ROT13 Encipherer

Use this utility to convert plain text into ROT13 text or vice-versa.


For a quick demonstration first ensure that you can see the large text area above, then try the links below:


ROT13 is a simple routine to obfuscate plain text. It is useful for hiding punch lines to jokes in newsgroup posts and to hide email addresses from spam bots. Don't use it for anything more demanding, it is not real encryption. If you want real encryption go to and download the free PGP client.

History Lesson

ROT13 employs a method popularly known as "Caesar's Cipher". Julius Caesar was known to send encoded messages to his subjects to prevent them being read during delivery. His method was to associate each letter of the alphabet with a number ('A' = 1, 'B' = 2, …). For each letter of his message he would determine this number and then add 3 to it. Using the same system, he would then determine the letter that represented this new number and write that letter down in his message instead of the meaningful original. So, 'A', with a value of 1, would be encoded to value of 4 and therefore letter 'D'. This way "CAESAR" would become "FDHVDU". If the new number was greater than the value for the last letter in the alphabet, then that value would be subtracted from the new number to wrap the letters back to the start of the alphabet. With 26 letters in the English alphabet once we get a number greater than 26 we simply subtract 26 and carry on as before. So encoding 'X' would give us 'A', 'Y' would become 'B' and finally 'Z' would equal 'C'.

On receipt of the message, Caesar's subjects would follow the reverse procedure to retrieve the original message. All very clever in the 1st century, but only useful to seven year olds in the 21st! ROT13 is no more cunning than the original Caesar's Cipher, except that the offset value to add is not 3 but 13. Since the English alphabet has 26 letters, adding 13 to a letter value will yield the same value as if we had actually subtracted 13 — because we wrap to the start of the alphabet again. The point here is that we can use the same routine for encoding as we do for decoding. E.g.: So on encoding we might have: 'A' + 13 = 'N', and decoding via subtraction as before we might have: 'N' - 13 = 'A'. But because we used 13 as our cipher offset, adding 13 to decode also returns us to our original character: 'N' + 13 = 'A'. In other words: 'A' + 13 + 13 = 'A'.



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