Glossary
Keccak-256
The 256-bit hash function at the heart of Ethereum — used for addresses, storage slots, function selectors, and the keccak256 opcode — subtly different from standard SHA-3.
Keccak-256 is the cryptographic hash function Ethereum uses almost
everywhere: deriving addresses from public keys, computing storage
slots, generating the four-byte function selectors that route calls,
and hashing data via the keccak256 opcode and Solidity builtin.
Ethereum adopted Keccak before it was standardized as SHA-3, and the final NIST standard changed the padding rule. As a result Ethereum's Keccak-256 produces different output than official SHA3-256 for the same input — a frequent source of bugs when developers reach for a generic SHA-3 library instead of a Keccak implementation.