Gabriel wants to send a confidential message over the Internet to Mia. Mia needs some way to decrypt the message, as well as a way to guarantee that Gabrieland not an imposterhas actually sent the message. First, Gabriel runs his message through an algorithm called a hash function. This produces a number known as the message digest. The message digest acts as a sort of "digital fingerprint" that Mia uses to ensure that no one has altered the message.