Why the French military cryptanalysis failed to break Enigma

Abstract

AbstractIn July 1939, when the French military discovered the possibility of breaking Enigma thanks to revelations from the Polish Cipher Service, it came as a complete surprise. Although the French secret services had known about the German machine for almost ten years, the military cryptologists based in Paris had quickly concluded that it was impossible to break it. Only the forced exile of Polish mathematicians in France after the 1939 campaign enabled the French to decipher Enigma from January 1940 until the June defeat. While the story of the Polish and British cryptological successes is now well known through academic and mainstream literature, the French failure has received virtually no attention until now. Using unpublished archives held at the Defense Historical Service in Vincennes, this study analyzes the reasons for this fiasco and paints a picture of French military cryptanalysis in the 1930s, quite different from the past success of French codebreakers in the First World War.Keywords: EnigmaFrench cryptanalysisWorld War II Disclosure statementNo potential conflict of interest was reported by the author(s).Notes1 De Lastours (Citation1998, 212).2 Kahn (Citation1996), Ollier (2004), Forcade (Citation2008).3 Some very short indications are given by Widman & Wik (Citation2021, 31), Canuel (Citation2013) and Faligot (Citation2001).4 Ribadeau-Dumas (Citation1975, 24).5 Service Historique de la Défense [Defense Historical Service – DHS], Vincennes, France.6 DHS, 1937, Feuille de renseignements concernant le Capitaine Braquenié (DHS).7 DHS, 1913, Relevé de notes (DHS).8 DHS, 1912, Feuillet individuel de campagne (DHS).9 DHS, no date, Relevé des notes (DHS).10 DHS, 1930, Feuillet individuel de campagne (DHS).11 Bertrand (DHS, 10).12 Bertrand (DHS, 147).13 DHS, 1936, Feuillet du personnel (DHS).14 Forcade (Citation2008, 179–180).15 Paillole (Citation2013, 130–140).16 Durand-Richard and Guillot (Citation2014, 160).17 Ribadeau-Dumas (Citation1975, 26–27).18 Rivet (Citation2010, 99).19 DHS, 1932, Résumé des notes antérieures à l’année 1932 (DHS).20 DHS, 1932, Feuillet du personnel (DHS).21 DHS, 1931, Feuillet individuel de campagne (DHS).22 DHS, 1933, Feuillet individuel de campagne (DHS).23 DHS, 1936, Feuillet individuel de campagne (DHS).24 DHS, 1921, Diplôme d’ingénieur des Travaux publics (DHS).25 DHS, 1937, Feuille de renseignements concernant le capitaine Henri Braquenié (DHS).26 DHS, 1937, Copie des notes inscrites au feuillet du personnel du capitaine de réserve Braquenié (DHS).27 DHS, 1936, Relevé des notes du capitaine Braquenié Henri (DHS).28 Kozaczuk (Citation1989, 328).29 DHS, 1938, Bulletin individuel de notes (DHS).30 Bertrand (DHS, 151).31 DHS, 1938, Bulletin de décès (DHS).32 Rivet (Citation2010, 198).33 DHS, no date, Fiche biographique du général de brigade Joubert des Ouches (DHS).34 DHS, 1935, Relevé des notes (DHS).35 DHS, no date, Fiche biographique du général de brigade Joubert des Ouches (DHS).36 DHS, 1909, Feuille de notes (DHS).37 DHS, 1940, Feuillet du personnel (DHS).38 DHS, 1938, Feuillet du personnel (DHS).39 Stengers (Citation2004, 451).40 DHS, 1938, Letter from Louis Rivet to Stewart Menzies, 14 December 1938 (DHS).41 Bertrand (Citation1973, 57).42 DHS, 1949, Contribution à l’étude de la Machine à Chiffrer “Enigma” (type Wehrmacht) par le S.R. de l’E.M.A. (de 1931–1942) (DHS, 151–152). In his memoirs, published in 1973, Bertrand mentioned only a French “expert,” deleting the reference to “Cne B.” It may have been due to a desire to spare Braquenié, still alive and totally unknown.43 Batey (Citation2019, 70), Turing (30, 152).44 Turing (2022, 152).45 Bertrand (Citation1973, 58).46 Erskine (Citation2004, 300).47 Rejewski (Citation2011, 66), Kozaczuk and Straszak (Citation2004, 236).48 Stengers (Citation2004, 464).49 DHS, 1939, Lettre du ministère de l’Air à Henri Braquenié (DHS).50 DHS, 1939, Copie des notes du feuillet du personnel (DHS).51 Ibid.52 DHS, 1939, Feuillet individuel de campagne, Joubert des Ouches (DHS).53 Kozaczuk (Citation1989, 318).54 Batey (Citation2019, 90).55 DHS, Letter from Louis Rivet to Alastair Denniston, 8 November 1939 (DHS).56 DHS, Letter from Alastair Denniston to Louis Rivet, 11 November 1939 (DHS).57 Kapera (Citation2015, 106).58 Kozaczuk (Citation1989, 320).59 Ibid. (322).60 Ibid. (321).61 DHS, 1940, Copie des notes feuille du personnel, (DHS).62 Ibid.63 DHS, 1949, Etudes et Résultats de la Recherche de Renseignement par les moyens techniques (1930–1942) (DHS, 40).64 DHS, 20 August 1940, Note du Colonel Louis Rivet (DHS).65 Kozaczuk (Citation1989, 323–328).66 DHS, 1945, Feuille de renseignements concernant le capitaine Henri Braquenié (DHS).67 Kapera (Citation2011, 63–67).68 DHS, 1949, Etudes et Résultats de la Recherche de Renseignement par les moyens techniques (1930–1942) (DHS, 46).69 Ibid. (41).70 Ibid.71 DHS, 1945, Feuille de renseignements concernant le capitaine Henri Braquenié (DHS).72 Ibid.73 Kozaczuk (Citation1989, 323).74 Ibid. (327).75 Widman and Wik (Citation2021).76 DHS, no date, Fiche biographique du général de brigade Joubert des Ouches (DHS).77 DHS, 1943, Feuillet du personnel (DHS).78 Ibid.79 Turing (2022, 306–307).80 DAA, 9 January 1943, Rapport n°662 du Commissaire divisionnaire (DAA).81 Ibid.82 DHS, 1943, Feuillet du personnel (DHS).83 DHS, 22 March 1945, Rapport particulier sur le capitaine Braquenié proposé pour le grade de commandant au titre de l’armée de l’Air (DHS).84 DHS, 13 July 1945, Feuillet individuel de campagne (DHS).85 Beaufre (Citation2020, 124–125).Additional informationNotes on contributorsJean-Charles FoucrierJean-Charles Foucrier is a research fellow at the French Air Force office of the Defense Historical Service (Vincennes, France). He holds a doctorate (PhD) in contemporary history from the ParisIV Sorbonne University and is a specialist in the Second World War and military aviation. He is the author of two books (La Strategie de la Destruction; La Guerre des Scientifiques) and a critical edition of General von Choltitz’s memoirs.