The PolyAlphabetic Cipher Table, sometimes called The Vigenere Square.
     I wrote this table in HTML because I could not find an example anywhere that could be printed legibly, or used to assist my grandson in the use of various cipher systems. Perhaps you can print and use this chart to encourage an inquisitive youngster to explore the world of encryption. Who knows, you may even be encouraging the next Super-Cryptologist!  Free use to all, links appreciated. Enjoy! The Old Man.
   
 
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*Note: The terms "Codes" and "Ciphers" do not mean the same, however this brief is not intended for the professional cryptologist, it is only intended to present one example of a PolyAlphabetic table to a general audience. This brief will not address the development of various cipher systems nor will it mention safeguards normally used in preparing, passing, receiving or storing cipher materials.
 
The PolyAlphabetic cipher table has been around in one form or another for at least 500 years. It is sometimes called the "Vigenere Square", recognizing a French fellow who used this method back in the middle ages. By using a good "Key" this method may be used to encipher a message with a fair amount of confidence for a short period of time. Even using the PolyAlphabetic Table, it would not take a competent cryptologist very long to decipher your message, whether or not he had the key or any genuine word in your message. Most languages have certain letters that occur more often than others, and most short words and connecting words can be guessed fairly easily. In short, if you use any form of substitution to encipher a message it should be for matters that only demand a short lifespan of secrecy. To increase the value of this type of substitution, you might use nulls, avoid words and letters on the high end of the frequency charts, and use the longest keyword you can think of... but remember, Keys cannot contain duplicate letters.
 
To use this table we will need a "Message" and a "Key". The Key must be known in advance by both the sender and the receiver, whereas the message will only be known by the sender until the receiver uses the Key and the enciphered message to decipher the message into plain text.
 
Capitalization and punctuation are not considered in this brief.
Message In Plain Text: "Banana". (Plain Text message will be known as "PT").
Key: "ug" ( No duplicate letters, does not have to be a real word).( Known as "Key" )
 
The Plain Text message "PT", we want to send is BANANA
The secret Key (or keyword) we have shared with our receiver is: UG
To encipher the Plain Text message write it out with enough space between the letters that you can keep track of the Key later. Below the message, write out the Key as many times as needed so that every letter in the message has a Key letter below it.
 
Message:  B A N A N A
Keyword: U G U G U G

 
  Now look at the PolyAlphabetic table. Find the first letter of the message (B) on the alphabet on the top row of the square. Then find the first letter of the keyword (U) on the column running down the left side of the square. Trace the two paths until you find the spot on the square where the B row meets the U column (V). Use this method to encode the entire message. Your coded message will read:
 
Enciphered Message: V G H G H G, you can now send this to your receiver.
 
To decipher the enciphered message, the receiver will write the keyword above the coded message: (just opposite of what you did to encipher it, right?)
Keyword: U G U G U G
Message: V G H G H G
 
  Find the first letter of the keyword (U) in the alphabet running down the left side of the square. Move your finger along this row until you meet the first letter of the coded message (V). Now run up from the V until you reach the top alphabet. This gives you the letter (B). Continue until you have deciphered the entire message.
 
Use the Vigenere Square to encipher and decipher a message with a friend. Use a long keyword or make up a long non-word and see how it works. Just remember to not use a Keyword with repeated letters, such as  B A N A N A ! (It has duplicate letters)
 
Credits and additional resources:
(1)nsa.gov/programs/mepp/hs/alg18.pdf
(2)witn.psu.edu/2217/gofigure.html
(3)adci.gov/cia/publications/pubs.html
(4)"Secrets of Making and Breaking Codes", Hamilton Nickels, Citadel Press, 1990.
(5)nolo.com/encyclopedia/articles/ilaw/pub_permission.html?