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Vigenere

 

Full Name: Blaise de Vigenere

Achievements: Developed the famous “uncrackable” code

Born: April 5, 1523, Saint-Pourcain, Auvergne, France

Died: February 19, 1596, Paris, France.

Aged 72

Field & Discipline: Cryptography

Language: French

Father: Jean de Vigenere

Mother: Marguerite du Lyon

Siblings: Names unknown

Spouse: Marie Varre

Children: Marie

Education: Private tutors; studies in Paris

Major Work:

"Traite des chiffres, ou Secrete maniere d'ecrire” (‘On Ciphers, or Secret Writing’)

Life & Times:

At 17, Vigenere was presented to the French court, and placed under the care of General Bayard, first Secretary of State to Francis I.  At 22, he was sent on a brief diplomatic mission to the Imperial Diet (Council) of Worms, Germany, after which he toured Europe for two years.  On his return to Paris he became secretary to the Duc de Nevers, with whom he remained for 15 years until the Duke died.  In 1566, the new king, Charles IX, sent Vigenere as a diplomat to Rome, where he stayed three years.  Through his dealings with the papal court, he became fascinated by the art of cryptography.   Nearly 100 years earlier, [influenced by] Alberti* had published "De componendi cifris" (On Encryption), which described the first known frequency table and the first cipher disk.   Alberti recommended that the setting of the disk be changed after enciphering three or four words, so inventing the concept of polyalphabeticity.  Vigenere came across a more recent book on the subject written in 1563 by Italian natural philosopher [influenced by] Porta*.

Porta's method of writing a cipher was "substitution", in which a given letter might be coded with any one of 11 different letters, and in which a single letter of the cryptogram might represent any one of 11 different letters in the original text.  Porta's code was theoretically perfect and unbreakable, but like so many others, was operationally impractical, since his system demanded that key-tables always be carried by the sender and the receiver, and errors in coding were easily made.

Back in Paris in 1569, Vigenere began work on a code book of his own  entitled "Traite des chiffres, ou Secrete maniere d'ecrire" (On Ciphers, or Secret Writing).  Finally published in 1586, it contained the "Vigenere tableau” (the Vigenere Table),  a method of double substitution, the first great cryptographic invention since Julius Caesar's time, and a system that ultimately drove almost all others from the field.  Vigenere's coding/decoding system (basically only an improvement on Della Porta's) depended on two essentials: 1) his Table (see below); and 2) an arbitrarily-chosen Key Word.

The Vigenere Table:

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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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

1) Vigenere's table consists of 26 horizontal alphabets. The top line is the normal alphabet, from A to Z.  The second line is still in proper alphabetical order, but begins with B, ending with A.  The third line begins with C, ending with AB and so on.  With this format a full, vertical alphabet, from A to Z, is created down the left side.

2) The second essential part of Vigenere's code system is the Key Word, a word chosen at random, but preferably one that can be easily memorized.  The Key Word is written above the message to be coded, letter for letter, repeating the Key Word as many times as needed.

For example:

To encode:

Message to be coded (or "plain text"): MEET ME AT MIDNIGHT 

Key Word: LURID

"LURID" is repeatedly written above the message,  as follows:

Key Word:  LURI   DL  UR  IDLURIDL

Message:    MEET ME  AT  MIDNIGHT

Follow along the horizontal alphabet at the top of the table until you reach the first letter of the message (M); follow down the left-hand vertical column until you reach the line which begins with the first letter of the Key Word (L).  Follow along that horizontal line.  The first letter of the code will be where the vertical line M and the horizontal line L intersect (at X).  Follow the same procedure with the second letter of the message (E) with the second letter of the Key Word (U).  These meet at "Y".  Go through the process again with the third letter, another E, and the third letter of the Key Word (R).   These meet at "V".  And so on. (Note that with this system, the two Es in MEET are represented by different letters.)

Coded message:

MEET ME  AT  MIDNIGHT

XYVB  PP  UK  ULOHZOKE

To decode:

Find the first letter of the same Key Word (L) on the vertical alphabet on the left side.  Follow horizontally along this line until you reach the first letter of the coded message (X).  Follow up from X along the vertical line until you reach the top line. The letter there (M) will be the first letter of the message.  And so on.

For many years Vigenere's system was called "the indecipherable cipher".   Using different Key Words, the same message may be coded in countless ways, making the code nearly impossible to break.

Extra Connections: The code was used by [code used by] John Wilkes Booth*, assassin of American President Abraham Lincoln.

Assessment: "The Vigenere Cipher" became the most effective code in Europe and remained unbreakable for centuries

Oneliner: Blaise de Vigenere, great cryptographer

Bibliography:

Wikipedia