Hash Functions

A cryptographic hash function takes an input of any size and produces a fixed-size output (the hash or digest). Key properties include: deterministic (same input always gives same output), pre-image resistant (can't reverse-engineer the input from the hash), collision resistant (extremely unlikely two different inputs produce the same hash), and avalanche effect (small input changes cause dramatic output changes). SHA-256, used in Bitcoin, produces a 256-bit hash. Keccak-256, used in Ethereum, is another widely used hash function.