A message digest is a fixed-length representation of a variable string of data. This fixed-length value is known as a message digest because it contains a potentially much smaller, yet unique, representation of the original message or data. By comparing the message digest generated in one place with the message digest generated in another place using what is supposed to be the same data, you can tell if the data has actually been changed while moving from one place to the other. If the two message digests are identical, the data has not changed. Thus, because of their fixed and relatively small size, message digests are very convenient to verify the integrity of the original data from which they are generated.
