Chaocipher

Abstract: John F. Byrne invented Chaocipher in 1918 and tried unsuccessfully for almost 40 years to interest the U.S. government in his cipher system. He offered a reward to anyone who could break his cipher but the reward was never claimed. In 1989, John Byrne, son of John F. Byrne, demonstrated Chaocipher to two Cryptologia editors to determine if it had any commercial value. After making some improvements and providing additional information they jointly issue a new challenge to would-be solvers.

Abstract: In this article, the authors present a new hillclimbing algorithm for a ciphertext-only cryptanalysis of Chaocipher in-depth messages. This algorithm is based on a “divide-and-conquer” approach and the use of the Index of Coincidence. It takes advantage of a major weakness in the design of the cipher. This previously unknown weakness may have been the reason why William F. Friedman, the inventor of the Index of Coincidence, rejected Byrne’s offer for the use of Chaocipher by the U.S. government. Additionally, the authors present a known-plaintext attack for short in-depth messages, as well as the solution for Lou Kruh’s and Cipher Deavours’s alternate Exhibit 5, also known as “Exhibit 6.” Finally, the authors reevaluate the security of the Chaocipher in view of those findings, with the conclusion that in its classic form, as designed by Byrne, the Chaocipher was a relatively weak cipher, despite Byrne’s rather strong assertions to the contrary.