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120 CHAPTER 7.
VIGENÈRE CIPHERS
Our eyes should be sufficiently trained by this point to detect the five Caesar
ciphers here. For example, the first two have the following fit:
First letters:
plain: 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
4 0 1 1 3 4 0 0 1 0 3 0 2 1 2 2 4 4 1 0 0 1 0 2 2 3
cipher: 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 Key is M.
Second letters:
plain: 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
3 0 2 2 1 3 3 2 1 0 1 1 1 2 1 0 1 3 0 2 1 3 1 4 2 1
cipher: 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 Key is A.
The other three letters of the keyword are quickly determined. (Do it!) 13
⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄
This example shows that if we can determine the length of the keyword of a
Vigenère cipher, the difficult work will be done, because breaking the Vigenère
cipher will be the same as breaking several Caesar ciphers. In other words, a
bit time-consuming, but pretty easy.
This is actually where Linquist’s method becomes useful. If one has a complete
Caesar cipher, running down the alphabet is a quick and simply-minded
of decrypting it. But when working on a Vigenère cipher we do not have consecutive
groups of letters, and so running down the alphabet looking for words
will not work. Further, there will be cases in which each group of similarlyencrypted
letters is quite small, making traditional frequency analysis hard.
Linquist’s method, however, is designed with small groups of letters in mind,
and will sometimes be helpful.
7.7 The Kasiski Test
Friedrick W. Kasiski (1805–81) was born in what is now Poland, entered East
Prussia’s Infantry in 1823 and retired 1852. In 1863 he published Die Geheimschriften
und die Dechiffrir-kunst, a book of only 95 pages. After his book
made no apparent impact, he quit cryptography and became an anthropologist
of some local fame. His book contained a method for determining the number of
letters in a Vigenère keyword, and by the beginning of the 20th century experts
considered polyalphabetic ciphers to be vulnerable.
13 The key is MARCEL and the plaintext is “Encode well or do not encode at all. In transmitting
cleartext, you give only a piece of information to the enemy, and you know what it is; in
encoding badly, you permit him to read all your correspondence and that of your friends.”
General Marcel Givierge, head of French cryptology during WWI, author of Course de
Cryptographie, 1925. [Kahn, pg 349]