The Vigenère Cipher

In the late 1500s, Blaise de Vigenère proposed a polyalphabetic system that is particularly difficult to decipher. His method used a combination of the Trimethius tableau and a key. The key determined which of the alphabets in the table the decipherer should use, but wasn't necessarily part of the actual message. Let's look at the Trimethius tableau again:

Let's assume you are encrypting a message using the key word "CIPHER." You would encipher the first letter using the "C" row as a guide, using the letter found at the intersection of the "C" row and the corresponding plaintext letter's column. For the second letter, you'd use the "I" row, and so on. Once you use the "R" row to encipher a letter, you'd start back at "C". Using this key word and method, you could encipher "How Stuff Works" this way:

Key

C

I

P

H

E

R

C

I

P

H

E

R

C

Plain

H

O

W

S

T

U

F

F

W

O

R

K

S

Cipher

J

W

L

Z

X

L

H

N

L

V

V

B

U


Your enciphered message would read, "JWL ZXLHN LVVBU." If you wanted to write a longer message, you'd keep repeating the key over and over to encipher your plaintext. The recipient of your message would need to know the key beforehand in order to decipher the text.

Vigenère suggested an even more complex scheme that used a priming letter followed by the message itself as the key. The priming letter designated the row the cryptographer first used to begin the message. Both the cryptographer and the recipient knew which priming letter to use beforehand. This method made cracking ciphers extremely difficult, but it was also time-consuming, and one error early in the message could garble everything that followed. While the system was secure, most people found it too complex to use effectively. Here is an example of Vigenère's system -- in this case the priming letter is "D":

Key

D

H

O

W

S

T

U

F

F

W

O

R

K

Plain

H

O

W

S

T

U

F

F

W

O

R

K

S

Cipher

K

V

K

O

L

N

Z

K

B

K

F

B

C


To decipher, the recipient would first look at the first letter of the encrypted message, a "K" in this case, and use the Trimethius table to find where the "K" fell in the "D" row -- remember, both the cryptographer and recipient know beforehand that the first letter of the key will always be "D," no matter what the rest of the message says. The letter at the top of that column is "H." The "H" becomes the next letter in the cipher's key, so the recipient would look at the "H" row next and find the next letter in the cipher -- a "V" in this case. That would give the recipient an "O." Following this method, the recipient can decipher the entire message, though it takes some time.

The more complex Vigenère system didn't catch on until the 1800s, but it's still used in modern cipher machines [source: Kahn].

In the next section, we'll learn about the ADFGX code created by Germany during World War I.