How to Use a Polybius Square Cipher

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Originally conceived by the Greek historian Polybius in the second century B.C., the Polybius square or Polybius checkerboard is one of the simplest tools in cryptography. You can use a Polybius square to encode a message that can then be deciphered only by someone who knows the arrangement of letters in the square. It's not secure enough to send military secrets, but it can be a fun way to learn about cryptography and to send secret messages to your friends.

Steps

Design a Polybius square (see image). Create an equal number of numbered rows and columns (see image). To determine how many rows and columns to make, find the square number (also called a perfect square) that is nearest to the number of letters in your alphabet. Then take the square root of that square number--this will be the number of rows and columns in your Polybius square. For example, in the English alphabet, there are 26 Write Different letters. The closest square number to 26 is 25, and the square root of 25 is 5, so you would need 5 columns and 5 rows, as shown in the image. Other languages use alphabets with more or fewer letters, so depending on the language used, the Polybius square may have more or fewer than 5 rows and columns. Write the letters of the alphabet in your grid. There should be one letter at the intersection of each row and column. In the simplest form, you can just write the alphabet out in order, but if you want to make the code harder to crack, you can mix up the letters. If there are too many letters in the alphabet to fit into the square, you can omit one or two (preferably these are uncommonly used letters). In the example, the letter "J" has been omitted. If there are too few letters to fill the grid, you can leave some spaces blank or use them for punctuation, emoticons, or whatever you like.

Write the message you want to encipher. Let's take a simple example: "I love wikiHow," though there's obviously no reason to hide the fact that you love wikiHow.

Replace the first letter of your message with the numbers that correspond to its coordinates. Look at your Polybius square, and find the row and column numbers of your first letter. In the example "I love wikiHow," the letter "I" is the first letter. "I" is located at the intersection of row 2 and column 4, so you would begin your coded message with the number 24 (the row number is usually written before the column number).

Repeat the previous step for all the letters in your message. "I love wikiHow" becomes "24 31 34 51 15 52 24 25 24 23 34 52." You could also get rid of the spaces (243134511552242524233452) or leave spaces only between words (24 31345115 52242524233452).

Decode the message by replacing each pair of coordinates with its corresponding letter. If you're sending the message to someone, you'll need to also send them the Polybius square. Send it to them separately to heighten the security of your message.