The Enigma machine looked like a chunky typewriter in a wooden box, but its arithmetic was monstrous. Press a key and an electrical signal snaked through a plugboard and a set of rotating wheels before lighting up a lamp showing the enciphered letter; every keypress moved the rotors, so the cipher changed with each letter. A standard German army Enigma, with its rotor choices, rotor settings and plugboard connections, offered on the order of 159 quintillion possible daily configurations. The Germans considered it unbreakable. They were wrong twice over — first in Warsaw, then at Bletchley Park.
The first break belongs to Poland. In 1932, mathematician Marian Rejewski of the Polish Cipher Bureau, working with Jerzy Różycki and Henryk Zygalski, reconstructed the wiring of the military Enigma using mathematics — permutation theory — plus intelligence supplied by a French-run German spy. The Poles read Enigma traffic for much of the 1930s and built machines to help, including a device they called the bomba. In July 1939, weeks before the invasion of Poland, they summoned British and French intelligence to a meeting near Warsaw at Pyry and handed over everything: reconstructed machines, methods, the lot. British codebreakers arrived thinking they were ahead; they left having been given a running start on the war.
Station X
Britain's Government Code and Cypher School moved in 1939 to Bletchley Park, a mansion and estate about fifty miles northwest of London, chosen partly for its railway links to Oxford, Cambridge and the capital. Codenamed Station X, it filled with an improbable mix of mathematicians, linguists, chess champions and crossword obsessives — at one point recruits were found via a Daily Telegraph crossword challenge. By the war's end roughly ten thousand people worked there and at its outstations, and about three-quarters of them were women, many operating the machinery around the clock in shifts.
Alan Turing and fellow Cambridge mathematician Gordon Welchman designed the British bombe, an electromechanical machine that could race through Enigma rotor settings and reject impossible ones. The bombe needed a starting wedge: a "crib," a guessed fragment of plaintext believed to lie somewhere in the message. Here German habits were the codebreakers' best friends — weather stations transmitting the word Wetter at predictable times, units signing off with stock phrases, and one exploitable design quirk: Enigma could never encipher a letter as itself. Welchman's addition of a "diagonal board" made the bombes dramatically more effective. Hut 6 attacked German army and air force Enigma; Hut 8, led for a time by Turing, took on the far harder naval traffic that decided the Battle of the Atlantic.
Naval Enigma demanded more than machines. Captured material — "pinches" — was vital, most famously from U-110 in May 1941. And when the U-boat fleet added a fourth rotor in February 1942, creating the traffic Bletchley called Shark, the Atlantic went dark for ten months while shipping losses soared. The blackout ended after sailors from HMS Petard seized codebooks from the sinking U-559 in the Mediterranean; two of them, Francis Fasson and Colin Grazier, drowned retrieving the documents and were posthumously awarded the George Cross.
Beyond Enigma
Bletchley's other great triumph is less famous and arguably greater. The German high command used not Enigma but the Lorenz teleprinter cipher — dubbed Tunny — for its most sensitive strategic traffic. Bletchley broke it without ever seeing the machine, and to attack it at speed, Post Office engineer Tommy Flowers built Colossus, operational in early 1944: the world's first programmable electronic digital computer, its logic run on thousands of thermionic valves. Colossus-derived intelligence helped confirm, before D-Day, that Hitler had swallowed the deception about where the invasion would land.
The intelligence flowing from all this was codenamed Ultra, and it was rationed with obsessive care so the Germans would never suspect their ciphers were compromised. The official historian Harry Hinsley later estimated Ultra shortened the war in Europe by around two years. Yet almost nobody knew. Staff signed the Official Secrets Act and stayed silent for decades — spouses often had no idea what their partners had done in the war — until F. W. Winterbotham's book The Ultra Secret cracked the silence open in 1974. Churchill is said to have called the Bletchley staff the geese that laid the golden eggs and never cackled. Turing, prosecuted in 1952 for homosexuality and dead by 1954, never lived to see his war work acknowledged; today his face is on the Bank of England £50 note, and Bletchley Park is a museum.
Quiz nuggets
- Polish mathematician Marian Rejewski first broke Enigma in 1932; Poland shared its work with Britain and France in July 1939.
- Enigma's key flaw: it could never encipher a letter as itself.
- Turing and Gordon Welchman designed the bombe; Hut 8 attacked naval Enigma, Hut 6 the army and air force traffic.
- Colossus (1944), built by Tommy Flowers to break the Lorenz cipher, was the first programmable electronic digital computer.
- Bletchley's work stayed secret until the 1970s; intelligence from it was codenamed Ultra.