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PLATFORM13

Enigma

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An electric typewriter that gives a different alphabet for every letter. Press a key and a current runs through a plugboard, three wired rotors and a reflector, then lights one of 26 lamps. Before each press the right-hand rotor turns one step, so typing AAAAA gives something like BDZGO. The same machine, set the same way, turns the lamps back into the message.

The machine

An Enigma I, lid open

This is the army and air force model with five rotors to choose from, reflector B and a plugboard on the front. Click or tap a key, or type on your own keyboard while the machine is on screen. Drag a thumbwheel up or down (or tap above or below its middle) to set the start letters. Join two plugboard sockets with a cable by tapping one and then the other. The settings panel chooses wheels and rings, and Inside view draws the path of the current at every press. Real army rotors had the numbers 01 to 26 in their windows; we show letters, as the navy's did.

Break it yourself

The lazy operator

The wheel order, ring settings and plugs for the day were printed on a monthly key sheet, and one has been captured. But every message also needed its own three-letter start position, chosen by the operator. He was supposed to choose at random. Bletchley Park found that many did not. Set the three windows: the board deciphers the intercept live, and it will read as German when you have his choice.

ARRIVED · CIPHER CRACKED

The story

The machine began as a business product. A Berlin engineer, Arthur Scherbius, patented a rotor cipher machine in 1918, and a company formed to sell it showed a commercial Enigma in 1923. Banks and firms were not much interested, but the German navy adopted a version in 1926 and the army followed in 1928. A plugboard on the front, which swapped pairs of letters on the way in and out, was added to the army machine around 1930 and made the number of settings astronomical: with three of the five rotors issued from 1938 and ten cables, about 159 million million million, before you count the rings.

Every key press steps the right-hand rotor. When it passes its notch it carries the middle one forward, and the middle rotor has a quirk: when it carries the left rotor forward it steps itself as well, so it moves on two presses in a row. This double step is in the engine on this platform, and it is the reason the period is a little shorter than 26 × 26 × 26.

Three mathematicians in Warsaw

The first people to read Enigma were not British. In Poland, the Cipher Bureau hired young mathematicians from Poznań University, and in December 1932 Marian Rejewski worked out the wiring of the army rotors by treating the machine as a set of permutations and solving equations about them, helped by key material that French intelligence had bought from a German informer. With Jerzy Różycki and Henryk Zygalski he then built methods to find the daily settings: a catalogue of rotor patterns, perforated sheets, and in 1938 the electromechanical bomba.

When the Germans added two more rotors at the end of 1938, the work became too big for the Polish bureau. In July 1939, five weeks before the invasion, the Poles met French and British codebreakers in the Kabaty forest at Pyry near Warsaw and handed over everything they had, including replica machines. Bletchley Park started from there.

Operators

The machine was strong; the people using it were human. Operators chose start positions like AAA, their girlfriend's initials, or three keys in a row. Bletchley's codebreakers called these lazy keys “cillies” and kept lists of each operator's habits. Fixed formats did the rest: weather reports, routine signals that said nothing had happened, and messages that began the same way every day.

How it was broken

Enigma was never defeated by trying every key. It fell to a mix of mathematics (Rejewski's permutation theory), captured documents and machines, and procedure: lazy start positions, repeated message keys in the early years, and stereotyped texts that gave codebreakers a probable word, or crib.

Its deepest weakness was built into the wiring. Because the current goes through the reflector and back by a different path, a letter can never come out as itself. That one fact told Bletchley where a crib could not sit, and it is what the bombe on the next platform exploits.