How the Enigma Machine Was Cracked

John Herivel discovered that German Enigma operators often made only slight shifts from default rotor settings, a habit he termed the 'Herivel tip'.

This laziness created discernible patterns in message headers, which allowed codebreakers to deduce daily ring settings and gain an early advantage in decryption efforts.

Codebreakers at Bletchley Park manually tested rotor orders and plug board settings, with a single message taking up to an hour to decode, a rate unsustainable given the hundreds of messages received daily.

The process was further complicated by multiple plug board swaps within messages, requiring the development of automated methods to manage the vast scale of intercepted communications.

His objective was to engineer a machine capable of systematically confirming or rejecting potential Enigma settings, thereby bypassing the arduous manual process.

This crucial work, aiming to streamline and accelerate the deciphering of enemy communications, was conducted in Hut 8, which served as his office throughout the war.

Turing capitalized on a fundamental flaw in the Enigma machine: a letter could never be encrypted as itself, a limitation he leveraged for decryption.

Codebreakers used 'cribs'—recurring phrases, like weather reports—to align suspected plaintext with encrypted messages, sliding the crib against the ciphertext until no letter mapped to itself.

This method provided a logical basis for the automated tests conducted by Turing's decryption machines, effectively turning a machine flaw into a strategic advantage.

Cribs were suspected plaintext phrases, like 'weather report,' identified from repeated patterns or slang in intercepted messages, critical for initiating logical tests against the Enigma.

The cribbing room at Bletchley Park specialized in identifying strong, consistent messages suitable for cribs, and Turing's tests assumed specific initial window settings to begin the decryption process.

A successful loop occurred when an input letter, such as 'R', emerged as 'R' after traversing three interconnected Enigma stages, indicating a potential match.

This specific loop formation signaled that the internal settings of the wired machines aligned with the original encryption parameters, providing a critical breakthrough in the decryption process.

Turing's critical insight demonstrated that back-to-back plug boards effectively canceled each other out.

This breakthrough simplified the problem by isolating rotor-only loops, dramatically reducing the mathematical complexity of solving Enigma settings and accelerating decryption efforts.

Turing collaborated with engineer Harold Keen to build 'The Bomb', named in tribute to the Polish 'Bomba' device that broke earlier Enigma versions.

The first prototype was completed in March 1940, a remarkable achievement given the complexity of the machine.

This machine used 36 Enigma machines working in parallel to automate Turing's logical tests, rapidly deriving the daily encryption key.

Each of the potential 100 stops required about 15 minutes to check, creating significant delays that risked the Enigma settings changing before a solution could be found.

He connected these wires, so if one went live, both would, eliminating redundant guesses and drastically reducing the number of false stops.

This modification successfully cut potential solutions from 100 to about four, thereby eliminating over 90% of false stops by August 1940 and making the Bombe a practical decryption tool.

The German Navy's addition of a fourth rotor to their Enigma machines forced Turing to develop new statistical methods for decryption.

Allies deployed 'gardening,' a tactic of strategically seeding mines in specific locations, to manipulate German communications and trigger predictable plaintext messages that aided codebreaking.

Codebreakers also critically relied on capturing key sheets from sinking U-boats to maintain decryption momentum, ensuring continuous access to vital intelligence.

The successful breaking of the U-boat Enigma by 1943 marked a critical strategic turning point in World War II, significantly impacting naval warfare.

Deciphered intelligence played a crucial role in the success of the D-Day landings in 1944 and the broader Allied invasion of Europe, with allies maintaining daily access to Nazi communications by the war's end.

Sir Dermot Turing contends that Alan Turing remained deeply involved in cutting-edge technology projects from 1945 to 1950, rather than entering a post-war downward spiral.

He argues against defining Turing's legacy solely by his tragic death, highlighting that his significant accomplishments in the post-war era are often undervalued and overlooked.

This advanced cipher machine boasted a key space of 10 to the power of 170, vastly exceeding Enigma's 10 to the power of 23, and remained classified throughout the Cold War era.