“Decoder rings” and children’s “code wheels” for sending and decoding secret messages generally used a simple method of encrypting (encoding) the alphabet by simply swapping one letter for another. Such a cipher is known as a Caesar cipher— named for Julius Caesar who is known to have used a similar type of code in his correspondences.

These types of ciphers are famously easy to “crack”. Even children are able to figure out quickly that a letter by itself is almost certainly the letter “I” or “A”. Repeated letter sequences that are three letters long might be the word “the” — especially if the first two letters (“th”) show up at the start of other words (“this”, “that”).

More clever children (who perhaps have played the game Hangman) might apply statistics — knowing that the vowels (especially the letter “e”) are the more common letters in English.

One simple such cipher involves replacing each letter in the alphabet with the letter thirteen places away (if you go off the end of the alphabet you wrap around back to the other end). This cipher is often called a Rot-13 cipher since you are “rot”-ating the alphabet thirteen places.
Rot13_vertical_tableSince there are twenty-six letters in the alphabet you don’t have to worry about remembering whether you are to count the letters to the left or to the right since the code works just as well in either direction. In fact, for the Rot-13 table to the right, I needed to only show the first thirteen letters on the left and the remaining thirteen on the right. See if you can figure out how to use it.

Here is a simple encrypted message you can try to decipher:

frperg zrffntrf

Even a short message like the above though can be tedious to decipher. Here is a text field you can enter text into and encode (or decode) using Rot-13. You can type either the original, unencrypted message in, or the Rot-13 encoded message. Clicking the Rot-13 button once will transform the text, clicking again will transform it again (in effect reverting it).