Enigma machine

• 1. Introduction

  ›

  The Enigma machine sat at the center of the deadliest conflict in human history, encoding the secrets of Nazi Germany in what its operators believed was an impenetrable cipher. German engineer Arthur Scherbius invented it at the end of World War I, and by the time the second war began, it encrypted the most top-secret messages across every branch of the German military. What nobody in Berlin suspected was that a small group of Polish mathematicians had already broken it years before the war even started. How did a machine considered so secure become one of the most catastrophic intelligence failures of the 20th century? And what role did a handful of mathematicians, spies, and captured codebooks play in shortening the war itself?

• 2. How the Machine Works

  ›

  Each rotor inside an Enigma machine is a disc approximately 10 centimetres in diameter, made from Ebonite or Bakelite, with 26 brass spring-loaded contact pins on one face and 26 corresponding circular plate contacts on the other. When rotors are mounted side by side on a spindle, the pins of one rotor press against the plate contacts of its neighbour, forming a live electrical circuit. Inside each rotor, 26 wires connect pins to contacts in a complex scrambled pattern unique to that rotor.

  The clever part is that the rotors move. With every keystroke, at least the rightmost rotor steps one position, rerouting every electrical pathway through the machine. A letter pressed once might light up Z; press the same letter again and a completely different lamp illuminates. This prevented the kind of frequency-analysis attacks that had cracked older ciphers for centuries.

  A component called the reflector completed the circuit by bouncing current back through the rotors along a different route. This made the machine self-reciprocal: two identically configured Enigmas could encrypt and decrypt each other's messages without any switching. But the reflector introduced a fatal quirk. No letter could ever encrypt to itself. The letter A would never produce A as output. That seemingly small property would later become one of the critical footholds codebreakers used to attack the cipher.

  Beyond the rotors, military versions added a plugboard, the Steckerbrett, which swapped pairs of letters before and after the rotor scrambling. The plugboard alone offered 150 trillion possible settings, contributing more cryptographic strength than an additional rotor. When combined with the rotor selection, ring settings, and starting positions, a three-rotor military Enigma offered nearly 159 quintillion possible configurations.

• 3. Polish Codebreakers First

  ›

  Marian Rejewski was a Polish mathematician and cryptologist at the Polish Cipher Bureau, and around December 1932 he did something German cryptographers believed impossible: he broke the Enigma's plugboard variant. Rejewski used the mathematical theory of permutations, combined with flaws in German military message procedures, to work out the unknown rotor wiring. He was helped by French-supplied material obtained by a German spy named Hans-Thilo Schmidt, who had handed Paris the daily keys used in September and October 1932.

  Rejewski worked alongside fellow mathematician-cryptologists Jerzy Rozycki and Henryk Zygalski. All three had been recruited from Poznan University, chosen specifically because the region had been held by Germany before World War I and its students spoke German. By January 1933 the Polish Cipher Bureau was reading German Enigma traffic.

  Over the following years, as Germany upgraded the machine and tightened its procedures, the Poles responded with innovations of their own. Rejewski invented the cyclometer, a device used to compile a catalogue of 100,000 entries. The team also invented Zygalski sheets and built an electromechanical device called the bomba to search automatically for rotor settings. By 1938 they had six bomby running. Then Germany added two more rotors to the army's machine, and the number of bomby needed increased tenfold overnight.

  With the situation becoming untenable and war approaching, the Poles made a decision that would shape the entire Allied intelligence effort. On the 26th and the 27th of July 1939, at Pyry just south of Warsaw, they brought French and British military intelligence representatives into the room and shared everything: their techniques, their Zygalski sheets, their cryptologic bomba designs, and each delegation received a Polish-reconstructed Enigma. Gordon Welchman, who later led Hut 6 at Bletchley Park, wrote that the effort would never have got off the ground without learning from the Poles "in the nick of time."

• 4. Allies Take It Further

  ›

  When Germany invaded France in June 1940, a remarkable and largely forgotten team of seven Spanish cryptographers relocated with the broader Allied effort. Known as Equipo D, or Team D, they were led by Antonio Camazon, the former head of the cipher service of the Spanish Republican Army during the Spanish Civil War. After the Republic fell in 1939, Camazon and his colleagues sought refuge in France and were recruited by French intelligence officer Gustave Bertrand. They worked at the PC Bruno centre near Paris alongside Polish cryptanalysts, performing manual decryption, rotor setting reconstruction, and message traffic analysis.

  Three intact Enigma machines had already been captured during the Norwegian campaign between April and June 1940, and starting on the 17th of May 1940 they were put into operation at Bletchley Park. British cryptologists there would go on to decrypt a vast number of German messages over the course of the war. The intelligence gathered was codenamed Ultra.

  In practice, breaking Enigma depended less on mathematical genius alone than on a string of German procedural failures. Operators repeated message keys, used predictable settings, and made mistakes that left detectable patterns. The Allies also captured critical hardware: codebooks printed in red water-soluble ink on pink paper by the German Navy, designed to dissolve if a vessel was sunk, sometimes did not dissolve in time. The Abwehr's version of the machine was broken on the 8th of December 1941 by Dilly Knox, who found that agents had been double-encoding messages in a simple cipher before Enigma transmission, which helped crack the daily settings.

  Many analysts have concluded that the flow of Ultra intelligence from decrypting Enigma, Lorenz, and other ciphers shortened the war substantially and may even have altered its final outcome.

• 5. Naval Enigma's Extra Complexity

  ›

  The German Navy's procedures for Enigma were far more elaborate than those of the Army or Air Force, and the Navy had always issued its machines with more rotors. The Naval Enigma started with six rotors to choose from, expanded to seven, and eventually reached eight. The additional rotors, marked VI, VII, and VIII, each had two notches rather than one, causing more frequent turnover and multiplying the possible settings considerably.

  On the 1st of February 1942, the Navy introduced a four-rotor version for U-boat traffic, called M4, with the network known to the Allies as Shark. The engineers fitted the extra rotor in the same physical space as the three-rotor version by splitting the reflector into a thinner reflector and a thin fourth rotor. The fourth rotor never stepped automatically but could be manually set to any of 26 positions.

  Naval communication also used separate codebooks before encryption. The Kurzsignalheft converted sentences into four-letter groups covering an enormous range of operational details: logistics, rendezvous with supply ships, harbour names, weather conditions, enemy positions. A separate codebook contained the Kenngruppen and Spruchschlüssel for message key identification. These layered systems made Naval Enigma substantially harder to attack than Army versions, and Allied cryptanalysts spent much of the war battling to keep pace with Naval upgrades.

• 6. Machines, Thefts, and Museums

  ›

  An estimated 40,000 Enigma machines were constructed in total. After the war ended, the Allies sold captured machines to developing countries, which still considered them secure, a fact the Allies declined to advertise given that they could read those countries' traffic.

  The effort to break Enigma was not publicly disclosed until 1973. In the decades since, surviving machines have been scattered across museums on several continents. The Deutsches Museum in Munich holds both three- and four-rotor German military variants alongside civilian versions. Two machines at the International Museum of World War II near Boston include a U-boat four-rotor model and one of fewer than ten surviving ten-rotor code machines; visitors can operate two three-rotor machines to encode and decode their own messages.

  At auction, prices in recent years have ranged from around US$40,000 to US$547,500 in 2017. In November 2025, Christie's Paris sold an Enigma M4 that had been used by Karl Donitz for 482,600 euros to an unidentified buyer.

  Not all surviving machines have stayed safe. A rare Abwehr Enigma designated G312 was stolen from the Bletchley Park museum on the 1st of April 2000. A man calling himself "The Master" sent a ransom note demanding 25,000 pounds and threatened destruction of the machine. The machine eventually arrived anonymously at the office of BBC journalist Jeremy Paxman, missing three rotors. In November 2000, an antiques dealer named Dennis Yates was arrested after telephoning a newspaper to arrange the return of the missing parts. In October 2001, Yates was sentenced to ten months and served three.

  In December 2020, German divers working for the World Wide Fund for Nature discovered a destroyed Enigma in Flensburg Firth, believed to be from a scuttled U-boat, which is being restored by the Archaeology Museum of Schleswig Holstein.