How to generate the correct hash for signatures?

Question

I want to create a hash in JavaScript for a signature. The hash needs to have the same output as this in solidity:

keccak256(abi.encodePacked(address, bytes32))

My solution in JavaScript so far produces a differing hash:

const types = ["address", "bytes32"];
const values = [myAddress, hash];
const message = ethers.utils.defaultAbiCoder.encode(types, values);
const hash = ethers.utils.keccak256(message);

How can I reproduce the code in Solidity in JavaScript?

Answer 1

The discussion above explains the problem with my code already. Although in pieces.

I am using abi.encodePacked in my solidity code and ethers.utils.defaultAbiCoder.encode in my JS code.

encodePacked gives additional padding to the input variables to get it to 32 bytes, while encode takes them as is.

TLDR: I have to use the same encoding in JS ans Solidity.

Answer 2

The main issue in your JavaScript code is related to how data is encoded before hashing. In Solidity, keccak256(abi.encodePacked(address, bytes32)) uses a packed encoding. However, the ethers.utils.defaultAbiCoder.encode method does not produce a packed encoding; instead, it follows the standard ABI encoding, which includes padding to 32 bytes for each value.

In order to get the same hash as keccak256(abi.encodePacked(address, bytes32)) in Solidity, you need to use a packed encoding. You can use the ethers.utils.solidityPack function to achieve this. Here’s how to modify your code to generate the correct hash:

const ethers = require('ethers'); // or import ethers if using modules

const types = ["address", "bytes32"];
const values = [myAddress, hash];

// Using solidityPack for packed encoding
const packedData = ethers.utils.solidityPack(types, values);
const correctHash = ethers.utils.keccak256(packedData);

console.log(correctHash);