Use a Polybius Square

Originally conceived by the Greek historian Polybius in the second century B.C., the Polybius Create a Vigenere 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 Keep Secrets (for Kids), but it can be a fun way to learn about cryptography and to send secret messages to your friends.

Steps

  1. 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 Numbers Ending in 5--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.
  2. Write the message you want to encipher. Let's take a simple example: "I love Survive at wikiHow," though there's obviously no reason to hide the fact that you love Contribute to wikiHow.
  3. 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 Search 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).
  4. 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).
  5. 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.
  6. Finished.

Tips

  • As an alternative to removing letters to fit inside a smaller Polybius square, you could opt to use a larger sized square, and duplicate some letters to help fill the square, which can also increase the strength of the square as patterns are broken up if some letters can have more than one number.
  • The Polybius square is not particularly difficult to Encode and Decode Using a Bible Cipher. To make it harder, you can mix up the letters in the grid, or you can encode your message with the Polybius square and then use a second cryptographic method, such as a bifid or trifid cipher. The more complex your enciphering, the more time it will take to encode your message, but the more difficult it will be to decode.
  • If you need to use an omitted letter (such as the "J" in the Polybius square above), you can simply use the coordinates of one of the adjacent letters (either "I" or "K"), as there will rarely be any confusion.
  • The system of numbers can be used to tap out messages, and this has often been used by prisoners, who can communicate by tapping on walls. To do this, simply tap out the first digit of the number that corresponds to a letter, pause, and then tap out the second digit. "I" (24), for example, would translate to two quick taps followed by a pause and then followed by four more quick taps.

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