The Chaocipher is a cipher

Cipher

In cryptography, a cipher is an algorithm for performing encryption or decryption — a series of well-defined steps that can be followed as a procedure. An alternative, less common term is encipherment. In non-technical usage, a “cipher” is the same thing as a “code”; however, the concepts...

 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

Cigar box

Cigar boxes are popular juggling props. Their rectangular shape resembles that of its namesake; they are sometimes padded on the ends and/or the sides with a felt-like material....

. He offered cash rewards for anyone who could solve it.

In May 2010 the Byrne family donated all Chaocipher-related papers and artifacts to the National Cryptologic Museum

National Cryptologic Museum

The National Cryptologic Museum is an American museum of cryptologic history that is affiliated with the National Security Agency . The first public museum in the U.S. Intelligence Community, NCM is located in the former Colony Seven Motel, just two blocks from the NSA headquarters at Fort...

 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

Cryptologia

Cryptologia is a journal in cryptography published quarterly since January 1977. Its remit is all aspects of cryptography, but there is a special emphasis on historical aspects of the subject. The founding editors were Brian J. Winkel, David Kahn, Louis Kruh, Cipher A. Deavours and Greg Mellen...

to whom John confided the underlying method in 1990.

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
• John F. Byrne's Chaocipher Revealed: An Historical and Technical Appraisal by Moshe Rubin, Cryptologia, Volume 35, Issue 4, October 2011