How to implement generalized meta transactions
We'll explore a powerful design for meta transactions based on 0x
Enabling meta transactions inside your contract is a powerful addition. Requiring users to hold ETH to pay for gas has always been and still is one of the biggest user onboarding challenges. Who knows how many more people would be using Ethereum right now if it was just a simple click?
But sometimes the solution can be added meta transaction capability inside your contracts. The implementation might be easier than you think.
Let's took at the high-level description first.
What are meta-transactions?
A meta transaction is a regular Ethereum transaction which contains another transaction, the actual transaction. The actual transaction is signed by a user and then sent to an operator or something similar, no gas and blockchain interaction required. The operator takes this signed transaction and submits it to the blockchain paying for the fees himself.
The contract ensures there's a valid signature on the actual transaction and then executes it.
High-level overview
If we want to support generalized meta-transactions in our contract, it can be done with a few simple steps. On a high-level, there are two steps to it.
Step 1: Verify the signature of the meta-transaction. We'll do this by creating a hash following the EIP-712 standard and ecrecover.
bool isValidSignature = ecrecover(hash(transaction), v, r, s) == transaction.signerAddressStep 2: Once verified, we can extract the actual transaction data. By using delegatecall on our current contract address, we execute a function in our current contract without doing a new contract call. Remember that delegatecall basically calls the contract's code but with the current contract's state. So by doing address(this).delegatecall we just execute the all in our current contract and we can pass the transaction data along.
(bool didSucceed, bytes memory returnData) = address(this).delegatecall(transaction.data);That's it for the most part. But there are some critical things to verify and also alternatives for the signature.
Let's look into more details.
Transaction execution in detail
As we've seen the heart of the execution is the delegatecall. This is where the actual transaction is executed. But to ensure proper execution, we have to make sure some things are correct.
Transaction Struct
First let's look at the data inside our Transaction struct on the right. It contains all the relevant requirements set by the user and the transaction itself which is to be executed as the bytes data. This is what's passed from the user to the operator to the contract.
struct Transaction {
uint256 salt;
uint256 expirationTimeSeconds;
uint256 gasPrice;
address signerAddress;
bytes data;
}Typed Transaction Hash
We'll further need to compute a hash over all this data. This is used for the signature scheme and to prevent double execution of the same transaction. For details about this look at the end in the signature explanation.
This is the transaction schema hash:
EIP712_TRANSACTION_SCHEMA_HASH = keccak256(
abi.encodePacked("Transaction(uint256 salt,uint256 expirationTimeSeconds,uint256 gasPrice,address signerAddress,bytes data)")
);This is the EIP712 schema hash and can be computed once in the constructor of the contract.
function _getTransactionTypedHash(
Transaction memory transaction
) private view returns (bytes32) {
return keccak256(abi.encodePacked(
EIP712_TRANSACTION_SCHEMA_HASH,
transaction.salt,
transaction.expirationTimeSeconds,
transaction.gasPrice,
uint256(transaction.signerAddress),
keccak256(transaction.data)
));
}
We can use that to compute the full typed transaction hash using keccak256 and abi.encodePacked.
By hashing all relevant values, we can ensure that only exactly what the original user signed would result in a successful transaction execution. If for example the operator was to change just 1 second inside expirationTimeSeconds, it would not work anymore.
This is just the first part of the hash, for the full details including the requirements for a secure signature, read the part about signatures below.
Setting correct msg.sender
If we just execute delegatecall, the msg.sender of the transaction would still be the operator of the meta transaction, not the original signer.
We can solve this by setting a context variable:
function _setCurrentContextAddressIfRequired(address contextAddress) private {
currentContextAddress = contextAddress;
}
function _getCurrentContextAddress() private view returns (address) {
return currentContextAddress == address(0) ? msg.sender : currentContextAddress;
}Everywhere you would use the msg.sender in your contract, you would now instead call _getCurrentContextAddress().
Preventing multiple wrapped transactions
Another thing we want to prevent is executing a meta-meta-transaction. (unless you want to be cool for no reason)
It serves no purpose and just wastes additional gas. So we can add the check before any transaction execution:
require(currentContextAddress == address(0), "META_TX: Transaction has context set already");Ensuring transaction requirements are met
We'll further make sure that all defined requirements are met:
- An expiration time is useful, so the user knows a transaction is not executed months later when he doesn't expect it anymore.
- A
transactionsExecutedmapping to ensure a meta transaction is only executed once. Note: Make sure to settransactionsExecuted[transactionHash] = trueafter a successful execution. - A defined gas price by the user. This might not be required in your system. Since the gas is paid by the operator, the only reason for enforcing some gas price would be if the value has some further effect inside the transaction. For example in 0x the gas price will affect the fee prices.
require(block.timestamp < transaction.expirationTimeSeconds, "META_TX: Meta transaction is expired");
require(!transactionsExecuted[transactionHash], "META_TX: Transaction already executed");
require(tx.gasprice == requiredGasPrice, "META_TX: Gas price not matching required gas price");Verifying the signature
Of course we only want to execute transactions with a valid signature. A naive solution may only take the transaction.data and sign that.
But...
- how do we ensure all additional transaction parameters are set correctly (expiration, salt, signer...) ?
- how do we prevent a signed transaction from being used multiple times?
The first part is easy, we create a hash over all those values as shown above with the _getTransactionTypedHash function. The second part is what EIP-712 is solving for us. You can see how we create a hash from the transaction data and additional EIP-712 data below:
function _getFullTransactionTypedHash(Transaction memory transaction) private view returns (bytes32) {
bytes32 transactionStructHash = _getTransactionTypedHash(transaction);
bytes32 EIP191_HEADER = 0x1901000000000000000000000000000000000000000000000000000000000000;
bytes32 schemaHash = keccak256(abi.encodePacked("EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)"));
uint256 chainId = 1; // mainnet
address verifyingContract = address(this);
bytes32 domainHash = keccak256(abi.encodePacked(
schemaHash,
keccak256(bytes("My Protocol Name")),
keccak256(bytes("1.0.0")),
chainId,
verifyingContract
));
return keccak256(abi.encodePacked(EIP191_HEADER, domainHash, hashStruct));
}We put additional information into our hash, so that a signed transaction may only be used for exactly this contract with the given chainId. For all the details, check out the EIP or my previous post on ERC20-Permit.
Okay now we have the full transaction hash and the signature from the user. We can obtain the three values r and s which are the elliptic curve signature values inside our signature by extracting the bytes32 values with a helper. The uint8 v value requires only a simple conversion.
Using ecrecover with our given signature and the transaction hash, we compute a signer address. If this address matches the set transaction.signerAddress, the signature is indeed valid.
function _isValidTransactionWithHashSignature(
Transaction memory transaction,
bytes32 txHash,
bytes memory signature
) private pure returns (bool) {
require(
signature.length == 66,
"META_TX: Invalid signature length"
);
uint8 v = uint8(signature[0]);
bytes32 r = _readBytes32(signature, 1);
bytes32 s = _readBytes32(signature, 33);
address recovered = ecrecover(txHash, v, r, s);
return transaction.signerAddress == recovered;
}function _readBytes32(
bytes memory b, uint256 index
) private pure returns (bytes32 result) {
require(
b.length >= index + 32,
"META_TX: Invalid index for given bytes"
);
// Arrays are prefixed by
// a 256 bit length parameter
index += 32;
// Read the bytes32 from array memory
assembly {
result := mload(add(b, index))
}
return result;
}This is the regular signature scheme. It works perfectly if you have a user signing his own transaction.
But what if you want to allow smart contracts to create valid signatures?
Advanced signature schemes
An arguably more advanced use case is having smart contracts sign meta transactions, but imagine a user has his funds inside a multi signature smart contract. This is already quite common for certain wallets. This user cannot sign a transaction with the EIP-712 scheme to create a valid v, r, s signature.
function isValidSignature(
bytes32 hash,
bytes memory signature
) public view returns (bytes4);where the return value is 0x1626ba7e for a valid signature. It is up to the smart contract developer to decide on how to implement the signature logic.
So how can we verify such a signature?
You can see an example implementation on the right. Using staticcall, we can ensure no further state modifications happen in the call. If the result succeeds and has a valid returnData length (this is very critical, see the previous 0x bug), we can check if the return value matches 0x1626ba7e.
function _staticCallEIP1271Wallet(
address verifyingContractAddress,
bytes memory data,
bytes memory signature
) private view returns (bool) {
bytes memory callData = abi.encodeWithSelector(
IEIP1271Wallet.isValidSignature.selector,
data,
signature
);
(bool didSucceed, bytes memory returnData)
= verifyingContractAddress.staticcall(callData);
require(
didSucceed && returnData.length == 32,
"META_TX: EIP1271 call failed"
);
bytes4 returnedValue = _readBytes4(returnData, 0);
return returnedValue == 0x1626ba7e;
}You may want to allow further methods for signatures like pre-signing or having operators that can sign on behalf of a user. See the existing types in 0x here for some inspiration.
Implementing it yourself
So far we've seen the critical parts for all the implementation. This should give you a good idea on how to implement this. I would further recommend looking at
- the 0x meta transaction implementation here
- the Openzeppelin EIP-712 support here
- the npm
eip-712lib to implement the signing part here
The Openzeppelin EIP-712 library is still a draft, but has additional support for the case of forks where the chain id could change. Also take a look at the 0x code where a lot of the implementation in this blog post comes from.
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You have mastered the basics of Solidity, created your first few useful projects and now want to get your hands on some real-world projects. Getting a Solidity developer job might be easier than you think. There are generally plenty of options to choose from and often times not a lot of...
Design Pattern Solidity: Mock contracts for testing
Why you should make fun of your contracts
Mock objects are a common design pattern in object-oriented programming. Coming from the old French word 'mocquer' with the meaning of 'making fun of', it evolved to 'imitating something real' which is actually what we are doing in programming. Please only make fun of your smart contracts if you...
Kickstart your Dapp frontend development with create-eth-app
An overview on how to use the app and its features
Last time we looked at the big picture of Solidity and already mentioned the create-eth-app. Now you will find out how to use it, what features are integrated and additional ideas on how to expand on it. Started by Paul Razvan Berg, the founder of sablier, this app will kickstart your frontend...
The big picture of Solidity and Blockchain development in 2020
Overview of the most important technologies, services and tools that you need to know
Now, I do not know about you, but I remember when I first started with Solidity development being very confused by all the tools and services and how they work in connection with one another. If you are like me, this overview will help you understand the big picture of Solidity development. As I...
Design Pattern Solidity: Free up unused storage
Why you should clean up after yourself
You may or may not be used to a garbage collectors in your previous programming language. There is no such thing in Solidity and even if there was a similar concept, you would still be better off managing state data yourself. Only you as a programmer can know exactly which data will not be used...
How to setup Solidity Developer Environment on Windows
What you need to know about developing on Windows
Using Windows for development, especially for Solidity development, can be a pain sometimes, but it does not have to be. Once you have configured your environment properly, it can actually be extremely efficient and Windows is a very, very stable OS, so your overall experience can be amazing. The...
Avoiding out of gas for Truffle tests
How you do not have to worry about gas in tests anymore
You have probably seen this error message a lot of times: Error: VM Exception while processing transaction: out of gas Disclaimer : Unfortunately, this does not always actually mean what it is saying when using Truffle , especially for older versions. It can occur for various reasons and might be...
Design Pattern Solidity: Stages
How you can design stages in your contract
Closely related to the concept of finite-state machines, this pattern will help you restrict functions in your contract. You will find a lot of situations where it might be useful. Any time a contract should allow function calls only in certain stages. Let's look at an example: contract Pool {...
Web3 1.2.5: Revert reason strings
How to use the new feature
A new Web3 version was just released and it comes with a new feature that should make your life easier. With the latest version 1.2.5, you can now see the the revert reason if you use the new handleRevert option. You can activate it easily by using web3.eth.handleRevert = true . Now when you use...
Gaining back control of the internet
How Ocelot is decentralizing cloud computing
I recently came across an ambitious company that will completely redefine the way we are using the internet. Or rather, the way we are using its underlying infrastructure which ultimately is the internet. While looking at their offering, I also learned how to get anonymous cloud machines, you...
Devcon 5 - Review
Impressions from the conference
I had a lot to catch up on after Devcon. Also things didn't go quite as planned, so please excuse my delayed review! This year's Devcon was certainly stormy with a big typhoon warning already on day 1. Luckily (for us, not the people in Tokyo), it went right past Osaka. Nevertheless, a lot of...
Devcon 5 - Information, Events, Links, Telegram
What you need to know
Devcon 5 is coming up soon and there are already lots of events available, information about Osaka and more. Here is a short overview: Events Events Calendar Events Google Docs Events Kickback Most events are in all three, but if you really want to see all, you will have to look at all three...
Design Pattern Solidity: Off-chain beats on-chain
Why you should do as much as possible off-chain
As you might have realized, Ethereum transactions are anything but cheap. In particular, if you are computing complex things or storing a lot of data. That means sometimes we cannot put all logic inside Solidity. Instead, we can utilize off-chain computations to help us. A very simple example...
Design Pattern Solidity: Initialize Contract after Deployment
How to use the Initializable pattern
There are a few reasons why you might want to initialize a contract after deployment and not directly by passing constructor arguments. But first let's look at an example: contract MyCrowdsale { uint256 rate; function initialize(uint256 _rate) public { rate = _rate; } } What's the advantage over...
Consensys Blockchain Jobs Report
What the current blockchain job market looks like
Consensys published their blockchain jobs report which you can checkout in their Blockchain Developer Job Kit. The most interesting aspects are Blockchain developer jobs have been growing at a rate of 33x of the previous year according to LinkedIns jobs report Typical salary is about...
Provable — Randomness Oracle
How the Oraclize random number generator works
One particularly interesting approach by Provable is the usage of a hardware security device, namely the Ledger Nano S. It uses a trusted execution environment to generate random numbers and provides a Provable Connector Contract as interface. How to use the Provable Randomness Oracle? Use the...
Solidity Design Patterns: Multiply before Dividing
Why the correct order matters!
There has been a lot of progress since the beginning of Ethereum about best practices in Solidity. Unfortunately, I have the feeling that most of the knowledge is within the circle of experienced people and there aren’t that many online resources about it. That is why I would like to start this...
Devcon 5 Applications closing in one week
Devcon 5 Applications closing
Watch out for the Devcon 5 applications. You only have one week left to apply either as Buidler Student Scholarship Press Devcon is by far the biggest and most impressive Ethereum conference in the world. And it's full of developers! I am especially excited about the cool location this year in...
Randomness and the Blockchain
How to achieve secure randomness for Solidity smart contracts?
When we talk about randomness and blockchain, these are really two problems: 1. How to generate randomness in smart contracts? 2. How to produce randomness for proof-of-stake (POS) systems? Or more generally, how to produce trusted randomness in public distributed systems? There is some overlap...