Cryptology - Unofficial St. Mary's College of California Web Site

72 CHAPTER 5. MONOALPHABETIC CIPHERS
We then encipher and decipher by translating from the plaintext to ciphertext
alphabets and back, as usual. In this example alphabet becomes SBUNSORY.
However it is not particularly easy to remember apparently random orderings
of 26 letters. So we will concentrate a couple of well-known methods that use a
key to develop the ciphertext alphabet’s order.
5.1 Keyword Ciphers
To use Keyword Cipher method to construct the ciphertext alphabet, pick a
keyword and write it down, ignoring repeated letters. Follow it with the letters
of the alphabet that have not yet been used.
Example: Find the alphabet pairs for the keyword COLLEGE.
Crossing out the letters that are making their second appearance leaves
COLEG. To encipher then we use the pair of alphabets
plaintext
ciphertext
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
C O L E G A B D F H I J K M N P Q R S T U V W X Y Z
Enciphering university then gives UMFVGRSFTY.
⋄
Clearly there is a problem with this cipher: it does a poor job of mixing the
ciphertext alphabet. Around 1580 Giovanni Battista Argenti suggested that one
also pick a keyletter and begin the keyword under that letter of the plaintext.
The Argentis, Giovanni and his nephew Matteo, form one of the great cryptology
families of the middle ages. After many years of trying, in 1590 Giovanni finally
became papal secretary of ciphers in Rome, only to quickly weaken from the
frequent necessary trips to Germany and France. Before dying on April 24,
1591, he passed his knowledge to his nephew Matteo who succeeded him and
held the office during the reign of the next five popes.
To use Giovanni’s method with keyletter p we would start COLEG under
pqrst, giving
plaintext
ciphertext
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
J K M N P Q R S T U V W X Y Z C O L E G A B D F H I
Then university is enciphered as AYTAPLETGH. This method mixes the ciphertext
alphabet better.