SKILL.md
Node.js Keccak-256
Ethereum uses Keccak-256, not the NIST-standardized SHA3 variant exposed by Node's crypto.createHash('sha3-256').
When to Use
- Computing Ethereum function selectors or event topics
- Building EIP-712, signature, Merkle, or storage-slot helpers in JS/TS
- Reviewing any code that hashes Ethereum data with Node crypto directly
How It Works
The two algorithms produce different outputs for the same input, and Node will not warn you.
import crypto from 'crypto';
import { keccak256, toUtf8Bytes } from 'ethers';const data = 'hello';
const nistSha3 = crypto.createHash('sha3-256').update(data).digest('hex');
const keccak = keccak256(toUtf8Bytes(data)).slice(2);
console.log(nistSha3 === keccak); // false
## Examples
### ethers v6
import { keccak256, toUtf8Bytes, solidityPackedKeccak256, id } from 'ethers';
const hash = keccak256(new Uint8Array([0x01, 0x02]));
const hash2 = keccak256(toUtf8Bytes('hello'));
const topic = id('Transfer(address,address,uint256)');
const packed = solidityPackedKeccak256(
['address', 'uint256'],
['0x742d35Cc6634C0532925a3b8D4C9B569890FaC1c', 100n],
);
### viem
import { keccak256, toBytes } from 'viem';
const hash = keccak256(toBytes('hello'));
### web3.js
const hash = web3.utils.keccak256('hello');
const packed = web3.utils.soliditySha3(
{ type: 'address', value: '0x742d35Cc6634C0532925a3b8D4C9B569890FaC1c' },
{ type: 'uint256', value: '100' },
);
### Common patterns
import { id, keccak256, AbiCoder } from 'ethers';
const selector = id('transfer(address,uint256)').slice(0, 10);
const typeHash = keccak256(toUtf8Bytes('Transfer(address from,address to,uint256 value)'));
function getMappingSlot(key: string, mappingSlot: number): string {
return keccak256(
AbiCoder.defaultAbiCoder().encode(['address', 'uint256'], [key, mappingSlot]),
);
}
### Address from public key
import { keccak256 } from 'ethers';
function pubkeyToAddress(pubkeyBytes: Uint8Array): string {
const hash = keccak256(pubkeyBytes.slice(1));
return '0x' + hash.slice(-40);
}
### Audit your codebase
grep -rn "createHash.sha3" --include=".ts" --include="*.js" --exclude-dir=node_modules .
grep -rn "keccak256" --include=".ts" --include=".js" . | grep -v node_modules