Alberti cipher

The Alberti Cipher was one of the first polyalphabetic ciphers.

Around 1466 described by Italian Leon Battista Alberti: The Alberti Cipher Disk (called the formula) used polyalphabetic substitution with mixed alphabets and variable period. This device is made with two concentric disks, attached by a common pin, which can rotate one with respect to the other.  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.

outer ring is fixed and called the base. The outer ring has upper case letters for the CIPHERTEXT.Alse there are the numbers 1,2,3,4. These can be used for pointing the index in the plaintext letter (at the end of words or lines) or can point at a plaintext letter to generate a null. The 24 characters are 1234ABCDEFGILMNOPQRSTVXZ. So 20 letters and 4 numbers.
Left out are H.J.K.U.W.Y.

inner ring is called the rotor. Inner ring contains lower case letters for the PLAINTEXT. The disk has the alphabet (minus the letters j, u, w) and the sign & (=et). So the inner ring contains 23 letters and &:  abcdefghiklmnopqrstvxyz&.  Left out are j , u and w

It is probably that you use Latin or Italian for the disk.

 

links:
http://en.wikipedia.org/wiki/Alberti_cipher
http://www.questia.com/PM.qst?a=o&d=89563736
http://architectura.cesr.univ-tours.fr/Traite/Auteur/Alberti.asp?param=en