Poseidon

An 'arithmetic-friendly' hash function designed for efficient computation inside zero-knowledge proof circuits. SHA-256 and Keccak-256 use bitwise operations that translate badly to circuits — a single SHA-256 evaluation costs tens of thousands of constraints. Poseidon uses field-friendly operations (additions and powers) that are 10-100x cheaper inside proofs. Used by zkSync, Polygon zkEVM, Filecoin proofs, and most modern ZK systems.