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114 CHAPTER 7.
VIGENÈRE CIPHERS
Giovanni Battista Porta (1535–1615) was probably the outstanding cryptographer
of the Renaissance. In addition, he organized the first association of
scientists, the “Academia Secretorum Naturae”. He was also a prolific writer:
between 1586–1609 published books on human physiognomy, meteorology, the
design of villas, astronomy, as well as 14 prose comedies.
In 1563 8 he published De Furtivis Libetarum Notis. Its four books dealt
with, respectively, ancient ciphers, modern ciphers, cryptanalysis, and linguistic
peculiarities, and encompassed all the cryptologic knowledge of the time.
According to Kahn, it is one of the few books of the period that is still readable.
(In it it appears that Porta nearly learned out how to break the Vigenère
ciphers. If he had, they probably would not have become popular, in which case
this chapter wouldn’t exist. But he didn’t quite, and so here we are.)
Porta’s contribution was two-fold. First, his book was very influential and
made the polyalphabetic ciphers popular. Second, he combined the letter-byletter
encipherments of Trithemius, the easily changed key of Belaso, and the
mixed alphabet suggested by Alberti. Rather than simply using the 26 regular
alphabets (from Trithemius’ table) as the ciphertext alphabets, his cipher
allowed for mixed alphabets.
7.4 Vigenère Ciphers
The Vigenère cipher is one of the most influential ciphers in history. It is simple
to use and easy to remember, and, because it is polyalphabetic, is much more
secure that the ciphers we have previously studied.
Examples:
(1) Encipher Meet me at the Met at the time ten twenty using the keyword
CODE.
plaintext
key
ciphertext
m e e t m e a t t h e m e t a t t h e t i m e t e n t w e n t
C O D E C O D E C O D E C O D E C O D E C O D E C O D E C O D
O S H X O S D X V V H Q G H D X V V H X K A H X G B W A G B W
Yes, this is a silly message. But notice how many times the e’s and t’s
and m’s go to different letters. There are 9 e’s and 10 t’s in the plaintext,
but in the ciphertext no letter appears more than 5 times.
(2) As his example of Vigenére, Parker Hitt used the key GRANT to encipher
All radio messages must hereafter be put in cipher. What
is the ciphertext
(3) Charles Dodgson (Lewis Carroll) reinvented the Vigenère cipher in 1868,
calling it the Alphabet Cipher. In his diary he used the keyword VIGILANCE
8 Why did the 1500’s have this sudden explosion of interest in cryptology ”The growth of
cryptology [in the west in the 1500’s] resulted directly from the flowering of modern diplomacy”
[Kahn, pg 108] since permanent ambassadors needed to send home regular reports.