Jonathan Lalou's blog

In his captivating 45-minute talk at Devoxx France 2022, Jean-Christophe Sirot, a cloud telephony expert from Sherweb, takes the audience on a historical journey through the cryptanalysis of the Enigma machine, used by German forces during World War II. Jean-Christophe weaves a narrative that blends espionage, mathematics, and technological innovation, highlighting the lesser-known contributions of Polish cryptanalysts like Marian Rejewski alongside Alan Turing’s famed efforts. His presentation, recorded in April 2022 in Paris, reveals how Enigma’s secrets were unraveled through a combination of human ingenuity and mathematical rigor, ushering cryptography into the modern era. This post summarizes the key themes, from early Polish breakthroughs to Turing’s machines, and reflects on their lasting impact.

The Polish Prelude: Cryptography in a Time of War

Jean-Christophe sets the stage in post-World War I Poland, a nation caught between Soviet Russia and a resurgent Germany. In 1919, during the Polish-Soviet War, Polish radio interception units, staffed by former German army officers, cracked Soviet codes, securing a decisive victory at the Battle of Warsaw. This success underscored the strategic importance of cryptography, prompting Poland to invest in codebreaking. By 1929, a curious incident at Warsaw’s central station revealed Germany’s use of Enigma machines. A German embassy official’s attempt to retrieve a misrouted “radio equipment” package—later identified as a commercial Enigma—alerted Polish intelligence.

Recognizing the complexity of Enigma, a machine with rotors, a reflector, and a plugboard generating billions of possible configurations, Poland innovated. Instead of relying on puzzle-solvers, as was common, they recruited mathematicians. At a new cryptography chair in western Poland, young talents like Marian Rejewski, Henryk Zygalski, and Jerzy Różycki began applying group theory and permutation mathematics to Enigma’s ciphers. Their work marked a shift from intuitive codebreaking to a systematic, mathematical approach, laying the groundwork for future successes.

Espionage and Secrets: The German Defector

The narrative shifts to 1931 Berlin, where Hans-Thilo Schmidt, a disgruntled former German officer, offered to sell Enigma’s secrets to the French. Schmidt, driven by financial troubles and resentment after being demobilized post-World War I, had access to Enigma key tables and technical manuals through his brother, an officer in Germany’s cipher bureau. Meeting French intelligence in Verviers, Belgium, Schmidt handed over critical documents. However, the French, lacking advanced cryptanalysis expertise, passed the materials to their Polish allies.

The Poles, already studying Enigma, seized the opportunity. Rejewski and his team exploited a flaw in the German protocol: operators sent a three-letter message key twice at the start of each transmission. Using permutation theory, they analyzed these repeated letters to deduce rotor settings. By cataloging cycle structures for all possible rotor configurations—a year-long effort—they cracked 70–80% of Enigma messages by the late 1930s. Jean-Christophe emphasizes the audacity of this mathematical feat, achieved with minimal computational resources, and the espionage that made it possible.

Turing and Bletchley Park: Scaling the Attack

As Germany invaded Poland in 1939, the Polish cryptanalysts shared their findings with the Allies, providing documentation and a reconstructed Enigma machine. This transfer was pivotal, as Germany had upgraded Enigma, increasing rotors from three to five and plugboard connections from six to ten, exponentially raising the number of possible keys. The Polish method, reliant on the repeated message key, became obsolete when Germany reduced repetitions to once.

Enter Alan Turing and the team at Bletchley Park, Britain’s codebreaking hub. Turing devised a new approach: the “known plaintext attack.” By assuming certain messages contained predictable phrases, like weather forecasts for the Bay of Biscay, cryptanalysts could test rotor settings. Turing’s genius lay in automating this process with the “Bombe,” an electromechanical device that tested rotor and plugboard configurations in parallel. Jean-Christophe explains how the Bombe used electrical circuits to detect inconsistencies in assumed settings, drastically reducing the time needed to crack a message. By running multiple Bombes, Bletchley Park decrypted messages within hours, providing critical intelligence that shortened the war by an estimated one to two years.

The Legacy of Enigma: Modern Cryptography’s Dawn

Jean-Christophe concludes by reflecting on Enigma’s broader impact. The machine, despite its complexity, was riddled with flaws, such as the inability to map a letter to itself and the exploitable key repetition protocol. These vulnerabilities, exposed by Polish and British cryptanalysts, highlighted the need for robust algorithms and secure protocols. Enigma’s cryptanalysis marked a turning point, transforming cryptography from a craft of puzzle enthusiasts to a rigorous discipline grounded in mathematics and, later, computer science.

He draws parallels to modern cryptographic failures, like the flawed WEP protocol for early Wi-Fi, which used secure algorithms but a weak protocol, and the PlayStation 3’s disk encryption, undone by poor key management. Jean-Christophe’s key takeaway for developers: avoid custom cryptography, use industry standards, and prioritize protocol design. The Enigma story, blending human drama and technical innovation, underscores the enduring importance of secure communication in today’s digital world.

Resources:

• Enigma by Dermot Turing

• Our Spy in Hitler’s Office by Paul Paillole

• The Code Book by Simon Singh

• The Codebreakers by David Kahn