THE IMITATION GAME

Breaking Enigma

How mathematics defeated the "unbreakable" cipher

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    ║     THE ENIGMA MACHINE                    ║
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    ║  │   Q W E R T Z U I O P          │  ║
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    ║  │    A S D F G H J K L           │  ║
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    ║  │     P Y X C V B N M            │  ║
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    ║         │ I │ II│III│  ← Rotors           ║
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The Challenge

The German Enigma machine was considered unbreakable. Each day's settings created one of 158 quintillion possible configurations. Every letter typed produced a different encryption, making frequency analysis useless.

The Insight

Turing realized that Enigma had a weakness: no letter could ever encrypt to itself. This seemingly minor property, combined with known message fragments ("cribs"), allowed mathematical elimination of impossible settings.

The Bombe

Turing designed the Bombe - an electromechanical device that automated the search for Enigma settings. Multiple rotors spun in parallel, testing possibilities until contradictions eliminated wrong answers. What would take humans millennia took the Bombe hours.

Facts About the Codebreaking

• Enigma had approximately 158,962,555,217,826,360,000 possible settings each day - far too many to test by brute force.

• The Bombe didn't 'crack' Enigma directly - it eliminated impossible settings, narrowing millions of possibilities to a handful.

• Turing's insight: Enigma could never encrypt a letter as itself. This weakness (no letter self-maps) enabled many attacks.

• The 'crib' technique used known or guessed plaintext. Weather reports often began with 'WETTER' (weather in German).

• Operators made predictable mistakes: using girlfriends' names, keyboard patterns, repeated setups. Human error broke machine security.

• The Polish cryptographers Marian Rejewski, Jerzy Rozycki, and Henryk Zygalski cracked early Enigma - they shared their work with Britain in 1939.

• Bletchley Park decoded an estimated 84,000 Enigma messages per month at peak operation.

• The work at Bletchley Park remained secret until 1974 - thirty years after the war. Turing died never able to speak of his greatest achievement.

• The Bombe had 36 Enigma-equivalent mechanisms running in parallel, checking multiple possibilities simultaneously.

• Naval Enigma (the 4-rotor version) was harder to crack. Turing's work on it was crucial to winning the Battle of the Atlantic.

The Impact

Intelligence from Enigma decrypts (codenamed "Ultra") influenced nearly every major campaign of the European war. Historians estimate it shortened the war by two years, saving millions of lives.

Turing received an OBE in 1945. The citation was vague. The reason was secret. It would remain so for another thirty years.