Chaocipher

The Chaocipher[1] is a cipher method invented by John Francis Byrne in 1918 and described in his 1953 autobiographical Silent Years.

Although numerous students of classical cryptanalysis attempted to solve the challenge messages over the years, none succeeded.

For 90 years, the Chaocipher algorithm was a closely guarded secret known only to a handful of persons.

The underlying algorithm is related to the concept of dynamic substitution[5] whereby the two alphabets are slightly modified after each input plaintext letter is enciphered.

A detailed description of the Chaocipher algorithm is available[4] as well as discussions of the deciphered plaintexts[6] and the solution to Byrne's challenge.

To illustrate the process we will encipher the first plaintext letter of Byrne's Exhibit 1 using the same alphabets and disk setting he used, as shown in Figure 1.

See the next section for instructions on performing Chaocipher's unique permuting of the two disks in preparation for enciphering the next plaintext letter.

In the final step (3), insert the extracted letter ("P") back into the alphabet at the nadir position.

In step (2) extract the letter tab at position zenith+2 ('N') from the disk, temporarily leaving a 'hole' (Figure 5c).

For the final step ( 4) insert the previously extracted letter tab ("N") back into the disk at the nadir position.

Henry E. Langen,[8] editor of The American Cryptogram Association's newsletter The Cryptogram between 1952–1956, was quoted as saying "He did explain that the machine is made up somewhat like a typewriter with two revolving disks with the alphabets arranged along the periphery in a complete disorder [...] With only two disks used, I am a bit confused as to how this can result in such utter chaotification of the plaintext message.

"[9] Although John F. Byrne presented Chaocipher challenge messages in his autobiographical "Silent Years", he never described how the system worked.