Hash Function

A cryptographic hash function is the algorithm that turns data of any size into a fixed-length output, or hash. To be usable in crypto it must be deterministic, fast to compute, practically impossible to reverse, and collision-resistant — meaning nobody can find two different inputs with the same output. Bitcoin uses SHA-256 and Ethereum uses Keccak-256; different chains and wallets pick different functions, which is why hashes are not interchangeable between them. Hash functions underpin mining puzzles, Merkle trees, address derivation and message signing. They are not encryption: hashing is one-way and has no key, so it protects integrity rather than confidentiality and can never be decrypted back.