After installing and enabling the Kibo keyboard, using it is quite easy. Simply type the message that you want to send privately, click the lock icon, and hit send. Your message will go to your recipient as a secret that they will uncover. For example, you may type "boy, is he a jerk," but the message will display as a random statement like "I didn't have Internet access then." Once your recipient sees the hidden message, they just hold down on it, select copy, and will then see your what you actually wrote.
Kibo works with many popular services such as Viber, WhatsApp, Facebook Messenger, WeChat, Skype, and more, including iMessage. You can also send non-secretive messages with Kibo, so it is useful as a regular keyboard as well. In addition, you can set up a passcode and use Touch ID for added security.