Rot-13 (short for rotate 13) is a simple letter substitution cipher that was popular in online forums for hiding spoilers, punchlines, offensive material, and other things from a casual glance. It works by replacing the current english letters in a message with those that are 13 positions ahead in the alphabet. For example, the letter a is replaced by n, b by o, c by p, etc. Numbers and punctuation are not encoded.