“The Chaocipher” is a devious cipher system invented in 1918 by John F. Byrne: allegedly, it was so complex that nobody could crack his challenge ciphertexts (even with the plaintext to refer to!), yet was so simple that its mechanism was claimed to comprise only two rotating disks small enough to fit in a cigar box, and could be operated by a ten-year-old (admittedly a diligent, determined and well-practised one) to encipher and decipher texts.

Hence, the Chaocipher’s long-standing mystery revolved around three questions:

Was the Chaocipher for real? (i.e. could something so simple really produce such tricksy ciphertext)? Was it more secure than, say, the Enigma machine? More to the point, is the Chaocipher actually an unbreakable cipher?

As of a few years ago, only three people knew the Chaocipher’s secrets – John Byrne Jr (the inventor’s son), and two Cryptologia editors (who saw it in 1990 but were sworn to silence). Yet as Chaucer noted, time and tide wait for no man (not even Cryptologia editors) – so there was a very real (and growing) possibility that the secrets of the Chaocipher might somehow get lost forever.

Hence last August, Moshe Rubin – who CM readers may well recall as the zesty Israeli software / crypto guy who not long before had set up the Chaocipher Clearing House website – decided to try to contact John Jr before it was too late, and so cold-called his way through the list of Byrnes living in Vermont. Before long, Moshe found himself in contact with Patricia Byrne (John Jr’s wife) from whom he discovered the sad news that her husband had passed away a year or two previously.

However, because Pat Byrne was already looking for a buyer for her husband’s cryptological material, Moshe put her in contact (via David Kahn) with David D’Auria, the chairman of the National Cryptological Museum’s Acquisitions Commitee. Somewhat surprisingly, after a couple of months Pat Byrne very generously decided to donate the whole set to the NCM, a terrific gesture which I (for one) highly appreciate (and I hope that you do too!)

And so it came to be that Moshe Rubin found himself allowed what he describes as “preview access to some of the material“. Although he found that the precise setup John Byrne Sr had employed was not immediately obvious from the material to hand, Moshe burnt a load of midnight oil (is elbow grease more or less inflammable?) before finally managing to reconstruct the original algorithm in all its subtly obfuscatory glory.

Just as Byrne had described, his Chaocipher used two rotors (with the plaintext alphabet on the right rotor and the ciphertext alphabet on the left rotor) BUT with both alphabets altered slightly (let’s call this process ‘twizzling’, for want of a better word) after processing each letter. I’ve hacked together a 30-second Chaocipher animation on YouTube to try to demonstrate Byrne’s twizzlification…

Rather than go through the fine details here, I’m happy to refer you to Moshe’s detailed (and very readable) description of the process here: the only significant difference between my video and his text is that because the rotors mesh (and hence physically rotate in opposite directions to each other), the numbering sequence on each rotor is reversed relative to the other – i.e. even though #1 is at the top of each rotor, #2 and #3 proceed clockwise on the right (plaintext) rotor but anticlockwise on the left (ciphertext) rotor. Whereas in his text, both numbering systems run in parallel to each other (which might confuse you, it certainly confused me a little).

Of course, the obvious practical weakness of the Chaocipher is that any errors in enciphering, transmission, and deciphering get near-irreversibly propagated through the rest of the message: which probably makes the whole system too fragile to use in wartime, however cryptographically secure it may be (and, answering the second question above, I suspect that it may well prove to be more complex than Enigma, for it really is quite a fiendish system).

But is it (practically) unbreakable? Well, the obvious answer would be that if it has now been released into the wild, you’d have thought someone in a three-letter-agency (or GCHQ, naturally) would have worked out a clever way in. However, I’m not 100% sure that has happened yet… so, interesting times.

All credit to Moshe Rubin, then, for his persistence and hard work bringing this cipher mystery into the light: he has a Cryptologia paper coming up, and plenty more work to do over coming months (or years?) fleshing out the behind-the-scenes story from the stack of Byrne’s papers now in the NCM. It’s a fascinating slice of cipher history, and I wish him the very best of luck with the inevitable book and selling the movie rights! 😉

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Update: I’ve added a follow-on Chaocipher post here, discussing the intriguing parallels between the Chaocipher and chaos theory…