A message digest is a function that ensures the integrity of a message. Message digests take a message as input and generate a block of bits, usually several hundred bits long, that represents the fingerprint of the message. A small change in the message (say, by an interloper or eavesdropper) creates a noticeable change in the fingerprint.

The message-digest function is a one-way function. It is a simple matter to generate the fingerprint from the message, but quite difficult to generate a message that matches a given fingerprint.

Message digests can be weak or strong. A checksum -- which is the XOR of all the bytes of a message -- is an example of a weak message-digest function. It is easy to modify one byte to generate any desired checksum fingerprint. Most strong functions use hashing. A 1-bit change in the message leads to a massive change in the fingerprint (ideally, 50 percent of the fingerprint bits change).