Chaocipher
The Chaocipher  is a cipher method invented by John Francis 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.

In May 2010 Byrne's daughter-in-law, Patricia Byrne, 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.

Operation 

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.

See also 
 Autokey cipher

References

Further reading 
 John F. Byrne's Chaocipher Revealed: An Historical and Technical Appraisal by Moshe Rubin, Cryptologia, Volume 35, Issue 4, October 2011
 Decoding Chaocipher Exhibits 2 & 3 by Esa Peuha (PDF). Retrieved April 23, 2014.
 Chaocipher Exhibit 5: History, Analysis, and Solution of Cryptologia's 1990 Challenge by Jeff Calof, Jeff Hill & Moshe Rubin, Cryptologia, Volume 38, Issue 1, January 2014, pages 1–25

External links
 The Chaocipher Clearing House - Basic resources and analyses for working on the Chaocipher
 The Crypto Forum - Discussion and analysis of the Chaocipher
 Cracking Chaocipher - A visual description of how Chaocipher works, and an explanation of cracking Exhibit 1
  Interesting ciphers and computer methods for solving - A review of Chaocipher.
 Famous Unsolved Codes and Ciphers - Elonka Dunin's famous site dedicated to unsolved codes and ciphers, including Chaocipher. Retrieved April 23, 2014

Classical ciphers