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Alberti Cipher

Tool to decrypt/encrypt Alberti. The Alberti cipher is a polyalphabetic system using two disks which can rotate.

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Alberti Cipher -

Tag(s) : Cryptography, Poly-Alphabetic Cipher

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# Alberti Cipher

## Alberti Encoder

Tool to decrypt/encrypt Alberti. The Alberti cipher is a polyalphabetic system using two disks which can rotate.

### How to encrypt using Alberti cipher?

Encryption uses a disk with two alphabets. 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.

Example: Alphabets are such as ABCDEFGHIJKLMNOPQRSTUVWXYZ is aligned with bcdefghijklmnopqrstuvwxyza

Example: 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.

Example: Alphabets are now aligned like this : ABCDEFGHIJKLMNOPQRSTUVWXYZ with defghijklmnopqrstuvwxyzabc, the new period begins, etc.

Example: The crypted message is edpgh

### How to decrypt Alberti cipher

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.

Example: Alphabets are such as ABCDEFGHIJKLMNOPQRSTUVWXYZ is aligned with bcdefghijklmnopqrstuvwxyza

Example: 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.

Example: Alphabets are now aligned like this: ABCDEFGHIJKLMNOPQRSTUVWXYZ with defghijklmnopqrstuvwxyzabc, the new period begins, etc.

Example: The original plain text is DCODE

### How to recognize an Alberti ciphertext?

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

### How to decipher Alberti without period or shift?

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

Around 1460.