no way to distinguish between direct calls and calls made from a contract's constructor, but at least this lets us identify and prevent some common user errors.
1import { SafeERC20 } from "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";Kopiraj
The ERC-20 standard(opens in a new tab) supports two ways for a contract to report failure:
- Revert
- Return
false
Handling both cases would make our code more complicated, so instead we use OpenZeppelin's SafeERC20
(opens in a new tab), which makes sure all failures result in a revert(opens in a new tab).
1/**2 * @title L1StandardBridge3 * @dev The L1 ETH and ERC20 Bridge is a contract which stores deposited L1 funds and standard4 * tokens that are in use on L2. It synchronizes a corresponding L2 Bridge, informing it of deposits5 * and listening to it for newly finalized withdrawals.6 *7 */8contract L1StandardBridge is IL1StandardBridge, CrossDomainEnabled {9 using SafeERC20 for IERC20;Prikaži sveKopiraj
This line is how we specify to use the SafeERC20
wrapper every time we use the IERC20
interface.
12 /********************************3 * External Contract References *4 ********************************/56 address public l2TokenBridge;Kopiraj
The address of L2StandardBridge.
12 // Maps L1 token to L2 token to balance of the L1 token deposited3 mapping(address => mapping(address => uint256)) public deposits;Kopiraj
A double mapping(opens in a new tab) like this is the way you define a two-dimensional sparse array(opens in a new tab).
Values in this data structure are identified as deposit[L1 token addr][L2 token addr]
.
The default value is zero.
Only cells that are set to a different value are written to storage.
12 /***************3 * Constructor *4 ***************/56 // This contract lives behind a proxy, so the constructor parameters will go unused.7 constructor() CrossDomainEnabled(address(0)) {}Kopiraj
To want to be able to upgrade this contract without having to copy all the variables in the storage.
To do that we use a Proxy
(opens in a new tab), a contract that uses delegatecall
(opens in a new tab) to transfer calls to a separate contact whose address is stored by the proxy contract (when you upgrade you tell the proxy to change that address).
When you use delegatecall
the storage remains the storage of the calling contract, so the values of all the contract state variables are unaffected.
One effect of this pattern is that the storage of the contract that is the called of delegatecall
is not used and therefore the constructor values passed to it do not matter.
This is the reason we can provide a nonsensical value to the CrossDomainEnabled
constructor.
It is also the reason the initialization below is separate from the constructor.
1 /******************2 * Initialization *3 ******************/45 /**6 * @param _l1messenger L1 Messenger address being used for cross-chain communications.7 * @param _l2TokenBridge L2 standard bridge address.8 */9 // slither-disable-next-line external-functionPrikaži sveKopiraj
This Slither test(opens in a new tab) identifies functions that are not called from the contract code and could therefore be declared external
instead of public
.
The gas cost of external
functions can be lower, because they can be provided with parameters in the calldata.
Functions declared public
have to be accessible from within the contract.
Contracts cannot modify their own calldata, so the parameters have to be in memory.
When such a function is called externally, it is necessary to copy the calldata to memory, which costs gas.
In this case the function is only called once, so the inefficiency does not matter to us.
1 function initialize(address _l1messenger, address _l2TokenBridge) public {2 require(messenger == address(0), "Contract has already been initialized.");Kopiraj
The initialize
function should only be called once.
If the address of either the L1 cross domain messenger or the L2 token bridge changes, we create a new proxy and a new bridge that calls it.
This is unlikely to happen except when the entire system is upgraded, a very rare occurrence.
Note that this function does not have any mechanism that restricts who can call it.
This means that in theory an attacker could wait until we deploy the proxy and the first version of the bridge and then front-run(opens in a new tab) to get to the initialize
function before the legitimate user does. But there are two methods to prevent this:
- If the contracts are deployed not directly by an EOA but in a transaction that has another contract create them(opens in a new tab) the entire process can be atomic, and finish before any other transaction is executed.
- If the legitimate call to
initialize
fails it is always possible to ignore the newly created proxy and bridge and create new ones.
1 messenger = _l1messenger;2 l2TokenBridge = _l2TokenBridge;3 }Kopiraj
These are the two parameters that the bridge needs to know.
12 /**************3 * Depositing *4 **************/56 /** @dev Modifier requiring sender to be EOA. This check could be bypassed by a malicious7 * contract via initcode, but it takes care of the user error we want to avoid.8 */9 modifier onlyEOA() {10 // Used to stop deposits from contracts (avoid accidentally lost tokens)11 require(!Address.isContract(msg.sender), "Account not EOA");12 _;13 }Prikaži sveKopiraj
This is the reason we needed OpenZeppelin's Address
utilities.
1 /**2 * @dev This function can be called with no data3 * to deposit an amount of ETH to the caller's balance on L2.4 * Since the receive function doesn't take data, a conservative5 * default amount is forwarded to L2.6 */7 receive() external payable onlyEOA {8 _initiateETHDeposit(msg.sender, msg.sender, 200_000, bytes(""));9 }Prikaži sveKopiraj
This function exists for testing purposes. Notice that it doesn't appear in the interface definitions - it isn't for normal use.
1 /**2 * @inheritdoc IL1StandardBridge3 */4 function depositETH(uint32 _l2Gas, bytes calldata _data) external payable onlyEOA {5 _initiateETHDeposit(msg.sender, msg.sender, _l2Gas, _data);6 }78 /**9 * @inheritdoc IL1StandardBridge10 */11 function depositETHTo(12 address _to,13 uint32 _l2Gas,14 bytes calldata _data15 ) external payable {16 _initiateETHDeposit(msg.sender, _to, _l2Gas, _data);17 }Prikaži sveKopiraj
These two functions are wrappers around _initiateETHDeposit
, the function that handles the actual ETH deposit.
1 /**2 * @dev Performs the logic for deposits by storing the ETH and informing the L2 ETH Gateway of3 * the deposit.4 * @param _from Account to pull the deposit from on L1.5 * @param _to Account to give the deposit to on L2.6 * @param _l2Gas Gas limit required to complete the deposit on L2.7 * @param _data Optional data to forward to L2. This data is provided8 * solely as a convenience for external contracts. Aside from enforcing a maximum9 * length, these contracts provide no guarantees about its content.10 */11 function _initiateETHDeposit(12 address _from,13 address _to,14 uint32 _l2Gas,15 bytes memory _data16 ) internal {17 // Construct calldata for finalizeDeposit call18 bytes memory message = abi.encodeWithSelector(Prikaži sveKopiraj
The way that cross domain messages work is that the destination contract is called with the message as its calldata.
Solidity contracts always interpret their calldata in accordance with
the ABI specifications(opens in a new tab).
The Solidity function abi.encodeWithSelector
(opens in a new tab) creates that calldata.
1 IL2ERC20Bridge.finalizeDeposit.selector,2 address(0),3 Lib_PredeployAddresses.OVM_ETH,4 _from,5 _to,6 msg.value,7 _data8 );Kopiraj
The message here is to call the finalizeDeposit
function(opens in a new tab) with these parameters:
Parameter | Value | Meaning |
---|---|---|
_l1Token | address(0) | Special value to stand for ETH (which isn't an ERC-20 token) on L1 |
_l2Token | Lib_PredeployAddresses.OVM_ETH | The L2 contract that manages ETH on Optimism, 0xDeadDeAddeAddEAddeadDEaDDEAdDeaDDeAD0000 (this contract is for internal Optimism use only) |
_from | _from | The address on L1 that sends the ETH |
_to | _to | The address on L2 that receives the ETH |
amount | msg.value | Amount of wei sent (which has already been sent to the bridge) |
_data | _data | Additional data to attach to the deposit |
1 // Send calldata into L22 // slither-disable-next-line reentrancy-events3 sendCrossDomainMessage(l2TokenBridge, _l2Gas, message);Kopiraj
Send the message through the cross domain messenger.
1 // slither-disable-next-line reentrancy-events2 emit ETHDepositInitiated(_from, _to, msg.value, _data);3 }Kopiraj
Emit an event to inform any decentralized application that listens of this transfer.
1 /**2 * @inheritdoc IL1ERC20Bridge3 */4 function depositERC20(5 .6 .7 .8 ) external virtual onlyEOA {9 _initiateERC20Deposit(_l1Token, _l2Token, msg.sender, msg.sender, _amount, _l2Gas, _data);10 }1112 /**13 * @inheritdoc IL1ERC20Bridge14 */15 function depositERC20To(16 .17 .18 .19 ) external virtual {20 _initiateERC20Deposit(_l1Token, _l2Token, msg.sender, _to, _amount, _l2Gas, _data);21 }Prikaži sveKopiraj
These two functions are wrappers around _initiateERC20Deposit
, the function that handles the actual ERC-20 deposit.
1 /**2 * @dev Performs the logic for deposits by informing the L2 Deposited Token3 * contract of the deposit and calling a handler to lock the L1 funds. (e.g. transferFrom)4 *5 * @param _l1Token Address of the L1 ERC20 we are depositing6 * @param _l2Token Address of the L1 respective L2 ERC207 * @param _from Account to pull the deposit from on L18 * @param _to Account to give the deposit to on L29 * @param _amount Amount of the ERC20 to deposit.10 * @param _l2Gas Gas limit required to complete the deposit on L2.11 * @param _data Optional data to forward to L2. This data is provided12 * solely as a convenience for external contracts. Aside from enforcing a maximum13 * length, these contracts provide no guarantees about its content.14 */15 function _initiateERC20Deposit(16 address _l1Token,17 address _l2Token,18 address _from,19 address _to,20 uint256 _amount,21 uint32 _l2Gas,22 bytes calldata _data23 ) internal {Prikaži sveKopiraj
This function is similar to _initiateETHDeposit
above, with a few important differences.
The first difference is that this function receives the token addresses and the amount to transfer as parameters.
In the case of ETH the call to the bridge already includes the transfer of asset to the bridge account (msg.value
).
1 // When a deposit is initiated on L1, the L1 Bridge transfers the funds to itself for future2 // withdrawals. safeTransferFrom also checks if the contract has code, so this will fail if3 // _from is an EOA or address(0).4 // slither-disable-next-line reentrancy-events, reentrancy-benign5 IERC20(_l1Token).safeTransferFrom(_from, address(this), _amount);Kopiraj
ERC-20 token transfers follow a different process from ETH:
- The user (
_from
) gives an allowance to the bridge to transfer the appropriate tokens. - The user calls the bridge with the address of the token contract, the amount, etc.
- The bridge transfers the tokens (to itself) as part of the deposit process.
The first step may happen in a separate transaction from the last two.
However, front-running is not a problem because the two functions that call _initiateERC20Deposit
(depositERC20
and depositERC20To
) only call this function with msg.sender
as the _from
parameter.
1 // Construct calldata for _l2Token.finalizeDeposit(_to, _amount)2 bytes memory message = abi.encodeWithSelector(3 IL2ERC20Bridge.finalizeDeposit.selector,4 _l1Token,5 _l2Token,6 _from,7 _to,8 _amount,9 _data10 );1112 // Send calldata into L213 // slither-disable-next-line reentrancy-events, reentrancy-benign14 sendCrossDomainMessage(l2TokenBridge, _l2Gas, message);1516 // slither-disable-next-line reentrancy-benign17 deposits[_l1Token][_l2Token] = deposits[_l1Token][_l2Token] + _amount;Prikaži sveKopiraj
Add the deposited amount of tokens to the deposits
data structure.
There could be multiple addresses on L2 that correspond to the same L1 ERC-20 token, so it is not sufficient to use the bridge's balance of the L1 ERC-20 token to keep track of deposits.
12 // slither-disable-next-line reentrancy-events3 emit ERC20DepositInitiated(_l1Token, _l2Token, _from, _to, _amount, _data);4 }56 /*************************7 * Cross-chain Functions *8 *************************/910 /**11 * @inheritdoc IL1StandardBridge12 */13 function finalizeETHWithdrawal(14 address _from,15 address _to,16 uint256 _amount,17 bytes calldata _dataPrikaži sveKopiraj
The L2 bridge sends a message to the L2 cross domain messenger which causes the L1 cross domain messenger to call this function (once the transaction that finalizes the message(opens in a new tab) is submitted on L1, of course).
1 ) external onlyFromCrossDomainAccount(l2TokenBridge) {Kopiraj
Make sure that this is a legitimate message, coming from the cross domain messenger and originating with the L2 token bridge. This function is used to withdraw ETH from the bridge, so we have to make sure it is only called by the authorized caller.
1 // slither-disable-next-line reentrancy-events2 (bool success, ) = _to.call{ value: _amount }(new bytes(0));Kopiraj
The way to transfer ETH is to call the recipient with the amount of wei in the msg.value
.
1 require(success, "TransferHelper::safeTransferETH: ETH transfer failed");23 // slither-disable-next-line reentrancy-events4 emit ETHWithdrawalFinalized(_from, _to, _amount, _data);Kopiraj
Emit an event about the withdrawal.
1 }23 /**4 * @inheritdoc IL1ERC20Bridge5 */6 function finalizeERC20Withdrawal(7 address _l1Token,8 address _l2Token,9 address _from,10 address _to,11 uint256 _amount,12 bytes calldata _data13 ) external onlyFromCrossDomainAccount(l2TokenBridge) {Prikaži sveKopiraj
This function is similar to finalizeETHWithdrawal
above, with the necessary changes for ERC-20 tokens.
1 deposits[_l1Token][_l2Token] = deposits[_l1Token][_l2Token] - _amount;Kopiraj
Update the deposits
data structure.
12 // When a withdrawal is finalized on L1, the L1 Bridge transfers the funds to the withdrawer3 // slither-disable-next-line reentrancy-events4 IERC20(_l1Token).safeTransfer(_to, _amount);56 // slither-disable-next-line reentrancy-events7 emit ERC20WithdrawalFinalized(_l1Token, _l2Token, _from, _to, _amount, _data);8 }91011 /*****************************12 * Temporary - Migrating ETH *13 *****************************/1415 /**16 * @dev Adds ETH balance to the account. This is meant to allow for ETH17 * to be migrated from an old gateway to a new gateway.18 * NOTE: This is left for one upgrade only so we are able to receive the migrated ETH from the19 * old contract20 */21 function donateETH() external payable {}22}Prikaži sveKopiraj
There was an earlier implementation of the bridge.
When we moved from the implementation to this one, we had to move all the assets.
ERC-20 tokens can just be moved.
However, to transfer ETH to a contract you need that contract's approval, which is what donateETH
provides us.
ERC-20 Tokens on L2
For an ERC-20 token to fit into the standard bridge, it needs to allow the standard bridge, and only the standard bridge, to mint token. This is necessary because the bridges need to ensure that the number of tokens circulating on Optimism is equal to the number of tokens locked inside the L1 bridge contract. If there are too many tokens on L2 some users would be unable to bridge their assets back to L1. Instead of a trusted bridge, we would essentially recreate fractional reserve banking(opens in a new tab). If there are too many tokens on L1, some of those tokens would stay locked inside the bridge contract forever because there is no way to release them without burning L2 tokens.
IL2StandardERC20
Every ERC-20 token on L2 that uses the standard bridge needs to provide this interface(opens in a new tab), which has the functions and events that the standard bridge needs.
1// SPDX-License-Identifier: MIT2pragma solidity ^0.8.9;34import { IERC20 } from "@openzeppelin/contracts/token/ERC20/IERC20.sol";Kopiraj
The standard ERC-20 interface(opens in a new tab) does not include the mint
and burn
functions.
Those methods are not required by the ERC-20 standard(opens in a new tab), which leaves unspecified the mechanisms to create and destroy tokens.
1import { IERC165 } from "@openzeppelin/contracts/utils/introspection/IERC165.sol";Kopiraj
The ERC-165 interface(opens in a new tab) is used to specify what functions a contract provides. You can read the standard here(opens in a new tab).
1interface IL2StandardERC20 is IERC20, IERC165 {2 function l1Token() external returns (address);Kopiraj
This function provides the address of the L1 token which is bridged to this contract. Note that we do not have a similar function in the opposite direction. We need to be able to bridge any L1 token, regardless of whether L2 support was planned when it was implemented or not.
12 function mint(address _to, uint256 _amount) external;34 function burn(address _from, uint256 _amount) external;56 event Mint(address indexed _account, uint256 _amount);7 event Burn(address indexed _account, uint256 _amount);8}Kopiraj
Functions and events to mint (create) and burn (destroy) tokens. The bridge should be the only entity that can run these functions to ensure the number of tokens is correct (equal to the number of tokens locked on L1).
L2StandardERC20
This is our implementation of the IL2StandardERC20
interface(opens in a new tab).
Unless you need some kind of custom logic, you should use this one.
1// SPDX-License-Identifier: MIT2pragma solidity ^0.8.9;34import { ERC20 } from "@openzeppelin/contracts/token/ERC20/ERC20.sol";Kopiraj
The OpenZeppelin ERC-20 contract(opens in a new tab). Optimism does not believe in reinventing the wheel, especially when the wheel is well audited and needs to be trustworthy enough to hold assets.
1import "./IL2StandardERC20.sol";23contract L2StandardERC20 is IL2StandardERC20, ERC20 {4 address public l1Token;5 address public l2Bridge;Kopiraj
These are the two additional configuration parameters that we require and ERC-20 normally does not.
12 /**3 * @param _l2Bridge Address of the L2 standard bridge.4 * @param _l1Token Address of the corresponding L1 token.5 * @param _name ERC20 name.6 * @param _symbol ERC20 symbol.7 */8 constructor(9 address _l2Bridge,10 address _l1Token,11 string memory _name,12 string memory _symbol13 ) ERC20(_name, _symbol) {14 l1Token = _l1Token;15 l2Bridge = _l2Bridge;16 }Prikaži sveKopiraj
First call the constructor for the contract we inherit from (ERC20(_name, _symbol)
) and then set our own variables.
12 modifier onlyL2Bridge() {3 require(msg.sender == l2Bridge, "Only L2 Bridge can mint and burn");4 _;5 }678 // slither-disable-next-line external-function9 function supportsInterface(bytes4 _interfaceId) public pure returns (bool) {10 bytes4 firstSupportedInterface = bytes4(keccak256("supportsInterface(bytes4)")); // ERC16511 bytes4 secondSupportedInterface = IL2StandardERC20.l1Token.selector ^12 IL2StandardERC20.mint.selector ^13 IL2StandardERC20.burn.selector;14 return _interfaceId == firstSupportedInterface || _interfaceId == secondSupportedInterface;15 }Prikaži sveKopiraj
This is the way ERC-165(opens in a new tab) works. Every interface is a number of supported functions, and is identified as the exclusive or(opens in a new tab) of the ABI function selectors(opens in a new tab) of those functions.
The L2 bridge uses ERC-165 as a sanity check to make sure that the ERC-20 contract to which it sends assets is an IL2StandardERC20
.
Note: There is nothing to prevent rogue contract from providing false answers to supportsInterface
, so this is a sanity check mechanism, not a security mechanism.
1 // slither-disable-next-line external-function2 function mint(address _to, uint256 _amount) public virtual onlyL2Bridge {3 _mint(_to, _amount);45 emit Mint(_to, _amount);6 }78 // slither-disable-next-line external-function9 function burn(address _from, uint256 _amount) public virtual onlyL2Bridge {10 _burn(_from, _amount);1112 emit Burn(_from, _amount);13 }14}Prikaži sveKopiraj
Only the L2 bridge is allowed to mint and burn assets.
_mint
and _burn
are actually defined in the OpenZeppelin ERC-20 contract.
That contract just doesn't expose them externally, because the conditions to mint and burn tokens are as varied as the number of ways to use ERC-20.
L2 Bridge Code
This is code that runs the bridge on Optimism. The source for this contract is here(opens in a new tab).
1// SPDX-License-Identifier: MIT2pragma solidity ^0.8.9;34/* Interface Imports */5import { IL1StandardBridge } from "../../L1/messaging/IL1StandardBridge.sol";6import { IL1ERC20Bridge } from "../../L1/messaging/IL1ERC20Bridge.sol";7import { IL2ERC20Bridge } from "./IL2ERC20Bridge.sol";Kopiraj
The IL2ERC20Bridge(opens in a new tab) interface is very similar to the L1 equivalent we saw above. There are two significant differences:
- On L1 you initiate deposits and finalize withdrawals. Here you initiate withdrawals and finalize deposits.
- On L1 it is necessary to distinguish between ETH and ERC-20 tokens. On L2 we can use the same functions for both because internally ETH balances on Optimism are handled as an ERC-20 token with the address 0xDeadDeAddeAddEAddeadDEaDDEAdDeaDDeAD0000(opens in a new tab).
1/* Library Imports */2import { ERC165Checker } from "@openzeppelin/contracts/utils/introspection/ERC165Checker.sol";3import { CrossDomainEnabled } from "../../libraries/bridge/CrossDomainEnabled.sol";4import { Lib_PredeployAddresses } from "../../libraries/constants/Lib_PredeployAddresses.sol";56/* Contract Imports */7import { IL2StandardERC20 } from "../../standards/IL2StandardERC20.sol";89/**10 * @title L2StandardBridge11 * @dev The L2 Standard bridge is a contract which works together with the L1 Standard bridge to12 * enable ETH and ERC20 transitions between L1 and L2.13 * This contract acts as a minter for new tokens when it hears about deposits into the L1 Standard14 * bridge.15 * This contract also acts as a burner of the tokens intended for withdrawal, informing the L116 * bridge to release L1 funds.17 */18contract L2StandardBridge is IL2ERC20Bridge, CrossDomainEnabled {19 /********************************20 * External Contract References *21 ********************************/2223 address public l1TokenBridge;Prikaži sveKopiraj
Keep track of the address of the L1 bridge. Note that in contrast to the L1 equivalent, here we need this variable. The address of the L1 bridge is not known in advance.
12 /***************3 * Constructor *4 ***************/56 /**7 * @param _l2CrossDomainMessenger Cross-domain messenger used by this contract.8 * @param _l1TokenBridge Address of the L1 bridge deployed to the main chain.9 */10 constructor(address _l2CrossDomainMessenger, address _l1TokenBridge)11 CrossDomainEnabled(_l2CrossDomainMessenger)12 {13 l1TokenBridge = _l1TokenBridge;14 }1516 /***************17 * Withdrawing *18 ***************/1920 /**21 * @inheritdoc IL2ERC20Bridge22 */23 function withdraw(24 address _l2Token,25 uint256 _amount,26 uint32 _l1Gas,27 bytes calldata _data28 ) external virtual {29 _initiateWithdrawal(_l2Token, msg.sender, msg.sender, _amount, _l1Gas, _data);30 }3132 /**33 * @inheritdoc IL2ERC20Bridge34 */35 function withdrawTo(36 address _l2Token,37 address _to,38 uint256 _amount,39 uint32 _l1Gas,40 bytes calldata _data41 ) external virtual {42 _initiateWithdrawal(_l2Token, msg.sender, _to, _amount, _l1Gas, _data);43 }Prikaži sveKopiraj
These two functions initiate withdrawals. Note that there is no needs to specify the L1 token address. L2 tokens are expected to tell us the L1 equivalent's address.
12 /**3 * @dev Performs the logic for withdrawals by burning the token and informing4 * the L1 token Gateway of the withdrawal.5 * @param _l2Token Address of L2 token where withdrawal is initiated.6 * @param _from Account to pull the withdrawal from on L2.7 * @param _to Account to give the withdrawal to on L1.8 * @param _amount Amount of the token to withdraw.9 * @param _l1Gas Unused, but included for potential forward compatibility considerations.10 * @param _data Optional data to forward to L1. This data is provided11 * solely as a convenience for external contracts. Aside from enforcing a maximum12 * length, these contracts provide no guarantees about its content.13 */14 function _initiateWithdrawal(15 address _l2Token,16 address _from,17 address _to,18 uint256 _amount,19 uint32 _l1Gas,20 bytes calldata _data21 ) internal {22 // When a withdrawal is initiated, we burn the withdrawer's funds to prevent subsequent L223 // usage24 // slither-disable-next-line reentrancy-events25 IL2StandardERC20(_l2Token).burn(msg.sender, _amount);Prikaži sveKopiraj
Notice that we are not relying on the _from
parameter but on msg.sender
which is a lot harder to fake (impossible, as far as I know).
12 // Construct calldata for l1TokenBridge.finalizeERC20Withdrawal(_to, _amount)3 // slither-disable-next-line reentrancy-events4 address l1Token = IL2StandardERC20(_l2Token).l1Token();5 bytes memory message;67 if (_l2Token == Lib_PredeployAddresses.OVM_ETH) {Kopiraj
On L1 it is necessary to distinguish between ETH and ERC-20.
1 message = abi.encodeWithSelector(2 IL1StandardBridge.finalizeETHWithdrawal.selector,3 _from,4 _to,5 _amount,6 _data7 );8 } else {9 message = abi.encodeWithSelector(10 IL1ERC20Bridge.finalizeERC20Withdrawal.selector,11 l1Token,12 _l2Token,13 _from,14 _to,15 _amount,16 _data17 );18 }1920 // Send message up to L1 bridge21 // slither-disable-next-line reentrancy-events22 sendCrossDomainMessage(l1TokenBridge, _l1Gas, message);2324 // slither-disable-next-line reentrancy-events25 emit WithdrawalInitiated(l1Token, _l2Token, msg.sender, _to, _amount, _data);26 }2728 /************************************29 * Cross-chain Function: Depositing *30 ************************************/3132 /**33 * @inheritdoc IL2ERC20Bridge34 */35 function finalizeDeposit(36 address _l1Token,37 address _l2Token,38 address _from,39 address _to,40 uint256 _amount,41 bytes calldata _dataPrikaži sveKopiraj
This function is called by L1StandardBridge
.
1 ) external virtual onlyFromCrossDomainAccount(l1TokenBridge) {Kopiraj
Make sure the source of the message is legitimate.
This is important because this function calls _mint
and could be used to give tokens that are not covered by tokens the bridge owns on L1.
1 // Check the target token is compliant and2 // verify the deposited token on L1 matches the L2 deposited token representation here3 if (4 // slither-disable-next-line reentrancy-events5 ERC165Checker.supportsInterface(_l2Token, 0x1d1d8b63) &&6 _l1Token == IL2StandardERC20(_l2Token).l1Token()Kopiraj
Sanity checks:
- The correct interface is supported
- The L2 ERC-20 contract's L1 address matches the L1 source of the tokens
1 ) {2 // When a deposit is finalized, we credit the account on L2 with the same amount of3 // tokens.4 // slither-disable-next-line reentrancy-events5 IL2StandardERC20(_l2Token).mint(_to, _amount);6 // slither-disable-next-line reentrancy-events7 emit DepositFinalized(_l1Token, _l2Token, _from, _to, _amount, _data);Kopiraj
If the sanity checks pass, finalize the deposit:
- Mint the tokens
- Emit the appropriate event
1 } else {2 // Either the L2 token which is being deposited-into disagrees about the correct address3 // of its L1 token, or does not support the correct interface.4 // This should only happen if there is a malicious L2 token, or if a user somehow5 // specified the wrong L2 token address to deposit into.6 // In either case, we stop the process here and construct a withdrawal7 // message so that users can get their funds out in some cases.8 // There is no way to prevent malicious token contracts altogether, but this does limit9 // user error and mitigate some forms of malicious contract behavior.Prikaži sveKopiraj
If a user made a detectable error by using the wrong L2 token address, we want to cancel the deposit and return the tokens on L1. The only way we can do this from L2 is to send a message that will have to wait the fault challenge period, but that is much better for the user than losing the tokens permanently.
1 bytes memory message = abi.encodeWithSelector(2 IL1ERC20Bridge.finalizeERC20Withdrawal.selector,3 _l1Token,4 _l2Token,5 _to, // switched the _to and _from here to bounce back the deposit to the sender6 _from,7 _amount,8 _data9 );1011 // Send message up to L1 bridge12 // slither-disable-next-line reentrancy-events13 sendCrossDomainMessage(l1TokenBridge, 0, message);14 // slither-disable-next-line reentrancy-events15 emit DepositFailed(_l1Token, _l2Token, _from, _to, _amount, _data);16 }17 }18}Prikaži sveKopiraj
Conclusion
The standard bridge is the most flexible mechanism for asset transfers. However, because it is so generic it is not always the easiest mechanism to use. Especially for withdrawals, most users prefer to use third party bridges(opens in a new tab) that do not wait the challenge period and do not require a Merkle proof to finalize the withdrawal.
These bridges typically work by having assets on L1, which they provide immediately for a small fee (often less than the cost of gas for a standard bridge withdrawal). When the bridge (or the people running it) anticipates being short on L1 assets it transfers sufficient assets from L2. As these are very big withdrawals, the withdrawal cost is amortized over a large amount and is a much smaller percentage.
Hopefully this article helped you understand more about how layer 2 works, and how to write Solidity code that is clear and secure.
Poslednja izmena: @lukassim(opens in a new tab), 26. април 2024.