Write a command line interface (CLI) application that reads a passphrase as input. Then gives the option to either encrypt a message and save it to a file, or read a message from a file, decrypt and show it.
A passphrase is just a string of characters or words. Passphrases are longer than usual passwords making them more secure. So you can think of a passphrase as essentially just a long password.
This is the project of Lasse Hansen and Anders Bernhard Madsen.