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Ethereum Keccak-256 Hashing

Your AI prevents Ethereum hashing bugs by using the correct Keccak-256 algorithm instead of Node's NIST SHA3 variant. It supports ethers, viem, and web3.js libraries for accurate selectors, signatures, storage slots, and address derivation.

affaan-m on GitHub

Curated by PromptsEdge from a public repo · MIT license. All credit goes to the author.

What is this skill?

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' },
);

Common patterns

typescript
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

typescript
import { keccak256 } from 'ethers';

function pubkeyToAddress(pubkeyBytes: Uint8Array): string {
  const hash = keccak256(pubkeyBytes.slice(1));
  return '0x' + hash.slice(-40);
}

Audit your codebase

bash
grep -rn "createHash.*sha3" --include="*.ts" --include="*.js" --exclude-dir=node_modules .
grep -rn "keccak256" --include="*.ts" --include="*.js" . | grep -v node_modules

Rule

For Ethereum contexts, never use crypto.createHash('sha3-256'). Use Keccak-aware helpers from ethers, viem, web3, or another explicit Keccak implementation.

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