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What is Keccak-256?
Keccak-256 is the hash function Ethereum uses throughout: it turns any data into a fixed 256-bit fingerprint. It is the original Keccak design that won the US SHA-3 competition, which differs slightly from the final SHA-3 standard, so the two give different outputs for the same input.
What it means for you
Your Ethereum address is the last 20 bytes of the Keccak-256 hash of your public key, and the mix of upper- and lower-case letters in an address is a Keccak-based checksum. A tool labelled SHA-3 will not reproduce Ethereum's hashes, so a mismatch there does not mean an address is wrong.
How it works
NIST selected Keccak as the winner of its SHA-3 competition, and FIPS 202 builds SHA3-256 and the extendable-output SHAKE functions from it with a sponge construction: input is absorbed into a large internal state, mixed by the Keccak-p permutation, and output is squeezed out. The Ethereum Yellow Paper specifies Keccak-256 as the winning competition entry rather than the final SHA-3 specification. Ethereum uses it to derive addresses from public keys, to reference nodes in its state trie and to identify contract code.
Sources: NIST FIPS 202: SHA-3 Standard, Ethereum Yellow Paper, ethereum.org: Ethereum accounts · checked 4 October 2026
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