Alberti cipher

The Alberti cipher is a method of secret writing devised in 1467 by the Italian architect and scholar Leon Battista Alberti, and it is counted among the first polyalphabetic ciphers, meaning that one plaintext letter can be replaced by different ciphertext letters at different points in a message. Alberti set out the method in his treatise De componendis cifris after a conversation with the papal secretary Leonardo Dati about the newly developed printing press. The device is a pair of concentric disks, a fixed outer one called the Stabilis and a rotating inner one called the Mobilis, each divided into 24 cells. The outer ring carries a plaintext alphabet and the numbers 1 to 4, which index a codebook of 336 phrases, and the inner ring carries a mixed ciphertext alphabet. The sender changes the alignment of the disks every few words and signals the change with a capital letter in the message, so the same letter is rarely enciphered the same way twice. Because of this, simple frequency analysis, then the only known attack, could not break it without knowledge of the method.