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EVM transaction data

In some case, we might want to add additional data to the transaction. However, the transaction only takes the following parameters:

  • from: the address of the sender
  • to: the address of the recipient
  • value: the amount of ether to send
  • data: the data to send
  • gas related parameters
info

Metamask - sendTransaction api

For plain transaction, it is ok to just put the additional data in the data field. However, when it comes to smart contract transaction, there is no way to attach additional data to the transaction but append it as suffix to the data field.

Take the ERC20 token approve function as an example, if we want to approve 100 tokens to another address with additional data, we can do the following:

// Assuming you already have a provider object to sign transaction
// The ethersjs version being used is "^5.5.4"
const erc20ABI = [
  "function approve(address _spender, uint256 _value) public returns (bool success)",
]
const tokenContract = new ethers.Contract('the-erc20-token-addr', erc20ABI, provider.getSigner());
const amount = ethers.utils.parseUnits('1', decimals);
// Each slot for EVM is 32 bytes, so we want to append 1 slot of data as additional data
const additionalSlot = '0000000000000000000000000000000000000000000000000000000000000001';
const approveData = tokenContract.interface.encodeFunctionData('approve', ['<target-contract-adress>', amount]);

const modifiedApproveData = approveData + additionalSlot;
console.log('approveData', modifiedApproveData);
const approveTransaction = await provider.getSigner().sendTransaction({
  to: tokenContract.address,
  data: modifiedApproveData
});

As known, the format of contract call data is function signature + parameters. Each of them is 32 bytes. As approved function has 2 parameters, so the data is function signature + parameter1 + parameter2 like below:

0x095ea7b330000000000000000000000008fe3842e0b7472a57f2a2d56cf6bce08517a1de000000000000000000000000000000000000000000000000000000000000f424
  • 0x095ea7b3 is the function signature of approve(address _spender, uint256 _value)
  • 0000000000000000000000008fe3842e0b7472a57f2a2d56cf6bce08517a1de is the address of the spender (32 bytes, 20 bytes for address and 12 bytes for padding 0)
  • 000000000000000000000000000000000000000000000000000000000000f424 is the amount (1 eth) to approve (32 bytes, 16 bytes for amount and 16 bytes for padding 0)

Then we append additional data 0000000000000000000000000000000000000000000000000000000000000001 to the end of the data. So eventually, the data is:

0x095ea7b330000000000000000000000008fe3842e0b7472a57f2a2d56cf6bce08517a1de000000000000000000000000000000000000000000000000000000000000f4240000000000000000000000000000000000000000000000000000000000000001