ROT13

Not really intended for security, just a mild amount of obscurity, ROT-13 is a system often used for posting spoilers to books and movies, or off-color content that might offend some readers, on newsgroups and forums. It consists of text in which the letters of the standard 26-letter English alphabet are rotated 13 positions (wrapping around the end/beginning of the alphabet). Other characters are unchanged. It doesn't matter whether you rotate forward or backward, since 13 is exactly half of the alphabet and will wind up in the same place either way. This also means that encrypting and decrypting are the same operation; doing it twice will leave the text exactly how it started. The result is text that looks "encrypted" and isn't obviously readable, but is easy to decrypt.