How the Enigma Machine Was Cracked During World War Two

158 Quintillion Possible Settings, and the Germans Thought That Was Enough

By 1939, every German military branch encrypted its communications with the Enigma machine—an electromechanical device the size of a typewriter that scrambled messages through a labyrinth of wired rotors, a plugboard, and a reflector. The machine offered approximately 158,962,555,217,826,360,000 possible configurations for each message. German intelligence calculated that even if an enemy tested one setting per second, checking them all would take longer than the age of the universe. They were right about the math. They were wrong about the assumption that brute force was the only option.

Inside the Machine: How Enigma Encrypted Messages

An Enigma operator typed a plaintext letter on the keyboard. Electrical current flowed through a plugboard (which swapped pairs of letters), then through three or four rotors (each wiring the 26 alphabet letters to different outputs), hit a reflector that sent the current back through the rotors by a different path, passed through the plugboard again, and lit up a ciphertext letter on the lampboard. Every keypress advanced the rightmost rotor by one position, changing the entire circuit.

That last line mattered enormously. The reflector meant a letter could never encrypt to itself. This single design choice—intended to simplify the machine—gave codebreakers their most reliable foothold.

Poland Breaks Enigma First: Rejewski's Mathematical Assault

The first Enigma break came not from Britain but from Poland. In 1932, mathematician Marian Rejewski of the Polish Cipher Bureau reconstructed the internal wiring of Enigma's rotors using intercepted ciphertext and a set of Enigma operating manuals obtained by French intelligence. His method was pure mathematics: group theory and permutation analysis.

Rejewski exploited a procedural weakness. German operators encrypted each message's three-letter rotor setting twice at the beginning of every transmission—a redundancy intended to prevent errors. This doubled key gave Rejewski enough structure to deduce rotor wiring through systems of permutation equations.

• Rejewski and colleagues Jerzy Różycki and Henryk Zygalski built electromechanical devices called "bombas" to automate parts of the decryption
• Zygalski developed perforated sheets that visually identified consistent rotor positions when stacked and held to light
• Poland read German Enigma traffic from 1933 to 1938 without the Germans suspecting any breach
• In July 1939, weeks before the German invasion, Poland shared its entire body of Enigma research with Britain and France

That transfer was one of the most consequential intelligence handoffs in history.

Bletchley Park: Britain's Secret Codebreaking Factory

The British Government Code and Cypher School relocated to Bletchley Park, a Victorian mansion in Buckinghamshire, in August 1939. At its peak, the operation employed nearly 10,000 people—mathematicians, linguists, chess champions, crossword enthusiasts, and a large corps of Women's Royal Naval Service (Wrens) who operated the decryption machinery.

The German military had increased Enigma's complexity after 1938, abandoning the doubled message key and expanding the rotor set. Poland's methods no longer worked directly. Bletchley Park needed new approaches at industrial scale.

Turing's Bombe: Automating the Impossible

Alan Turing arrived at Bletchley Park on September 4, 1939—the day after Britain declared war. Building on Rejewski's bomba concept, Turing designed an electromechanical machine called the bombe. It exploited "cribs"—guessed plaintext fragments that analysts expected to appear in intercepted messages. Weather reports, for instance, predictably contained the word "Wetterbericht" (weather report). Rigid military formatting gave codebreakers reliable cribs daily.

The bombe tested possible rotor configurations by checking whether a crib-ciphertext pairing produced contradictions. Because no letter could encrypt to itself, any configuration where a crib letter matched its ciphertext counterpart was immediately eliminated. This cascading elimination reduced the search space by orders of magnitude.

• The first bombe, named "Victory," began operating in March 1940
• By war's end, over 200 bombes ran continuously at Bletchley Park and outstations
• Each bombe simulated 36 Enigma machines running simultaneously
• Wrens operated the bombes in three shifts around the clock

Colossus: The World's First Programmable Electronic Computer

The Enigma machines used by the German Army and Air Force were not the only target. The German High Command used the Lorenz SZ40/42 cipher machine—far more complex than Enigma—for strategic communications. To attack Lorenz, engineer Tommy Flowers built Colossus, a machine using 1,500 vacuum tubes (later 2,400 in Colossus Mark II) that could process 5,000 characters per second.

Colossus Mark II became operational on June 1, 1944—five days before D-Day. It confirmed that Hitler believed the Normandy invasion would target Calais, giving Allied commanders confidence that their deception plan was working.

Intelligence Impact: Shortening the War

Historians estimate that Ultra intelligence—the codename for information derived from breaking Enigma and other Axis ciphers—shortened World War Two by approximately two years and saved millions of lives. The evidence is compelling across multiple theaters.

• The Battle of the Atlantic: Decrypted U-boat patrol orders allowed convoys to reroute, reducing Allied shipping losses dramatically after mid-1943
• North Africa: Ultra revealed Rommel's supply routes, enabling targeted attacks that starved the Afrika Korps
• D-Day: Confirmed the success of Operation Fortitude's deception, which kept German reserves away from Normandy
• Eastern Front: Shared intelligence (carefully disguised to protect the source) aided Soviet operations

Decades of Silence: The Secret Kept Until the 1970s

Churchill reportedly called the Bletchley Park codebreakers "the geese that laid the golden eggs and never cackled." The British government classified all Ultra-related information under the Official Secrets Act. Colossus machines were dismantled. Thousands of veterans went to their graves without ever discussing their wartime work.

The secret held for thirty years. In 1974, Group Captain Frederick Winterbotham published The Ultra Secret, the first public account. Historians had to rewrite entire chapters of the war. Battles previously attributed to tactical brilliance or luck turned out to have been informed by decrypted enemy communications.

Alan Turing, prosecuted in 1952 for homosexuality and subjected to chemical castration, died in 1954 at age 41. The full scope of his wartime contribution remained classified for another two decades. In 2013, Queen Elizabeth II granted Turing a posthumous royal pardon. The man who helped save millions of lives had been destroyed by the country he served—and the country didn't even know what it owed him.