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8.5.
VIGENÈRE’S AUTO KEY CIPHER 149
8.5 Vigenère’s Auto Key Cipher
In a conversation on that subject [unbreakable
ciphers] which I had with the late Mr. Davies
Gilbert, President of the Royal Society, [he and I]
each maintained that he possessed a cipher which
was absolutely inscrutable. On comparison, it
appeared that we had both imagined the same
law.
Charles Babbage, discussing Auto-key ciphers
Passages from the Life of a Philosopher
So we hope to produce and use really long keywords. But producing good
ones is hard, and exchanging them even harder. What can we do
Cardano had among the first ideas along this line. In modern terminology
his idea is to encipher each word with a progressive key starting with the first
letter of the previous word. Of course, there must be some way of enciphering
the first word, and Belaso suggested picking a key letter.
Example: Encipher Lets attack them Friday using the keyletter J.
So Lets is enciphered with JKLM (recall that a progressive key moves one
letter forward each step), attack with LMNOPQ, and so on.
plaintext l e t s a t t a c k t h e m f r i d a y
key J K L M L M N O P Q A B C D T U V W X Y
ciphertext U O E E L F G O R A T I G P Y L D Z X W
So the ciphertext is UOEEL FGORA TIGPY LDZXW.
⋄
What is the security of this method It seems better than the Vigenère
method since the keyword is not repeated. One possible problem is that since
t starts so many words, many words much of the plaintext will be enciphered
with key TUVWXY.... If word length is preserved in the ciphertext then we may
simply decipher each word using the key TUVWXY... until we get lucky and find
one for which this works. Since almost 16% of all words begin with T, we’ll
likely only have to try 3 or 4 until we get lucky. And once we have one word
we are done for it will tell us how to decipher the next word. So this is a good
idea, but we need a way to make it less predictable.
At this point Blaise de Vigenère (1523–1596) finally enters the picture.
As a youth Vigenère received an excellent education, attending the Diet of
Worms as a very young secretary and traveling throughout Europe in diplomatic
missions. In 1549 he was sent to Rome, where he apparently read Trithemius,
Belaso, Cardano and Porta, among others, and studied with the experts of the
papal curia. After a career in diplomacy (for the Duke of Nevers and King
Charles IX) in 1570 he married a much younger wife, quit court life, gave his
annuity to the poor, and devoted himself to writing. And he wrote about