For the recipient to be able to decrypt the message, they need to have a copy of the key.  This is a complicated process to explain, so we will show you how it works on the example of paint instead of math..  To create an encryption for a private message you and your friend first have to decide on a color. In this example we picked yellow. This information is made public. [Pause] Next, you both pick a color separately, but this time keep it private. Now, you and your friend mix your private color with the public color so you both get a new color that’s different form each other.