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

Chapter 7
Vigenère Ciphers
It was the amateurs of cryptology who created
the species. The professionals, who almost certainly
surpassed them in cryptanalytic expertise,
concentrated on the down-to-earth problems of
the systems that were then in use but are now
outdated. The amateurs, unfettered to these realities,
soared into the empyrean of theory.
David Kahn
The Codebreakers
We’ve seen four types of monoalphabetic ciphers:
Caesar Ciphers: shift the letters of the alphabet by some fixed amount.
Decimation Ciphers: multiply by some fixed amount.
Linear Ciphers: multiply and shift.
Keyword Ciphers: use a keyword and columns to make a new alphabet.
In these ciphers each plaintext letter is replaced throughout the entire message
by the same ciphertext letter. Because of this one-to-one correspondence, frequency
analysis allows anyone to decide which letter is posing as which, and
hence to decrypt the message.
How, then, should we make a better cipher We must change this one-forone
replacement and find a way to cause multiple letters to represent the same
letter. We must (re)invent polyalphabetic ciphers.
Apparently, the first monoalphabetic cipher was thought up by one person
in one sudden intellectual burst. Not so for the family of polyalphabetic ciphers.
To fully develop this idea took four people: an architect, a cleric, a courtier and
a scientist, none of whom were cryptologists by profession.
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