Depends, how good are you at creative writing? I can think of a lot of messages you might send to someone that you'd want to be private that aren't nefarious plots. Weird fan fiction. Deviant porn. Messages exchanged with a secret mistress. Depending on the situation, you might even want to give them a fake copy of your nefarious plot. Include more than one extra set of messages if you like and give them whatever keys you like in whatever order is appropriate.