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.
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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.
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."
- What is Chaocipher?, The Chaocipher Clearing House, retrieved August 8, 2010
- Byrne , J. F. 1953 . Silent Years: An Autobiography with Memoirs of James Joyce and Our Ireland . New York : Farrar, Straus, and Young (Reprinted in 1975 by Octagon Books, a division of Farrar, Straus, and Giroux).
- Chaocipher Machine and Papers Archived 2010-07-22 at the Wayback Machine National Cryptologic Museum, retrieved July 2, 2010
- Rubin, Moshe (July 2, 2010). "Chaocipher Revealed: The Algorithm" (PDF). Retrieved July 3, 2010.
- Substitution Cipher with Pseudo-Random Shuffling: The Dynamic Substitution Combiner. Ritter, T. 1990. Cryptologia. 14(4): 289-303. Retrieved July 2, 2010
- Rubin, Moshe (August 8, 2010). "Chaocipher Revealed: Deciphering Exhibit #1" (PDF). Retrieved August 9, 2010.
- Cowan, Mike (December 2010). "Chaocipher: Solving Exhibits 1 and 4" (PDF). Archived from the original (PDF) on 2016-11-01.
- Henry E. Langen biographical information, The Chaocipher Clearing House, retrieved July 2, 2010
- ?, ?. "The Tragic Story of J.F. Byrne." PurpleHunt.com. 1998. Aug 17, 2007 Archived August 7, 2007, at the Wayback Machine.
- 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
- 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