Nodejs Keccak256

---
name: nodejs-keccak256
description: "Use when prevent Ethereum hashing bugs in JavaScript and TypeScript. Node's sha3-256 is NIST SHA3, not Ethereum Keccak-256, and silently breaks selectors, signatures, storage slots, and address derivation. Triggers on \"nodejs-keccak256\", \"nodejs keccak256\", \"keccak256\"."
metadata:
  origin: ECC direct-port adaptation
version: "1.0.0"
---

# 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.

```javascript
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

```typescript
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

```typescript
import { keccak256, toBytes } from 'viem';

const hash = keccak256(toBytes('hello'));
```

### web3.js

```javascript
const hash = web3.utils.keccak256('hello');
const packed = web3.utils.soliditySha3(
  { type: 'address', value: '0x742d35Cc6634C0532925a3b8D4C9B569890FaC1c' },
  { type: 'uint256', value: '100' },
);