TheCodeBreakers
The encipherer
repeats this process with all the letters of the plaintext. To decipher, the
clerk begins with the keyletter, runs in along the ciphertext alphabet
until he strikes the cipher letter, then follows the column of letters
upward until he emerges at the plaintext letter at the top. For example:
key TYPETYPETYPET
plain nowisthetimef
cipher OMLMLRWIMGBIY
key YPETYPETYPET
plain orallgoodmen
cipher MGEEJVSHBBIG
Polyalphabetic ciphers were, when used with mixed alphabets and
without word divisions, unbreakable to the cryptanalysts of the
Renaissance. Why, then, did the nomenclator reign supreme for 300
years? Why did cryptographers not use the polyalphabetic system
instead?
Apparently because they disliked its slowness and distrusted its
accuracy. Encipherment in a polyalphabetic system, with its need to
annaabi.ee 
