Hash chain

A hash chain is the successive application of a cryptographic hash function to a piece of data. In computer security it is a method for producing many one-time keys from a single key or password, and it can also record the chronology of data's existence, for non-repudiation, by applying the hash function in turn to additional pieces of data. A hash chain of length 4 over a string x is the hash function applied four times to x. Leslie Lamport suggested hash chains as a password protection scheme in an insecure environment: a server stores the far end of a chain, the user supplies the previous link each time, and because the hash function is one-way an eavesdropper cannot reverse it to obtain an earlier link. Binary hash chains, used with hash trees, combine two hash values at a time, and Winternitz chains, used in hash-based cryptography, apply a chain function repeatedly to a secret key. A hash chain resembles a blockchain, but a blockchain is meant to support distributed agreement around a public ledger and carries rules for data and permissions.