Chose a key, share it with your friend(s) & you can now start sending your texts or emails safely.
For a better protection, be sure to pick a key with a reasonnable lenght (10+ characters).
The crypto used in this application is quite strong for a daily classic use, but absolutely not unbreakable by professionals. (It's actually a classic Vigenere cypher)
Please, feel free to contact me for any bug or information ! :)
Happy safe exchange !