What is Hash? A hash function is a mathematical process that takes data of any size, performs an operation on it, and returns a hash, which is data of a fixed size. Hashing algorithm takes an input and generates a unique hexadecimal output of a fixed length. Hashing only goes in one direction: you can't take the fixed-length data output and recreate the input string. Bitcoin blockchain uses the SHA256 algorithm. Ethereum uses Keccak-256 in a consensus engine called Ethash.