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

150 CHAPTER 8. POLYALPHABETIC CIPHERS
everything, everything under the sun, including Traicté des Comètes, which
helped to destroy the superstition that comets come from an angry God trying
to warn a wicked world to stop sinning.
In 1586 he wrote the 600 page Traicté des Chiffres which, beyond discussions
of magic and the mysteries of the universe, was a comprehensive overview of
cryptography. Of immediate interest to us, he wanted an easily remembered
but really long and non-repeating key. When enciphering a message, what is
the nearest long possible key The message itself!
Auto Key Algorithm: Pick a priming key of one or more letters.
Encipher the plaintext as a Vigenère cipher in which the actual key is the
priming key followed by the message.
Examples:
(1) Encipher My vengeance is at hand, key D.
plaintext m y v e n g e n c e i s a t h a n d
key D M Y V E N G E N C E I S A T H A N
ciphertext P K T Z R T K T P G M A S T A H N Q
The ciphertext is PKTZR TKTPG MASTA HNQ.
(2) Encipher Beware the hand of fate with key = TO.
plaintext b e w a r e t h e h a n d o f f a t e
key T O B E W A R E T H E H A N D O F F A
ciphertext U S X E N E K L X O E U D B I T F Y E
The ciphertext is USXEN EKLXO EUDBI TFYE.
(3) Decipher XZCYY INTCQ AQX, key = M.
To decipher we must creep along, remembering that the next keyletter is
the previous plaintext letter.
ciphertext X Z C Y Y I N T C Q A Q X
key M L
plaintext l
ciphertext X Z C Y Y I N T C Q A Q X
key M L O
plaintext l o
ciphertext X Z C Y Y I N T C Q A Q X
key M L O O
plaintext l o o