Name: Vigenère
Type: Substitution
Ease of use: Easy / Intermediate / Difficult
Breaking the code: Easy / Intermediate / Difficult
The Vigenère cipher is a polyalphabetic substitution cipher. It is invented by Giovan Batista Belaso, but later brought back to life by Blaise de Vigenère, hence the name. Blaise de Vigenère is thus misattributed for this cipher, but he later helped create a more secure variant, using the Autokey method. The cipher uses a series of Caesar's Shifts to encode or decode a message. The cipher is very popular as a field cipher, because to many beginners the cipher may seem to be unbreakable, yet it is rather easy to implement.
The Vigenère Cipher uses a keyword and a Tabula Recta, or cipher tableau, to encode and decode a message. Below is the Vigenère cipher tableau:
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
A A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
B B C D E F G H I J K L M N O P Q R S T U V W X Y Z A
C C D E F G H I J K L M N O P Q R S T U V W X Y Z A B
D D E F G H I J K L M N O P Q R S T U V W X Y Z A B C
E E F G H I J K L M N O P Q R S T U V W X Y Z A B C D
F F G H I J K L M N O P Q R S T U V W X Y Z A B C D E
G G H I J K L M N O P Q R S T U V W X Y Z A B C D E F
H H I J K L M N O P Q R S T U V W X Y Z A B C D E F G
I I J K L M N O P Q R S T U V W X Y Z A B C D E F G H
J J K L M N O P Q R S T U V W X Y Z A B C D E F G H I
K K L M N O P Q R S T U V W X Y Z A B C D E F G H I J
L L M N O P Q R S T U V W X Y Z A B C D E F G H I J K
M M N O P Q R S T U V W X Y Z A B C D E F G H I J K L
N N O P Q R S T U V W X Y Z A B C D E F G H I J K L M
O O P Q R S T U V W X Y Z A B C D E F G H I J K L M N
P P Q R S T U V W X Y Z A B C D E F G H I J K L M N O
Q Q R S T U V W X Y Z A B C D E F G H I J K L M N O P
R R S T U V W X Y Z A B C D E F G H I J K L M N O P Q
S S T U V W X Y Z A B C D E F G H I J K L M N O P Q R
T T U V W X Y Z A B C D E F G H I J K L M N O P Q R S
U U V W X Y Z A B C D E F G H I J K L M N O P Q R S T
V V W X Y Z A B C D E F G H I J K L M N O P Q R S T U
W W X Y Z A B C D E F G H I J K L M N O P Q R S T U V
X X Y Z A B C D E F G H I J K L M N O P Q R S T U V W
Y Y Z A B C D E F G H I J K L M N O P Q R S T U V W X
Z Z A B C D E F G H I J K L M N O P Q R S T U V W X Y
The table has the plaintext alphabet on the top row and 26 alphabets below it, shifting one position to the left on each row. The left most column is the key column.
The encoding is done by first looking up the keyword letter in the left key column and the plaintext letter in the top plaintext row. The letter where these two intersect is the ciphertext letter.
To decode, you look up the letter from the keyword again in the left column, then move to the right until you get to the ciphertext letter. The letter above it in the top row is the original plaintext letter.
The Vigenère cipher can be cryptanalyzed looking for repeating patterns indicating the keyword length. Taking these intervals normal frequency analysis can be performed.
To show you the encoding process, I'll encode the message 'Attack their flank now' using the keyword 'BREAD' for you
Plaintext: ATTACK THEIR FLANK NOW
Keyword: BREADB READB READB REA
First look up the first letter of the keyword, the 'B', in the left column and the first letter of the plaintext, the 'A', in the top row. They intersect at the 'B', so that will be our first ciphertext letter. Do the same for the second letter of the keyword and plaintext, the 'R' and 'T' respectively. You should find the 'K' now. Continue until you processed all plaintext letters, repeating the keyword when necessary. The complete ciphertext will be:
Plaintext: ATTACK THEIR FLANK NOW
Keyword: BREAD
Ciphertext: BKXAFL KLELS WPAQL ESW
Formatted: BKXAF LKLEL SWPAQ LESW
Continue to the tool for this cipher or go back to the list of ciphers.