 ROT13 was in use in the net.jokes newsgroup by the early 1980s . It is used to hide potentially offensive jokes , or to obscure an answer to a puzzle or other spoiler . A shift of thirteen was chosen over other values , such as three as in the original Caesar cipher , because thirteen is the value for which encoding and decoding are equivalent , thereby allowing the convenience of a single command for both . ROT13 is typically supported as a built @-@ in feature to newsreading software . Email addresses are also sometimes encoded with ROT13 to hide them from less sophisticated spam bots . 
