Alberti cipher


The Alberti Cipher created in In 1467 by an Italian architect Leon Battista Alberti was one of the first polyalphabetic ciphers. In the opening pages of his treatise he explained how his conversation with the papal secretary Leonardo Dati about a recently developed movable type printing press led to the development of his cipher wheel.

Description

The Alberti Cipher Disk described by Leon Battista Alberti in his treatise :it:De cifris|De Cifris embodies the first example of polyalphabetic substitution with mixed alphabets and variable period. This device, called Formula, was made up of two concentric disks, attached by a common pin, which could rotate one with respect to the other. The larger one is called Stabilis , the smaller one is called Mobilis . 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.
For an unequivocal study of this cipher two chapters of De Cifris are herewith reproduced in English.

Cryptanalysis

’s invention revolutionized encryption. Compared to previous ciphers of the time the Alberti Cipher was impossible to break without knowledge of the method. This was because the frequency distribution of the letters was masked and frequency analysis - the only known technique for attacking ciphers at that time - was no help. It was also more convenient than the Vigenère cipher.

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