A hash function, otherwise known as a one-way hash function, takes an arbitrary message of arbitrary length and creates an output (a hash) of a fixed length. The main characteristics of a cryptographic hash function are that given a message, it is easy to compute the hash; given the hash, it is difficult to compute the message; and that given a message, it is difficult to find a different message that would produce the same hash (this is known as a collision). A keyed hash function takes a message and a secret key as input and outputs a fixed-length hash.