Code 8 : Beaufort Cipher

To encrypt a plaintext message using the Vigenère Cipher, one locates the row with the first letter to be encrypted, and the column with the first letter of the keyword. The ciphertext letter is located at the intersection of the row and column. This continues for the entire length of the message.
A Beaufort cipher uses the same alphabet table as the Vigenère cipher, but with a different algorithm. To encode a letter you find the letter in the top row. Then trace down until you find the keyletter. Then trace over to the left most column to find the enciphered letter. To decipher a letter, you find the letter in the left column, trace over to the keyletter and then trace up to find the deciphered letter.
Some people find this easier to do than finding the intersection of a row and column.

What is the Beaufort Cipher?

Choose a keyword. Write the plaintext in horizontal rows under the keyword, with each row being the same length as the keyword. Use a tableau, using the row that corresponds to the keyword letter, and the column that corresponds to the plaintext letter.

Example

Plaintext: Whoever has made a voyage up the Hudson must remember the Kaatskill mountains.
Keyword: RIPVANWINKLE
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 
Z Z Y X W V U T S R Q P O N M L K J I H G F E D C B A
Y Y X W V U T S R Q P O N M L K J I H G F E D C B A Z
X X W V U T S R Q P O N M L K J I H G F E D C B A Z Y
W W V U T S R Q P O N M L K J I H G F E D C B A Z Y X
V V U T S R Q P O N M L K J I H G F E D C B A Z Y X W
U U T S R Q P O N M L K J I H G F E D C B A Z Y X W V
T T S R Q P O N M L K J I H G F E D C B A Z Y X W V U
S S R Q P O N M L K J I H G F E D C B A Z Y X W V U T
R R Q P O N M L K J I H G F E D C B A Z Y X W V U T S
Q Q P O N M L K J I H G F E D C B A Z Y X W V U T S R
P P O N M L K J I H G F E D C B A Z Y X W V U T S R Q
O O N M L K J I H G F E D C B A Z Y X W V U T S R Q P
N N M L K J I H G F E D C B A Z Y X W V U T S R Q P O
M M L K J I H G F E D C B A Z Y X W V U T S R Q P O N
L L K J I H G F E D C B A Z Y X W V U T S R Q P O N M
K K J I H G F E D C B A Z Y X W V U T S R Q P O N M L
J J I H G F E D C B A Z Y X W V U T S R Q P O N M L K
I I H G F E D C B A Z Y X W V U T S R Q P O N M L K J
H H G F E D C B A Z Y X W V U T S R Q P O N M L K J I
G G F E D C B A Z Y X W V U T S R Q P O N M L K J I H
F F E D C B A Z Y X W V U T S R Q P O N M L K J I H G
E E D C B A Z Y X W V U T S R Q P O N M L K J I H G F
D D C B A Z Y X W V U T S R Q P O N M L K J I H G F E
C C B A Z Y X W V U T S R Q P O N M L K J I H G F E D
B B A Z Y X W V U T S R Q P O N M L K J I H G F E D C
A A Z Y X W V U T S R Q P O N M L K J I H G F E D C B

Encipher step

R I P V A N W I N K L E
W H O E V E R H A S M A
D E A V O Y A G E U P T
H E H U D S O N M U S T
R E M E M B E R T H E K
A A T S K I L L M O U N
T A I N S

Ciphertext:
VBBRF JFBNS ZEOEP AMPWC JQWLK EIBXV IVBQT LAEDR OMSRU DHURI WDQFL XBWRR YIHII

Vulnerabilities of Beaufort Cipher

Doing an analysis of Periodic ICs can help reveal potential keyword lengths, as can the Kasiski Examination.

How to Crack a Beaufort Cipher

The Kasiski Examination can be performed: look for repeated sequences in the ciphertext and factor the distance between them. This gives potential keyword lengths. Placing the ciphertext into columns with the width of those lengths allows one to perform a frequency analysis on each column to help determine which alphabet was used for that column.

Introduction 

The Beaufort cipher, created by Sir Francis Beaufort, is a polyalphabetic substitution cipher that is similar to the Vigenère cipher, except that it enciphers characters in a slightly different manner.

The Algorithm 

The 'key' for a beaufort cipher is a key word. e.g. 'FORTIFICATION'

The beaufort cipher uses the following tableau (the 'tabula recta') to encipher the plaintext:

 
    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

To encipher a message, repeat the keyword above the plaintext:

FORTIFICATIONFORTIFICATIONFO
DEFENDTHEEASTWALLOFTHECASTLE

(The following assumes we are enciphering the plaintext letter D with the key letter F) Now we take the letter we will be encoding, and find the column on the tableau, in this case the 'D' column. Then, we move down the 'D' column of the tableau until we come to the key letter, in this case 'F' (The 'F' is the keyword letter for the first 'D'). Our ciphertext character is then read from the far left of the row our key character was in, i.e. with 'D' plaintext and 'F' key, our ciphertext character is 'C'.

So, the ciphertext for the above plaintext is:

FORTIFICATIONFORTIFICATIONFO
DEFENDTHEEASTWALLOFTHECASTLE
CKMPVCPVWPIWUJOGIUAPVWRIWUUK

Deciphering is performed in an identical fashion, i.e. encryption and decryption using the beaufort cipher uses exactly the same algorithm.

This process can be compared to the Vigenère cipher, which uses a different algorithm, but the same tableau, for finding the ciphertext characters.

Source http://crypto.interactive-maths.com, http://www.braingle.com , http://www.secretcodebreaker.com, http://substitution.webmasters.sk/simple-substitution-cipher.php