www.MarkTAW.com/technology/Rot13EncoderDecoder.html (printable version)
Rot13 Encoder/DecoderROT13, or Rotation 13 is a simple form of encryption often used online when you're about to reveal movie or video game spoilers and don't want someone to casually see what you've written. It takes the characters you type and moves them up/down by 13 characters. Since the english alphabet is 26 characters, encryption and decryption are exactly the same procedure.
The people who monitor your e-mail, instant messgener conversations, etc. aren't expecting you to be using ROT13, so any keywords that normally get flagged won't raise a flag, and the conversation will look like gibberish to them. Even if they're aware of ROT13, you'll be in the minority by using it, and they likely won't go through the trouble to decrypt your messages.
The great thing about this script is it's entirely client side, so your unencrypted message never travels across the network. Only if they have a keystroke logger will they have your unencrypted message.
This script originally appeared on http://tools.geht.net/rot13.html
Message Board: http://www.marktaw.com/forum/list.php?f=1
page first created on Monday, November 25, 2002
this site and it's contents copyright Mark Wieczorek