Hash

A hash is the fixed-length output produced by running data of any size through a cryptographic hash function — a mathematical one-way fingerprint that uniquely summarizes the input. The same input always yields the same hash, but changing even one character produces a completely different result, and there is no practical way to work backwards from a hash to the original data or to find two different inputs that share one.

Blockchains use hashes everywhere. Every transaction has a hash serving as its ID; every block header contains the hash of the previous block, which is what literally chains blocks together — altering an old transaction would change its block's hash, breaking every subsequent link and making tampering instantly detectable. Merkle trees of hashes let light clients verify a single transaction without downloading everything, and proof-of-work mining is a race to find a block hash below a target value. For example, hashing the word "hello" with SHA-256 (Bitcoin's hash function) always produces the same 64-character hexadecimal string, while "Hello" produces an unrelated one.

A common misconception is that hashing is encryption; encryption is reversible with a key, whereas hashing is designed to be irreversible and carries no key at all. Different chains use different functions — Bitcoin uses SHA-256, Ethereum uses Keccak-256 — but all rely on the same properties: determinism, collision resistance, and the avalanche effect that makes outputs unpredictable.