Chaocipher

The Chaocipher is a mechanical polyalphabetic cipher invented by John F. Byrne in 1918, who claimed to have built an unbreakable device fitting inside a cigar box. He posted several public challenges through the 1920s and 30s (book Silent Years, 1953), offering a reward to anyone who could break his cipher — no one succeeded in his lifetime.

The exact algorithm stayed secret for 90 years. It wasn’t until 2010, after Byrne and his wife had died, that his family handed the complete archive (notebooks, prototypes, plaintexts of the challenges) to the Marshall Library of Diplomacy in Lexington, Virginia. Modern cryptanalysis was then able to reconstruct the mechanism.

Principle

The two discs

Chaocipher relies on two circular alphabets stacked one above the other, conventionally called left (plain) and right (cipher). Each holds the 26 letters in a secret order (the key).

To encrypt one letter:

1. Find the plaintext letter on the left disc — its position gives you the cipher letter at the same position on the right disc.
2. Permute the left disc: rotate it to bring the cipher letter to position 1, then swap the letters at positions 14 and 15 (centred insertion).
3. Permute the right disc: rotate to bring the plain letter to position 2, then swap positions 13 and 14.

Every character mutates both discs — hence the name chao-cipher: controlled chaos between every operation.

Why it’s powerful

• Both discs change at every character, so two identical plaintext letters never encrypt the same way.
• The evolution is deterministic and reversible: with the initial state of the two discs, you can recover everything.
• The key space is 26! × 26! ≈ 1.6 × 10⁵³ → astronomically large.

Why Byrne kept it secret

Byrne thought — perhaps rightly — that the mechanical simplicity of his machine was its main commercial asset. He tried to sell it to the US Army, AT&T, the State Department: all refused, despite the apparent resilience, because his demos were poorly presented and he refused to disclose any internal detail before signing.

He then chose to publish his challenges in his autobiography Silent Years (1953), hoping the academic community would break the silence. No reply — partly because his challenge texts were too short (200–300 characters) for statistical analysis to converge.

Modern break

In 2010, after the Byrne archive was made public, Moshe Rubin published a complete cryptanalysis. The method:

1. Reconstruct the two initial alphabets via probable-word attacks (cribs against the challenges).
2. Verify that the dynamic permutations indeed lead to the published ciphertexts.
3. Break the remaining challenges in a few hours of computation.

Conclusion: Chaocipher is resistant to crib-free attacks, but vulnerable to crib + computer attacks. Byrne was right about the tools of his era, wrong about modern tools.

Legacy

• Device replica — the crypto-historian community has rebuilt several prototypes in wood and 3D-printed plastic for demonstration.
• Literary inspiration — Byrne shows up in several espionage novels (e.g. The Decipher by Jonathan Holt) as the prototype of the “maverick genius” of amateur cryptography.
• Pedagogy — Chaocipher has become a textbook test case for cryptanalysis students, illustrating how hard it is to break a cipher whose algorithm is unknown.

In CipherChronicle

Chaocipher illustrates the strategic value of algorithmic secrecy — exactly what Kerckhoffs (1883) flagged as bad practice. Companion grids can replay one of Byrne’s historical challenges, providing two starting alphabets and a partial crib, to give the player the experience of a probable-word attack.