Chaocipher
John Byrne's 1918 disk cipher — disclosed after 92 years, then cryptanalyzed
Published key-recovery attacks by Lasry, Rubin, Kopal and Wacker (2016).
Why This Matters
John Byrne — Irish-American writer and friend of James Joyce — invented the Chaocipher in 1918 and spent four decades trying to convince the US government and AT&T to adopt it. Nobody would license it without seeing how it worked, and Byrne refused to reveal it. His 1953 autobiography Silent Years included challenge ciphertexts that defeated codebreakers for nearly a century. In 2010, his family donated his papers to the National Cryptologic Museum and the algorithm was finally published. It turned out to be elegantly simple: two rotating disks that permute themselves after each letter.
Byrne carried his concept in a small box he called "The Chao" for over 50 years. William Friedman politely declined to evaluate it. The US Navy declined. AT&T declined. After Byrne's death in 1960, the device and notes passed to his son and then sat in a closet until 2010. When Moshe Rubin published the algorithm in Cryptologia, working from the family’s donation to the National Cryptologic Museum, the cryptographic community confirmed: Byrne's system was genuinely strong for a hand cipher, with no obvious break given only ciphertext.
Two 26-letter alphabets, called the "left" (ciphertext) and "right" (plaintext) wheels, are independently scrambled. To encrypt one letter:
1. Find the plaintext letter on the right wheel. 2. The ciphertext letter is at the same position on the left wheel. 3. Rotate both wheels so the touched letters are at the "zenith" (position 0). 4. Permute the LEFT wheel: take the letter at position 1, slide it into position 13. 5. Permute the RIGHT wheel: rotate it one position, then take the letter at position 2 and slide it into position 13. 6. Repeat for the next plaintext letter.The wheels evolve continuously, so the same plaintext letter rarely encrypts to the same ciphertext letter twice in a row.
The Byrne family donated the Chaocipher papers to the National Cryptologic Museum in 2010, making the algorithm public after 92 years. Disclosure itself was not a cryptanalytic break; it allowed researchers to study the keyed construction.
A known-plaintext key-recovery method was published in 2010. In 2016, George Lasry, Moshe Rubin, Nils Kopal, and Arno Wacker published a hill-climbing attack against multiple messages encrypted with the same initial settings, plus a known-plaintext attack on short in-depth messages. Their Cryptologia paper establishes weaknesses in the classic cipher; it does not claim a general ciphertext-only break of every isolated message.
| Concept from Chaocipher | Modern Evolution |
|---|---|
| Self-modifying state | Modern stream ciphers (RC4, ChaCha20) update internal state per byte |
| Algorithm secrecy is not security | Kerckhoffs' principle, finally proven on Byrne's case |
| Simple components, complex evolution | Two wheels + two permutations = strong mixing |
| Exhibit | 46 of 169 |
| Era | Modern · 1918 |
| Trust | Broken |
| Inventor | John F. Byrne |
| Year | 1918 (disclosed 2010) |
| Key Type | Two scrambled 26-letter alphabets |
| Famous fact | Held secret for 92 years; public cryptanalysis followed the 2010 disclosure |
| Modern Lesson | Self-modifying ciphers can be strong with simple parts |