The stepping
Rotors are only as strong as the rhythm of their turning — and the simplest rhythm is the one easiest to predict.
How the wheels move
A rotor machine substitutes one letter for another, then advances its rotors so the next letter faces a different wiring configuration entirely. That advancement — the stepping — is what prevents the cipher from collapsing into a simple one-alphabet-for-another substitution. Without it, every letter in the message would be enciphered by the same static wiring, and a single session of frequency analysis would break it before afternoon.
The most obvious stepping arrangement is odometer-style: the rightmost rotor advances one position with each keypress; when it completes a full revolution, it notches the middle rotor forward one step; the middle rotor in turn notches the left. Twenty-six steps, twenty-six steps, twenty-six steps. Predictable, mechanical, and — in the Enigma — not quite what was implemented.
The Enigma used a notch-and-pawl mechanism, and this introduced a peculiarity known as the double-stepping anomaly. When the middle rotor sits at its own notch position, pressing a key causes it to step simultaneously with the left rotor, regardless of whether it had just advanced the previous keypress. The middle rotor therefore steps twice in succession under certain conditions — once when it notches the left, and once when it is itself notched by the right. This means the middle rotor's behavior deviates from pure odometer logic at a predictable but irregular moment. Cryptanalysts at Bletchley Park mapped this anomaly precisely and factored it into their reconstruction of rotor positions.
Other machines chose different solutions. The Lorenz SZ 40 and SZ 42, used by the German High Command for teleprinter traffic, drove two sets of wheels — the chi wheels and the psi wheels — with independent, irregular stepping patterns governed by a third set of motor wheels. The psi wheels did not advance on every character at all; they stepped only when the motor wheels permitted. The result was a far more complex period and a much flatter statistical profile — on paper. In practice, an operator error in August 1941 handed Bletchley an enormous stretch of traffic enciphered twice with nearly identical settings, which allowed Bill Tutte to reconstruct the entire wheel configuration from scratch.
The lesson threading through both machines is identical: irregular stepping raises the period and flattens the statistics, which is genuinely valuable. But the stepping pattern itself is fixed by the machine's design and known to any analyst who has ever seen a capture or a technical manual. What protected messages was not the stepping logic but the daily settings — the choice of rotors, their starting positions, and for Lorenz, the pin patterns on every wheel. The mechanism bought complexity; procedure was supposed to buy secrecy.
