Every program that interacts with people responds to a specific set of commands. The set of commands and the syntax for entering them is called the user interface and varies from one program to another. The DOS operating system makes a distinction between internal and external commands.
As far as I remember, message-digest functions distill the information contained within a file (small or large) into a single fixed-length<span>number, typically between 128 and 256 bits in length. This function is designed to protect the integrity of a piece of data to define changes to any part of a message.</span>
