Moshe Rubin just emailed me to let me know that his extensive October 2011 Cryptologia article “John F. Byrne’s Chaocipher Revealed: An Historical and Technical Appraisal” (vol. 35 issue 4, pp.328-379 [!!!]) can currently be viewed and downloaded for free from Taylor & Francis (who publish Cryptologia), via the “Download full text” button there.
If (like me) you’re into both the social and technical aspects of historical cryptography, it’s a cracking old read, covering both Byrne’s life and his numerous attempts to get the US military to accept his “Chaocipher” invention. Yet Moshe’s article is far from all ra-ra-pro-Byrne stuff: it also makes clear…
* the system’s inherent fragility (because each step changed the state of the two rotors, it suffered from near-worst-case error propagation);
* Byrne’s cryptographic inexperience (the way that he proposed concealing the indicator settings was far from secure); and
* Byrne’s cryptologic naivety (he believed that the flat letter distribution of the ciphertext made it explicitly unbreakable).
If you’ve read Ratcliff’s “Delusions of Intelligence” (a book the GCHQ Historian recommended I read, thanks for that!), you’ll know that this last mindset was precisely what the various German agencies using the Enigma machine suffered from: and if Chaocipher had been extensively used by the Allies in WW2, who’s to say that Hitler’s fragmented array of codebreaking agencies wouldn’t have eventually found a way of breaking into it, just as they did with virtually all the Allies’ low-to-medium-echelon ciphers?
One thing that strikes me most about the whole saga is that even though Byrne (who sometimes wrote under the anagrammatic pseudonym “J. F. Renby”, I was amused to see) seems to have envisaged Chaocipher as an expensive-to-build set of mechanical rotors, I think it is actually very easy to use with two Scrabble alphabets arranged in horizontal rows. (OK, Scrabble wasn’t devised until the 1930s, but my basic point still stands regardless). All the sliding operations (zenith / nadir, etc) then become immediately straightforward, arguably far more so than if you were using a machine to do the same.
Regardless of whether or not Scrabble tiles are the best way to Chaocipherify your plaintext, I’d argue that what sets Byrne’s cryptographic ideas apart most is the way he conceptualized his crypto system in terms that mesh peculiarly well with modern computer science: in fact, it’s quite hard to describe it at all without lapsing into contemporary CompSciSpeak. It’s almost as if Byrne were projecting himself forward into a software world: but then again, one of the chapters of his autobiography was SciFi, so perhaps the future was where he felt most at home!