Press the green flag to play. If you really want to you can try your own "messages" but you will have to modify the code because I can't be bothered :)
Watch Scratch Cat's explanation of the RSA encryption system. Also feel free to admire his amazing maths abilities. This actually does perform all the calculations, if you don't believe me try replaying it. Also the received message is actually calculated as well, it isn't just copying over the message - check the script.