A cryptographic hash function is a transformation that takes an input ( or ` message ' ) and returns a fixed-size string , which is called the hash value ( sometimes called a message digest , a digital fingerprint , a digest or a checksum ) . A cryptographic hash function is a deterministic procedure that takes an arbitrary block of data and returns a fixed-size bit string , the ( cryptographic ) hash value , such that an accidental or intentional change to the data will change the hash value . The data to be encoded is often called the `` message , '' and the hash value is sometimes called the message digest or simply digest .