Hall of Codebreakers
Codebreakers, cipher designers and the teams behind important cryptanalytic achievements
Cryptanalytic results depend on evidence, methods, teamwork and operating conditions.
Person: Abu Yusuf Yaqub ibn Ishaq al-Kindi, Baghdad
Technique: Frequency analysis
Al-Kindi’s surviving treatise gives an early systematic account of frequency analysis. A fixed letter substitution preserves counts, allowing language statistics and word patterns to guide a search. That does not mean every ciphertext has enough information for a unique solution.
Letter frequencies vary by language, genre and sample length. Comparing them can suggest candidate substitutions; words and higher-order patterns help test those guesses. The most common symbol need not encode the most common letter in a short message.
Person: Thomas Phelippes, agent of Sir Francis Walsingham
Technique: Frequency analysis + contextual knowledge
Mary's encrypted correspondence with Babington Plot conspirators was intercepted and decoded. She was executed February 8, 1587. The break is one of history's clearest examples of cryptanalysis determining a human life.
Person: Commandant Étienne Bazeries (1846–1931), French Army cryptanalyst
Technique: Syllabic-substitution analysis on a 587-symbol nomenclator, with contextual cribbing against historical Louis XIV correspondence
In the 1890s Bazeries reconstructed a Rossignol-era nomenclator using syllable hypotheses, contextual readings and repeated groups. The royal systems developed during the seventeenth century; Bonaventure should not be credited with creating one alongside his father in the 1620s. Kahn describes homophones, nulls and an erasure instruction affecting the previous sign. These special entries complicate analysis, but no one trap explains the whole history of the decipherment.
Bazeries’s decipherment of Louis XIV correspondence also led him to propose an identity for the Man in the Iron Mask. That historical interpretation is disputed; deciphering a letter does not settle every inference about its subject. He also designed a cipher cylinder and published Les Chiffres Secrets Dévoilés (1901 edition).
The Great Cipher break is the canonical example of cryptanalysis as historiography: the tool was invented to decrypt diplomacy in real time, but its lasting payoff was unlocking a 200-year-old archive. See the exhibit: Hall II · Great Cipher (17th century).
People: Room 40, British Naval Intelligence
Technique: Code book reconstruction from partial recoveries
Germany secretly offered Mexico an alliance against the United States. Britain decoded it, waited strategically, then shared it with Washington. The telegram helped bring America into WWI. The war ended 20 months later.
People: Marian Rejewski, Jerzy Różycki, Henryk Zygalski
Technique: Permutation group mathematics + known message indicators
Marian Rejewski, Jerzy Różycki and Henryk Zygalski combined mathematical analysis, intercepted indicators and intelligence supplied through French cooperation to reconstruct and attack military Enigma. They shared their results with French and British colleagues near Warsaw in July 1939. See GCHQ’s account of that meeting.
Rejewski's breakthrough came from analyzing the message indicator system — the first six characters of each Enigma message. By collecting enough indicators, he found patterns that revealed the mathematical structure of the rotors. Pure mathematics, applied to an engineering problem, by a 27-year-old mathematician.
People: Alan Turing, Gordon Welchman, engineering teams and bombe operators
Technique: Crib-based electromechanical search
British bombe machines tested constraints derived from possible plaintext, building on the Polish breakthrough and later Allied work. ULTRA intelligence contributed to wartime operations. Estimates of how much it shortened the war are counterfactual judgments, not measured results.
People: John Tiltman (message/key recovery), Bill Tutte and the Research Section (structure), Tommy Flowers and team (engineering)
Technique: Statistical wheel-pattern analysis
Tiltman recovered messages and key from a repeated-key transmission in 1941. Tutte and the Research Section inferred the machine’s structure by early 1942. Flowers and his team later built Colossus, operational in February 1944, to accelerate statistical wheel-setting.
Person: William F. Friedman, US Army Signal Intelligence
Technique: Statistical analysis + machine reconstruction
The SIS team reconstructed PURPLE’s logical behavior and achieved its major breakthrough in September 1940. It produced valuable Japanese diplomatic intelligence before Pearl Harbor, but those diplomatic messages did not disclose the operational attack plan. Diplomatic and naval intelligence must not be treated as interchangeable.
Breaking an encryption system does not guarantee access to every relevant message, a correct translation or the right interpretation. Intelligence collection, distribution and operational decision-making are separate stages.
People: 420 Navajo Marines, US Marine Corps
Cipher: Navajo language plus a constructed military code
Navajo Marines used a specially developed military vocabulary and alphabet-spelling code as well as their language. The original 29 code developers received Congressional Gold Medals in 2001; later code talkers received silver medals. The code’s reported wartime success is a historical result, not a mathematical proof of secrecy.
Person: Joan Elisabeth Lowther Clarke (later Murray), 1917–1996
Technique: Banburismus — Bayesian sequential analysis of Enigma indicators
Recruited from Cambridge mathematics in 1940, Clarke became one of the most productive cryptanalysts in Hut 8 and rose to deputy head — extraordinary in a service that paid female staff at clerical rates and barred them from "officer" titles. She was a fluent practitioner of Banburismus, the Bayesian sequential procedure invented by Turing to slash the Bombe's workload by ranking which wheel orders to test first. On the most stubborn naval traffic she shared the load with Turing himself; on shorter runs she worked alone.
Clarke’s Naval Enigma work contributed to intelligence used in the Atlantic campaign. Access varied by network and period, including the 1942 U-boat blackout. She received the MBE in 1946. Her postwar career included a move to Scotland after marriage in 1952, a return to Cheltenham in 1962, formal retirement in 1977 and later clerical work. The linked biography distinguishes those dates and supplies the sources.
Person: William Thomas "Bill" Tutte, 1917–2002, Cambridge graph theorist
Technique: Statistical period-finding on a 4,000-character depth produced by an operator's August 1941 mistake
After a repeated-key Lorenz transmission in August 1941, John Tiltman recovered plaintext and key material. Tutte and the Research Section analysed the resulting stream and inferred the twelve-wheel structure by early 1942. This work distinguished five chi wheels, five psi wheels and two motor wheels, enabling later statistical attacks without first possessing the machine.
Tutte’s statistical work helped make machine-assisted Tunny attacks possible. Colossus was a collective engineering and operational achievement, and manual work continued alongside it. Tutte later made major contributions to graph theory and combinatorics. His codebreaking role remained little known for many years; the Tutte Institute for Mathematics and Computing in Canada commemorates his work.
See also: Hall VII · Lorenz exhibit for the machine; Story 6 above for Tommy Flowers and Colossus.
Person: Elizebeth Smith Friedman, 1892–1980
Technique: Hand cryptanalysis of commercial codes, polyalphabetics, and one-part nomenclators across 12,000+ messages
Elizebeth Friedman developed cryptanalytic methods at Riverbank and later led important Coast Guard work against smuggling traffic. Her team’s work included expert testimony and wartime analysis of clandestine communications. American cryptanalysis had earlier practitioners too; it was not invented by one couple.
During WWII Friedman’s Coast Guard team analyzed clandestine networks in the Western Hemisphere. Its cryptanalytic contributions are distinct from the investigative work that disrupted those networks. Her archival papers and the biographical sources linked in the exhibit help recover that history.
Person: Agnes Meyer Driscoll, 1889–1971, US Navy civilian cryptanalyst
Technique: Analysis of Japanese naval codes and additive encipherment; manual codebook reconstruction
Driscoll joined the Navy in 1918 and became an important analyst of Japanese naval systems, including the Red Book, Blue Book and early JN-25. She helped train later Navy cryptanalysts. Her JN-25 work preceded the Pacific teams’ 1942 Midway intelligence successes; wartime assignments also included work against German Enigma.
Driscoll worked on the Navy’s CM cipher machine before leaving briefly for the Hebern company in 1923. Her career combined machine knowledge and manual analysis. See the NSA biographical account.
Person: Leopold Samuel "Leo" Marks, 1920–2001, Head of Codes, Special Operations Executive
Technique: Prepared keying material, letter one-time pads and communications security checks
Marks worked on SOE agents’ communications, including efforts to recover messages corrupted by operator errors and reduce the need for dangerous retransmission. Prepared keys and agreed security checks addressed different operational risks; no indicator scheme guaranteed survival under error, interception or coercion.
Marks advocated prepared worked-out keys and letter one-time pads as improvements to SOE communications. These are distinct methods, and silk describes a carrier rather than a cryptographic construction. His memoir, Between Silk and Cyanide: A Codemaker’s War, 1941–1945 (1998 edition), gives his retrospective account. A poem-derived transposition key is not a random one-time pad.
See also: Hall IX · One-Time Pad for the mathematics of letter one-time pads; silk could also carry other kinds of prepared keying material.
People: Meredith Gardner and the US Army Signal Intelligence Service
Technique: Depth attack on reused pad pages
VENONA analysts exploited duplicated Soviet pad material. These messages used decimal code groups and no-carry arithmetic, so subtracting aligned ciphertext digits cancels the reused pad; it is not literally bytewise XOR. Recovering code groups and identifying people required further analysis. See the NSA’s VENONA history.
The pad-production and distribution process violated the one-use condition. Negative results against a correctly used one-time pad say nothing about traffic encrypted with duplicated pages. VENONA’s partial recoveries came from this operational weakness, not a contradiction of Shannon’s theorem.
People: David Oranchak, Sam Blake, Jarl Van Eycke
Technique: Computational search + n-gram statistical scoring
The Z340 solution announced in December 2020 combined computer searches, homophonic substitution analysis and recognition of a transposition structure with irregularities. David Oranchak, Sam Blake and Jarl Van Eycke solved the message; this did not identify the sender.
Person: Frank B. Rowlett, US Army Signal Intelligence Service
Technique: Stepping-switch analysis using traffic and related plaintext evidence
Frank Rowlett led day-to-day SIS work against PURPLE with colleagues including Genevieve Grotjan and William Friedman. Statistical relationships in traffic and related plaintext evidence helped reconstruct the machine’s behavior without a captured PURPLE machine.
Rowlett also contributed to the development of SIGABA. No successful enemy wartime cryptanalytic break of that system is publicly documented; that is narrower than a guarantee that the design can never be attacked.
Person: Genevieve Grotjan Feinstein, SIS junior cryptanalyst
Technique: Pattern recognition across aligned worksheets
On the afternoon of September 20, 1940, Grotjan asked Rowlett to look at a worksheet she had prepared. Aligning intercept plaintext-ciphertext pairs, she had identified a recurring relationship in the consonant cipher alphabet — the alignment that revealed how Purple's stepping switches were wired. Rowlett later described the moment as “one of the great breaks of the war.” Grotjan had been with SIS for less than two years.
Grotjan also contributed to the work that became VENONA. She left government service in 1947, before NSA was established, and later taught mathematics. See the NSA biography.
Person: Boris C. W. Hagelin, Swedish-born cipher-machine designer
Contribution: Pin-and-lug rotor mechanics for hand-portable cipher machines
Boris Hagelin developed commercially important pin-and-lug machines, including the family from which the M-209 was derived. Crypto AG’s later supplier-compromise history is a separate question from the public cryptanalysis of particular Hagelin mechanisms. The Hagelin exhibit distinguishes those issues.
People: Kevin Knight (USC ISI), Beáta Megyesi and Christiane Schaefer (Uppsala University)
Technique: Transcription, statistics and manual cryptanalysis
Knight, Megyesi and Schaefer combined transcription, statistical exploration and manual cryptanalysis to decipher the Copiale manuscript in 2011. Early automatic hypotheses did not directly solve it: recognizing how ordinary Roman letters and abstract symbols functioned was crucial. See their original paper.
People: Moshe Rubin (algorithm reconstruction, 2010), George Lasry (cryptanalytic verification)
Technique: Documentary reconstruction + simulated annealing
Byrne kept Chaocipher’s mechanism secret. The family’s 2010 donation made its algorithm available; later published key-recovery work studied attacks under specified message and crib conditions. Algorithm disclosure, recovery of a particular key and a general claim of security are different results.
Codebreaker Exhibits
The episodes above describe selected codebreaking efforts. The linked exhibits below provide fuller biographies and context; not every person or team has a separate page.
Joseph Rochefort led Station HYPO’s team. Partial JN-25 recoveries and traffic analysis helped Nimitz assess Japanese plans before Midway, alongside reconnaissance and operational decisions.
Arne Beurling’s 1940 reconstruction of the Siemens T52, with assistance and subsequent daily-key recovery distinguished from the initial breakthrough.
Dilly Knox worked on commercial and military Enigma variants, including Italian naval and Abwehr traffic, with colleagues whose contributions the exhibit identifies.
Herbert Osborne Yardley founded America's Black Chamber in 1919, broke Japanese diplomatic codes at the 1921 Washington Naval Conference, and then infamously published the secrets in his 1931 bestseller.
Mavis Batey worked with Dilly Knox and colleagues on Italian naval Enigma. Their decrypts helped inform the British response before Cape Matapan in March 1941.
Ellis’s January 1970 non-secret encryption report, Cocks’s 1973 construction, and Williamson’s 1974 key-agreement work, disclosed publicly in 1997.
David Kahn brought cryptographic history to a wide audience with the original 1967 edition of The Codebreakers; the reading credit below identifies the revised edition used here. The exhibit distinguishes its scholarship, influence and publication controversies.
Elonka Dunin is a game designer and cryptography author known for documenting unsolved cryptograms and studying Kryptos.
George Lasry, computational cryptanalyst and co-decipherer of Mary Stuart’s lost letters, published in 2023 with Norbert Biermann and Satoshi Tomokiyo.
Agnes Meyer Driscoll helped develop U.S. naval cryptanalysis and trained later analysts. Her work included Japanese naval codes, early JN-25 and cipher-machine analysis.
Bill Tutte and the Research Section inferred the Lorenz structure by early 1942, analysing key material recovered by John Tiltman from a 1941 repeated-key transmission.
Elizebeth Smith Friedman led important American cryptanalytic work against smuggling and wartime clandestine communications, with a team whose work remained little known for years.
Joan Clarke, Hut 8 cryptanalyst and expert practitioner of Banburismus, helped reduce the search for Naval Enigma settings.
Leo Marks worked to improve SOE field communications, advocating prepared keys and letter one-time pads and writing The Life That I Have.
Mary Stuart’s letters from 1578–1584: 57 deciphered letters, about 50,000 words, published by Lasry, Biermann and Tomokiyo in 2023.
Sources and Reading
David Kahn, The Codebreakers: The Comprehensive History of Secret Communication from Ancient Times to the Internet, revised edition (1996), for the historical survey and Great Cipher account. Leo Marks, Between Silk and Cyanide: A Codemaker’s War, 1941–1945, 1998 edition, for his retrospective SOE account. Individual exhibits identify archival and institutional sources for their biographies; the summaries here preserve those sources’ limits.