Keccak256 Hash Workbench

What is Keccak256?

Keccak256 is a cryptographic hash function from the Keccak family, producing a 256-bit (32-byte) output from any input of arbitrary length. It is the primary hash function used in Ethereum and the EVM, chosen for its security properties, resistance to length-extension attacks, and efficient hardware implementation.

In Solidity, keccak256 is a built-in function: keccak256(bytes memory input) returns (bytes32). It appears throughout smart contract development for computing function selectors, event topic hashes, storage slot keys, and commitment schemes.

Keccak256 vs SHA3-256: A Critical Difference

Many developers confuse keccak256 with SHA3-256. They are NOT the same. Both are based on the same Keccak sponge construction, but NIST modified the padding domain separator when standardizing SHA-3 in FIPS 202. Ethereum adopted the original Keccak padding (0x01), not the NIST SHA-3 padding (0x06).

How Ethereum Uses Keccak256

• 1
  Account addresses: Ethereum addresses are the last 20 bytes of keccak256(publicKey), where publicKey is the uncompressed 64-byte ECDSA public key.
• 2
  Function selectors: bytes4(keccak256("transfer(address,uint256)")) = 0xa9059cbb. The EVM uses this 4-byte prefix in calldata to dispatch calls to the correct function.
• 3
  Event topic hashes: topic[0] in EVM logs is the full 32-byte keccak256(eventSignature). Unlike function selectors, the full hash is used to avoid log filtering collisions.
• 4
  Storage slot calculation: In mappings, the storage slot for a key is keccak256(abi.encode(key, baseSlot)). Understanding this is critical for storage layout analysis and attack vector discovery.
• 5
  Transaction & block hashes: keccak256 is used to hash transaction data and build Merkle trees for block integrity throughout the Ethereum protocol.
• 6
  CREATE2 address derivation: keccak256(0xff ++ deployerAddr ++ salt ++ keccak256(initcode)), last 20 bytes = deterministic contract address.

Code Examples

// Solidity
bytes32 hash = keccak256(abi.encodePacked("Hello, World!"));
bytes4 selector = bytes4(keccak256("transfer(address,uint256)"));

// Storage slot for mapping key
bytes32 slot = keccak256(abi.encode(userAddress, uint256(0)));

// ethers.js v6
import { keccak256, toUtf8Bytes, id } from "ethers";

// Hash a UTF-8 string
const hash = keccak256(toUtf8Bytes("Hello, World!"));

// Shorthand: hash a UTF-8 string (id = keccak256 of UTF-8)
const selector = id("transfer(address,uint256)").slice(0, 10);

// web3.js
const hash = web3.utils.keccak256("Hello, World!");

// Function selector
const selector = web3.eth.abi.encodeFunctionSignature("transfer(address,uint256)");
// Returns "0xa9059cbb"