3.3.5 How is information integrity assured?

When you put together your message to me and encrypt it with my public key, you also have your computer calculate something called a ``message digest'' and attach it to the message. The message digest is a small piece of data that results from performing a particular mathematical calculation on your message. It acts as the digital fingerprint of your message -- the message digest of every message is different.

When I receive your message and decrypt it with my private key, I also calculate the message digest of your message. Then, I compare the digest I generated to the one you sent with your message. If they match, then I know that I received exactly what you sent.