Alberti Cipher

Encryption uses a disk with two alphabets, one fixed (stabilis) one moving (mobilis). By rotating a disk, it shifts an alphabet to the next letter.

To encrypt, the disk is set in one position, the initial shift (which can be zero) corresponds to the number of letters shifted at the beginning.

Example: The disk is composed of ABCDEFGHIJKLMNOPQRSTUVWXYZ for the large outer ring, and abcdefghijklmnopqrstuvwxyz for the small inner ring. A is in line with a, B is inline with b, etc. Rotating a disk by 2, then A is in line with c, and the initial shift becomes 2.

Each letter of the plain text found on the outer ring, is replaced by the corresponding one (the one aligned) in the inner ring.

By default, every 4 characters (4 = period), the disk is rotated clockwise of 1 letter (1 = periodic increment), this changes the substituting alphabet.

Example: Encrypt DCODE with the parameters: initial shift: 1, periodic increment: 2, period: 3.
Alphabets are such as ABCDEFGHIJKLMNOPQRSTUVWXYZ is aligned with bcdefghijklmnopqrstuvwxyza.
The period begins, D is coded by e, C by d, O by p, the period (length 3) ends, the disk is rotated by 2 letters.
Alphabets are now aligned like this : ABCDEFGHIJKLMNOPQRSTUVWXYZ with defghijklmnopqrstuvwxyzabc, the new period begins, etc.
The encrypted message is edpgh

Le decryption needs the disk (or the 2 alphabets) and the parameters: initial position, period and shift.

To cipher a message, the disk is set with the corresponding initial shift. Each letter is identified on the inner disk, and is coded by the letter aligned in the outer disk.

By default, every 4 characters (4 = period), the disk is rotated counter-clockwise of 1 letter (1 = periodic increment).

Example: The parameters are: initial shift: 1, periodic increment: 2, period: 3, and the cipher message edpgh.
Alphabets are such as ABCDEFGHIJKLMNOPQRSTUVWXYZ is aligned with bcdefghijklmnopqrstuvwxyza
The period begins, e is decoded by D, d by C, p by O, the period (length 3) ends, the disk is rotated by 2 letters.
Alphabets are now aligned like this: ABCDEFGHIJKLMNOPQRSTUVWXYZ with defghijklmnopqrstuvwxyzabc, the new period begins, etc.
The original plain text is DCODE

The ciphered message has a polyalphabetic index of coincidence.

In its original version, the message has only these letters: ABCDEFGHIKLMNOPQRSTVXYZ and &, no J, U or W

The mention of De Cifris refers to the treaty published by Alberti presenting his cipher.

One can crack Alberti but brute-forcing the period first, then initial shift and periodic increment. Use the 'Bruteforce attack' button.

Leon Battista Alberti would have invented this wheel around 1460.