Keeping a machine secret

What was public and what was not

Enigma was sold commercially from the early 1920s. Anyone could buy one. The circuits, the mechanical logic, the very principle of rotor substitution — none of it was hidden. Patent filings made the design available to anyone who looked. The German military adopted a modified version, added a plugboard, changed the reflector arrangement, and then classified not the machine but its settings: the daily wheel order, the ring positions, the plugboard pairs. The device was an open secret; the key list was not.

This was a reasonable position, and in one sense it was sound. A cipher machine is an algorithm made of metal, and publishing an algorithm does not break it if the key space is large enough. Knowing how Enigma worked did not tell an interceptor which wheels were installed in which order, or how the plugboard was wired for that day. The British at Bletchley Park learned the machine's internals in detail from the Poles in 1939, just before the war; it was the daily settings that drove the entire intelligence effort.

Yet the military version's specific modifications — the extra plugboard, the particular reflector — were themselves treated as sensitive, and with reason. Each departure from the commercial model narrowed the problem for anyone trying to reconstruct what they were facing. Polish cryptanalysts at the Biuro Szyfrów worked out the military wiring through mathematical inference and a fortunate source of documents; they did it precisely because the differences from the known commercial machine gave them a constrained target.

The lesson the Enigma story actually teaches is not that the machine was broken but that operator habit compounded every structural weakness the design already contained. The settings were the secret, and the settings were betrayed, daily, by the people using them. Keeping a machine secret turns out to mean keeping its operators disciplined — and that is a procedural problem, not an engineering one.