The Chaocipher is a cipher method invented by J. F. Byrne in 1918 and described in his 1953 autobiographical Silent Years. He believed Chaocipher was simple, yet unbreakable. Byrne stated that the machine he used to encipher his messages could be fitted into a cigar box. He offered cash rewards for anyone who could solve it.
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In May 2010 the Byrne family donated all Chaocipher-related papers and artifacts to the National Cryptologic Museum in Ft. Meade, Maryland, USA. This led to the disclosure of the Chaocipher algorithm.
How Chaocipher works
The Chaocipher system consists of two alphabets, with the "right" alphabet used for locating the plaintext letter while the other ("left") alphabet is used for reading the corresponding ciphertext letter. The underlying algorithm is related to the concept of dynamic substitution whereby the two alphabets are slightly modified after each input plaintext letter is enciphered. This leads to nonlinear and highly diffused alphabets as encryption progresses.
Deciphering is identical to enciphering, with the ciphertext letter being located in the "left" alphabet while the corresponding plaintext letter being read from the "right" alphabet.
A detailed description of the Chaocipher algorithm is available as well as discussions of the deciphered plaintexts and the solution to Byrne's challenge.
Points of interest
Henry E. Langen, editor of The Cryptogram during that time, 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."
Until 2010 at least three people knew how it works: Byrne's son John, and two of the editors of Cryptologia to whom John confided the underlying method in 1990.