Alberti Cipher Disk
The Alberti cipher disk, sometimes called a formula disk is a cipher (known as the Alberti cipher) was described by Leon Batista Alberti in his treatise "De Cifris" (1467), it is a very early example of polyalphabetic substitution.
Encode / Decode Tool
Getting started:

  The outer circle is called the stabilis disk because it cannot be moved, the inner disk is the mobilis and it is used to transition form one cipher alphabet to the next.
  Check out the methods for examples and explanations.
  Keep in mind that the top letter (first letter in the text fields) for both disks is always taken as the first selected letter.

  To view the other methods check out "More Oprtions > Method".
  Use UPPERCASE letters when writing your cleartext, this is because the outer disk is in capital letters.
  Batista's disk is missing H, J, K, U, Y so those letter cannot be used as plaintext.


Rules
Before attempting to use an Alberti disk cipher read through these requirements.
 Two concentric disks with an equal number of different letters are assigned.
 The outer disk represents the cleartext letters available with at least one predefined null value.
 The inner disk corresponds to your ciphertext.
 Both disks must have an equal amount of letters.
 All letters must be different (upper case and lower case can be used).
 Encoder and decoder must have prearranged:
 A copy of the disks to be used.
 Several outer disk null letters.
 One inner disk starter letter.
 For the second method only; one outer disk index letter.
Tips
Even when used correctly the security provided by this cipher is not enough to keep information safe, please keep that in mind and follow the guidelines below:
 Never use more than 10 letters without changing the alphabet.
 Change alphabets at random intervals using a 6 (or at most 10) sided die.
 Use your null letters to superencode from a codebook.
 Leave out letters of your cleartext intentionally.
 Make your message as short as possible.
 Add a null in the ciphertext to make your cipher stronger.
 Save this page and use it offline.
 Two concentric disks with an equal number of different letters are assigned.

Method
At least two methods are known to encode and decode an Alberti disk cipher.
Here are examples of the first and second methods of enciphering in which will use the following presets:

 Stabilis disk:
"ABCDEFGILMNOPQRSTVXZ1234"  Mobilis disk:
"gklnprtuz&xysomqihfdbace"
 Stabilis disk:
First method:
 Encoding:
 Cleartext:
"IL DVCE E MORTO".  We chose "g" as our inner ring starting letter and the numbers 1, 2, 3 and 4 as nulls.
 Before we start we add nulls and cipher letters randomly to the cleartext so it becomes: "gIL1DVCEmE3MORTO"
 Position the starting letter under the letter "A" of the outer ring to start encoding.
 "A" becomes the first letter of our ciphertext to show our decoders where to start.
 "Auzbnhlp" is the first portion of ciphertext using the current alphabet.
 We now change the starting letter to the letter "m".
 So "R" (above the "m") is added to the ciphertext and it becomes "AuzbnhlpR".
 Finally "AuzbnhlpRfsekptk" is the full ciphertext.
 Cleartext:
 Decoding:
 Provided that:
 We have the Stabilis and Mobilis disk setup.
 We know that our starting letter is "g".
 And that 1, 2, 3 and 4 are nulls.
 We receive the ciphertext: "AuzbnhlpRfsekptk".
 Position the Mobilis disk so the starting letter is under the first letter of the Stabilis disk, "A".
 The second letter is "u" corresponding to the cleartext letter "I".
 Continue to decipher the letters accordingly until you find another letter from the Stabilis row, "R".
 Next set your starting letter to the "R" position and proceed to decipher.
 Finally the text "gIL1DVCEmE3MORTO" will emerge.
 By removing the Mobilis letters and nulls we get the cleartext: " IL DVCE E MORTO".
 Provided that:
 Encoding:
Second method:
 Encoding:
 Cleartext:
"PIV TRVPPE VENGONO".  We choose "g" as our starting letter and use "1, 2, 3 and 4" to indicate a change of alphabet.
 Additionally an index in the outer disk is chosen. For example "A".
 We rearrange our cleartext to "PIVTR2VP3EVE1NGO4NO" (TRVPPE is reduced to TRVPE for security) adding numbers randomly to the text.
 Upon encoding after adding the enciphered number you move the disk so the numbers value is under the chosen outer index "A".
 This will result in the ciphertext: "suhimam&deseqkalmgk".
 Cleartext:
 Decoding:
 Suppose we know that:
 We have the Stabilis and Mobilis disk setup.
 "g" is our starting letter
 "A" is our outter index.
 All numbers are nulls.
 And we receive the ciphertext: "suhimam&deseqkalmgk".
 Looking up the values in the disk, we find that "s" is "P" and so on...
 Until we reach "a" which is the number "2" indicating that "a" must be moved under the outer index "A".
 By repeating this process we get "PIVTR2VP3EVE1NGO4NO".
 Finally by removing the nulls we arive at "PIVTRVPEVENGONO".
 Suppose we know that:
 Encoding:
