Alberti Cipher Disk

The Alberti cipher disk, also called formula, is a cipher disc which was described by Leon Battista Alberti in his treatise De Cifris of 1467. The device embodies the first example of polyalphabetic substitution with mixed alphabets and variable period, and 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. A translation of Alberti's description is given under Alberti cipher.

A lowercase letter on the smaller ring is used as an index.

In this example the letter g in the inner ring is chosen as an index and is moved under an uppercase letter (in this case A) of the stationary ring. The alphabets in use are (see figure):

ABCDEFGILMNOPQRSTVXZ1234 Stationary disk

gklnprtuz&xysomqihfdbace Movable disk

Dispatch: “La guerra si farà ...”

_LAGVER2RA_ Plaintext

AzgthpmamgQ Ciphertext

The key letters A and Q are included in the cryptogram. The small letter a resulting from the encipherment of the number 2 is a null and must be discarded in the decipherment.

After enciphering a few letters a different uppercase letter (Q) is inserted in the cryptogram and the movable disk is accordingly rotated obtaining a new combination:

QRSTVXZ1234ABCDEFGILMNOP Stationary disk

gklnprtuz&xysomqihfdbace Movable disk

The encipherment will resume thus:

_SIFARÀ Plaintext

Qlfiyky Ciphertext.

The same procedure will be continued with different key letters through the end of the message.

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