The Vigenère square method of encryption uses a keyword instead of a key number. The keyword should be easy to remember for those who are supposed to know it and hard to guess for those who aren’t. This keyword is used with the square at the right. Let’s say the keyword is “trees”, and the plaintext message is “the end is near”. The encoder would begin by writing one keyword letter over each message letter as shown in Table 5, repeating the keyword as many times as necessary. They would then get each cipher text character by looking up the keyword character and the plain text character on Table 6. (Since Table 6 is symmetric, it doesn’t really matter whether you put the keyword character on top and the plain text character on the side or vice versa.) t and t gives M, r and h gives Y, e and e gives I, etc.

Notice that W appears three times in the cipher text. The first time, however, it represents a d, the second time an s and the final time it replaces an n. This is called a poly-alphabetic encryption scheme, since different letters can be represented with the same letter, and the same letter can be represented by different letters. For example, the first plain text e is represented by I, but the last one (in the word near) is represented by W.

To decipher the message if you know the keyword, simply get in the column of the letter of the keyword and move down until you find the letter of the cipher text. The letter at the far left is the plain text character. For example, knowing the message above was encrypted with the keyword “trees”, you’d get in the t column and move down until you reach M. In the first column the letter t is the plain text character.

BREAKING THE CODE

To decipher the message if you don’t know the keyword is difficult at best, and is beyond the scope of this booklet. The method involves statistics, perseverance and luck in trying to guess the length of the keyword. Once you know the length of the keyword, write the encrypted message in that many columns and use the trick in breaking additive codes on each column.