Glossary
Poseidon
A ZK-friendly hash function designed to be cheap inside SNARK and STARK circuits — orders of magnitude fewer constraints than SHA-256 or Keccak.
Poseidon, published in 2019, was designed specifically for use inside zero-knowledge proof systems. Traditional hashes like SHA-256 and Keccak operate on bits — translating bit operations into circuit constraints is enormously expensive. Poseidon operates over a finite field directly, requiring orders of magnitude fewer constraints.
The hash is the default choice inside SNARK and STARK circuits that need internal hashing: ZK rollups (Polygon zkEVM, Aztec, zkSync), privacy protocols (Railgun), and many ZK-identity systems use Poseidon. The successor Poseidon2 reduces constraints further.