Leon Battista Alberti Describes "The Alberti Cipher"

Italian author, artist, architect, poet, priest, linguist, philosopher, cryptographer and general Renaissance humanist polymath Leon Battista Alberti wrote De Cifris describing the first polyalphabetic substitution with mixed alphabets and variable period. Compared to previous ciphers of the period, the Alberti Cipher was impossible to break without knowledge of the method. This was because the frequency distribution of the letters was masked and frequency analysis - the only known technique for attacking ciphers at that time - was no help. To facilitate the encryption process employed the first mechanical device, known as the Alberti cipher disk, also called formula. 

The cipher disk "is made up of two concentric disks, attached by a common pin, which can rotate one with respect to the other.

"The larger one is called Stabilis [stationary or fixed], the smaller one is called Mobilis [movable]. The circumference of each disk is divided into 24 equal cells. The outer ring contains one uppercase alphabet for plaintext and the inner ring has a lowercase mixed alphabet for ciphertext.

"The outer ring also includes the numbers 1 to 4 for the superencipherment of a codebook containing 336 phrases with assigned numerical values. This is a very effective method of concealing the code-numbers, since their equivalents cannot be distinguished from the other garbled letters.

" The sliding of the alphabets is controlled by key letters included in the body of the cryptogram" (Wikipedia article on Alberti cipher disk, accessed 03-30-2012).