Chaocipher is a symmetric encryption method developed by John Francis Byrne in 1918. Byrne believed the algorithm to be unbreakable. Until his death, Byrne unsuccessfully tried to convince government agencies to use his method, as he had no intention of releasing details of the algorithm to the public. In 2010, his family donated the inventor’s left-behind papers to the National Cryptologic Museum at Ft. Meade, U.S.A., making the algorithm public for the first time.
In its original form, Byrne had planned his method as a mechanical device, but never built it. A version made of wood and cardboard was made by his son.
The device consists of two side-by-side discs, each with the 26 letters of the alphabet around its circumference in a variable order. The discs rotate and are connected in such a way that when one disc rotates in one direction (clockwise or counterclockwise), the other disc rotates in the opposite direction.
One disk is the alphabet for the plain text. The second disk is the alphabet for the ciphertext.
The principle of Chaocipher is that both alphabets are permuted after each encryption or decryption of a letter. Since this permutation depends on the previously encrypted or decrypted letter, this method is a polyalphabetic monographic substitution cipher with a self-synchronizing stream cipher.