1932 · Warsaw, PolandThe first break: mathematics
Poland sits squeezed between Germany and the Soviet Union, and it knows it. Its Cipher Bureau does something no country has tried before: instead of hiring language experts to break codes, it hires mathematicians.
One of them is Marian Rejewski, twenty-seven years old, quiet, brilliant. In December 1932 he is handed the impossible job: figure out the wiring inside the German military Enigma — without ever seeing one. He has three things: intercepted gibberish messages, a branch of mathematics called permutation theory, and a stack of documents that a French spy bought from a German traitor (settings sheets, sold for cash by a man named Hans-Thilo Schmidt).
In about three months of pencil-and-paper work, Rejewski deduces exactly how the secret rotors are wired. Historians still call it one of the greatest feats of codebreaking ever. Poland then quietly builds its own copies of Enigma and starts reading German military mail — for most of the 1930s.
Rejewski solved the machine like a giant logic puzzle. He noticed the Germans made a small, lazy mistake in how they used Enigma (they typed a key twice at the start of each message), and mathematics turned that tiny slip into a crowbar. Lesson of this whole story: machines don't leak secrets — habits do.
Rejewski works with two other young mathematicians, Jerzy Różycki and Henryk Zygalski. Together they invent tools with wonderful names: the cyclometer, Zygalski sheets (stacks of perforated cardboard you slide over a light table until holes line up), and in 1938 the bomba — a machine that tests Enigma settings automatically. A machine to fight a machine.
July 1939 · A giftFive weeks before the war
By 1939 Germany has made Enigma tougher — more rotors to choose from, more plugboard cables — and Poland, about to be invaded, can no longer keep up alone. So in July 1939, in a forest hut near Warsaw, the Poles do something extraordinary: they hand their British and French allies everything. Working replica machines. All their methods. Years of hard-won knowledge, given away for free.
Five weeks later, Germany invades Poland and the Second World War begins. Rejewski, Różycki and Zygalski escape and keep breaking codes for the Allies from France. Their head start saved the British years of work — a debt Britain only fully acknowledged decades later.
1939–1941 · Bletchley Park, EnglandCodebreaking becomes a factory
An hour north of London stands a Victorian country house called Bletchley Park. Behind its gates, Britain gathers an odd army: mathematicians, chess champions, crossword-puzzle wizards, linguists, engineers. At its peak, about 9,000 people work here in round-the-clock shifts — and roughly three quarters of them are women, operating machines, running intercept stations, and cracking codes.
The most famous mind at Bletchley is Alan Turing — a shy, marathon-running mathematician who had already, at 24, imagined the theoretical blueprint for every computer you've ever used. With engineer Gordon Welchman, Turing designs the Bombe: a two-metre-tall machine full of spinning drums that imitates dozens of Enigmas at once, powering through settings that would take humans lifetimes.
The Bombe needs a hint to start from — a crib, a guess at some words in the message. Luckily the Germans are wonderfully predictable. Weather stations transmit WETTER (weather) every morning at the same time. One outpost signs off every single day with "nothing to report." And Enigma has that one fatal flaw — a letter can never encrypt to itself — which lets codebreakers slide a crib along a message and cross off every position where a letter would have to match itself. (Try this yourself in File 03.)
The Bombe didn't "solve" the message. It rapidly threw away wrong answers — millions of them — until only a few possible settings survived. Humans checked those few by hand. Guess smart, let the machine grind, check the survivors: that's still how a lot of computing works today.
1941–1943 · The AtlanticThe U-boat war
The hardest Enigma belongs to the German navy — stricter procedures, and from 1942 a four-rotor machine. This one really matters: U-boats are sinking the convoy ships carrying food and fuel to Britain, and Britain cannot survive without them.
Breaking naval Enigma takes more than brains — it takes daring. British sailors board sinking submarines and armed trawlers to grab code books and key sheets before the sea claims them; some drown doing it. Every captured key sheet means weeks of readable messages — and convoys quietly steered around waiting U-boat packs.
Decoded Enigma intelligence gets a codename: Ultra. It is so precious that the Allies sometimes let bad things happen rather than reveal they can read German mail. A secret is only useful while the other side doesn't know you have it.
1945 and afterThe secret that stayed secret
Historians estimate that Ultra intelligence shortened the war in Europe by around two years, saving millions of lives on all sides. And then — silence. Everyone at Bletchley had signed the Secrets Act. The Bombes were dismantled. For thirty years, veterans told no one, not even their families. The world only learned the story in 1974, when the secrecy was finally lifted.
That silence had cruel costs. Alan Turing, who had done as much as anyone to win the war, was prosecuted in 1952 — not for anything to do with secrets, but simply because he was gay, which was treated as a crime in Britain at the time. He died two years later, aged 41, his war work still unknown to the public. Britain formally apologised in 2009 and pardoned him in 2013; his face is now on the £50 note. Marian Rejewski survived the war, returned to Poland, and worked as an ordinary accountant — his colleagues never suspecting that the quiet man at the next desk had out-thought the Third Reich.