Meeting of Pyry

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The deciphering of the German Enigma machine was a central topic at the Pyry meeting

The Pyry meeting was a top secret get-together of Polish, French and British cryptanalysts . It happened at the end of July 1939 in the Kabaty Forest of Pyry , near the Polish capital and about a month before the German invasion of Poland .

At this secret conference, the Poles revealed to their astonished allies the methods and equipment they had successfully developed since 1932 , with which they had succeeded in deciphering the communications encrypted by the German Reichswehr and later by the Wehrmacht with the help of the Enigma key machine . With this boost, the British cryptanalysts in Bletchley Park, England, were able to launch another attack on the German machine when the war broke out, which subsequently enabled them to decipher the encrypted German radio messages almost continuously.

prehistory

The Polish cryptanalyst Marian Rejewski (1932)

After the invention of the Enigma in 1918 by the German Arthur Scherbius , this innovative type of machine encryption was used by the Reichswehr of the Weimar Republic from the mid-1920s, initially on an experimental basis and increasingly regularly from 1930. Germany's neighbors, above all France, Great Britain and Poland, followed this with suspicion, especially when the National Socialist rule began in 1933 and this key machine established itself as a standard procedure in the course of the armament of the Wehrmacht . While it failed French and the British, in the encryption break and they classified the Enigma as "unbreakable," the 27-year-old Polish mathematicians succeeded Marian Rejewski in his work in the charge of Germany Unit BS4 of the Polish Biuro Szyfrów (BS) (German : "Chiffrenbüro"), the first break-in in 1932 (see also: Deciphering the Enigma ). To do this, he and his colleagues Jerzy Różycki and Henryk Zygalski exploited a serious procedural error that the Germans had made.

To secure transmission to ensure was, at the time of the authorized recipient of a message to the decryption needed message key (also see: Enigma radio message ) asked twice in a row and encrypted to the beginning of a message written ( " message key duplication "). Thus the first and fourth, the second and fifth as well as the third and sixth ciphertext letters were each assigned to the same plain text letter. With the help of two specially built machines, called Zyklometer and Bomba , which embodied two or three times two Enigma machines connected in series and each offset by three rotational positions, the Polish cryptanalysts were able to determine for each of the six possible roller positions at which roller positions the observed assignment the letter pairs was possible and so narrow the search space considerably. After analyzing several radio messages, the correct message key was found.

After the Germans changed their process technology on September 15, 1938 and three months later with the introduction of rollers IV and V increased the number of possible roller layers from six (= 3 · 2 · 1) to sixty (= 5 · 4 · 3) the Poles could no longer keep up, and the Enigma was safe again. In view of the impending danger, the Polish General Staff , headed by Generał brygady (German: Brigadegeneral ) Wacław Stachiewicz, at the suggestion of Pułkownik (German: Colonel ) Tadeusz Pełczyński , decided to give all of the knowledge about the deciphering methods of the German rotor key machine to the Polish allies and had the British and French invited to the Polish capital by the BS in July 1939.

Attendees

Some of the men from different countries had never met before. They had different educational and cultural backgrounds, different mother tongues and mastered different foreign languages. While almost all Poles (except their mother tongue), due to their birthplace in the then Austria-Hungary or in the German Empire, spoke excellent German, some French and no English, the French spoke no English and only a little German, and the British participants French and some German. English was therefore out of the question as the conference language, and Polish was also ruled out. French was the only native language left, but the Polish participants had little command of it. Curiously enough, German of all things was the ideal conference language.

Period

The Hotel Bristol in Warsaw, where the British delegation stayed (photo July 2006)

There are slightly different details about the exact date of the meeting. It is undisputed that it took place in the last full week of July ( calendar week 30 ) in 1939 . According to the custom at the time, calendar weeks began on Sunday and ended on Saturday. Thus, the relevant period of the meeting is between Sunday, July 23rd, and Saturday, July 29th, 1939. The conference presumably lasted two or three days and began on either Monday the 24th or Tuesday 25th, or on Wednesday, July 26th, 1939.

 Juli 1939 (KW 30)
 So Mo Di Mi Do Fr Sa
 23 24 25 26 27 28 29

Two of the participants in the meeting, Gustave Bertrand and Marian Rejewski, indicate July 25th and 26th, i.e. Tuesday and Wednesday. Bertrand remembers these two days in his book and Rejewski also gives exactly the same period. Although both participants were participants in the meeting and this information can therefore be regarded as primary sources, it should be taken into account that they gave this data from memory more than thirty years after the meeting. Many historians have adopted this information, according to the German cryptologist and author of one of the most important German-language textbooks on cryptology, Friedrich L. Bauer , whereby he only gives the first day of the meeting, July 25, i.e. Tuesday. The renowned American historian and cryptologist David Kahn, on the other hand, names Monday, July 24th, as the day of arrival, on which the two French conference participants Bertrand and Braquenié arrived in Warsaw by express train , while the three British delegates arrived by plane. Accordingly, the conference started on Monday. Furthermore, the meeting would have ended on Tuesday, July 25, 1939. Another report that Rejewski wrote in Polish during the war, in 1940, contradicts this period. Here he noted July 26, 1939, Wednesday, as the day of the meeting. Other sources, on the other hand, indicate July 24-26, 1939, a period of three days from Monday to Wednesday.

Train route of the Nord-Express , which Denniston and Knox very likely used for their journey on July 24th and 25th, 1939 from London via Berlin to Warsaw (status 1939, routes in blue deviating from before 1933)

Fortunately, the notebook including the diary of another participant in the conference, namely Alastair Denniston, has been preserved. In the book Codebreakers - The inside story of Bletchley Park , the crucial page of the relevant calendar week is shown as a photo. For Monday, July 24th, “WARSAW” (German: “Warsaw”) is entered there in capital block letters. A time is probably written just above it, followed by “Vict”. This could be the departure time and station, namely Victoria Station in central London . "Bristol Hotel" is registered for July 25th. From this it can be concluded that Denniston began his journey on Monday in London and reached Warsaw on the following Tuesday, where, as is undisputedly known, he stayed at the Hotel Bristol . It would be logical that, possibly after a small talk in the hotel on Tuesday, the conference took place on Wednesday and then another day, Thursday, was added as the final day.

The renowned British historian and cryptologist Ralph Erskine therefore suspects July 26 and 27, 1939 to be the two days of the conference. Mavis Batey , a close colleague of Dilly Knox from BP, mentions July 26-27, 1939. The entry and exit stamps on Denniston's passport, which fortunately have also been preserved, fit this. The Polish stamps document July 25th as the day of entry and July 28th, 1939 as the day of departure and one page further in the passport you can see the German visa of July 28th, 1939 from the passport control in Neu-Bentschen station , the former border station between the German Reich and the Republic of Poland , obviously from his return journey when crossing the Polish-German border (see also: passport under web links ).

In a report written shortly after the war, Denniston noted Wednesday, July 26, 1939 as the most important day of the meeting: "The 26th (Wednesday) was The Day." Denniston also explains (with regard to times and means of transport differently than David Kahn presented ): "Knox & I went by train as we wished to see Germany probably for the last time, the other two separately by air. We left on the 24th & were met by the Poles & Bertrand & lodged at The Bristol. We were there for work on the 26th & 27th & leaving on the 28th. I was back in London by Sunday the 30th. ”(German:“ Knox and I took the train because we probably wanted to see Germany for the last time, the other two came separately by plane. We left on the 24th and met the Poles and Bertrand and stayed at the Bristol . We had our work meetings on the 26th and 27th and left on the 28th. I was back in London on Sunday the 30th. ")

course

The Kabaty forest in the snow
In the Polish Enigma replica, of which at least 15 were made in the mid-1930s, buttons (1), lamps (2) and sockets (7), like the German Enigma-C , were simply arranged alphabetically.
The three young Polish cryptanalysts
Rejewski, Różycki and Zygalski (approx. 1928–1932)

In his report “How News was Brought from Warsaw at the end of July 1939”, which was written shortly after the war and which is published as Appendix 1 in Ralph Erskine's Cryptologia article , Alastair Denniston describes the course of the meeting. According to this, the participants were picked up by their Polish hosts at 7:00 a.m. on the morning of July 26th and driven to the nearby Kabaty Forest, about 20 km south of their hotel, to a well camouflaged and guarded military facility. What Denniston did not know at the time was the newly established headquarters of Section BS4, responsible for Germany, of the Polish Cipher Office (Polish code name Wicher , German "Sturm"), which was located in the Saxon Palace (Polish: Pałac Saski ) until 1937 . was housed in the middle of Warsaw.

After the formal greeting by the superiors of the BS, by Langer and by the "Grand Chef" Colonel Mayer, the boss of BS4, Maksymilian Ciężki, gave a detailed lecture of about three hours on the Polish methods of deciphering the Enigma used by the German army . Possibly because of language problems or because of too many details, the explanations given were difficult to understand for the guests. Even Dilly Knox, who, as a proven deciphering expert with many years of experience with the Enigma, should have followed suit, was anything but amused, while his boss, Alastair Denniston, openly admitted that he had not understood anything. A subsequent visit to the basement, where the Poles kept their Enigma replicas and operated electromechanical deciphering machines, especially the Bomby , did little to change this . Knox kept his iron face and said nothing. It was only on the way back in the car that his anger broke out when, on the assumption that no one could speak English, he cursed that the Poles were playing with the wrong cards and that what they portrayed as successes in deciphering was actually based on documents they had obtained through espionage or treason. The situation became more and more embarrassing for Denniston as Bertrand, who did not understand English, of course also saw that Knox was obviously angry about a subject where the British had failed but the Poles were successful. Denniston also remembers the rest of the day as a real nightmare . Knox remained aloof and silent while Denniston discussed the situation extensively with Bertrand and Sandwith. They decided to leave as early as possible the next day.

But the next day turned out differently than expected. After the more formal first day, there were now a number of informal working meetings and intensive discussions among colleagues. The discussions between Dilly Knox and the three young Polish cryptanalysts, Marian Rejewski, Jerzy Różycki and Henryk Zygalski were particularly fruitful. Not only did they get on very well professionally, they also quickly got closer to each other on a human level. This is how Knox, who on that day was the opposite of himself from the day before, earned the hearts and admiration of his much younger Polish colleagues. Conversely, he benefited from her direct and informal manner and learned things about the Enigma and its weaknesses that he had never suspected.

Since the early 1930s, Knox had tried in vain to find out the unknown wiring sequence of the entry roller of the machine used by the German military, which was important for the intended deciphering of the Enigma. He was aware that the Germans had simply connected the entry roller to the keyboard for the civilian Enigma in the order in which the letters appear on the keyboard , i.e. the first contact of the entry roller with Q, the second with W, the third with E and so on. With the Enigma I used for military purposes, however, as he quickly recognized, this sequence did not apply. The Germans had obviously decided on a different wiring order. The big question was for which one? Since this problem was of paramount importance for the envisaged deciphering of the Enigma, Dilly, who was known to his colleagues in Bletchley Park for creating strange but concise terms, coined the name as an abbreviation for the unknown wiring sequence of the entry roller of the military Enigma I Name " QWERTZU " according to the order of the first seven letters on the Enigma keyboard.

 Q   W   E   R   T   Z   U   I   O 
   A   S   D   F   G   H   J   K 
 P   Y   X   C   V   B   N   M   L 

As a synonym for QWERTZU, the term “diagonal” (English for “diagonal”) was also used in BP . This also meant the sequence of letters in which the individual letter keys of the Enigma I were connected to the entry roller.

Dilly Knox herself worked on this problem for many years, though not exclusively, until 1939 without finding a solution. Nor did his BP colleagues Tony Kendrick , Peter Twinn and even the great Alan Turing come up with the wiring chosen by the Germans, which in view of the immense number of possibilities for the QWERTZU did not surprise anyone. None of the cryptanalysts mentioned was even remotely suspected of being an “unimaginative dullard” . On the contrary, they were all valued at BP as extremely intelligent and creative minds. Still, none of them found the right QWERTZU.

Dilly Knox only obtained this important information through Marian Rejewski, after he had first asked him the question (according to Mavis Batey in French): "Quel est le QWERTZU?" (German: "What is the QWERTZU?" What is the wiring sequence for the entry roller? ”). This had tormented him for so long. Rejewski's answer was ingeniously simple: "ABCDEFG ..."

That simple answer must have hit Knox almost like a blow. It is said that he initially reacted angry and probably angry with himself. However, his mood later turned into the opposite, into euphoria . The English codebreaker Peter Twinn , who also worked for BP, reports that Dilly, after driving back from the meeting with Bertrand to the hotel in Warsaw, enthusiastically sang in French: “Nous avons le QWERTZU, nous marchons ensemble” (German: “ We have the QWERTZU, we will march together ”).

In retrospect, of course, the solution seems ridiculously simple, but from the point of view of the British specialists it was also the stupidest of all the options the Germans had chosen. With an astronomically large number of more than 400 quadrillion possibilities, the German cryptographers actually simply selected the usual alphabetical order as QWERTZU. From the British point of view, this was so absurd and so stupid, and therefore so unlikely, that they had never seriously considered this case. Marian Rejewski, on the other hand, had already guessed this order intuitively and correctly in 1932, thus creating the basis for the historically significant Allied Enigma decipherments (code name: " Ultra ") during the Second World War.

consequences

The concept of the British Turing bomb (here a replica in Bletchley Park, operated by a " Wren " ) goes much further than that of the Bomba

For the British Codebreakers , the wide range of assistance and the boost they received from their Polish allies were undoubtedly extremely valuable, possibly even decisive, in order to “get off the starting blocks” in the first place. In particular, the knowledge of the wiring of the Enigma rollers and the functionality and structure of the Bomba was extremely important and helpful for the British. The English mathematician and cryptanalyst Gordon Welchman , who was one of the leading minds of the British code breakers in Bletchley Park, expressly praised the Polish contributions and assistance by writing: “... had they not done so, British breaking of the Enigma might well have failed to get off the ground. " (German:" ... had they [the Poles] not acted like this, the British break of the Enigma might not have gotten off the starting blocks at all. ")

Shortly after the Pyry meeting, also in 1939, and undoubtedly inspired by the extremely valuable Polish information, the English mathematician and cryptanalyst Alan Turing invented the Turing bomb named after him . Shortly afterwards, this was improved significantly by his compatriot and colleague Welchman through the invention of the diagonal board (German: "Diagonal board "). Both the Polish word "bomba" and the French word "bombe" used by the British mean the same thing in English, namely "bomb" (German: "bomb"). However, it would be wrong to conclude from the similarity of names of the two cryptanalytic machines and the close technical and chronological connection that the Turing bomb was little more than a slightly modified British replica of the Polish bomba . On the contrary, the cryptanalytic concept of the British bomb differs significantly from that of the Bomba and goes far beyond that. Apart from the name and the same target as well as the technical commonality of using several Enigma roller sets within the machine and running them through all 17,576 possible roller positions, there are hardly any similarities between the Polish and the British machine.

The decisive disadvantages of the Bomba , which Turing deliberately avoided during its development, were its dependence on the German procedural error of the duplication of the slogan key and on as many unplugged letters as possible. After the Germans fixed these errors, the bomba was useless. The British bomb, on the other hand, did not depend on the duplication of the spell key and could therefore continue to be used without restriction until the end of the war, even after Turing had foreseen the dropping of the spell key duplication. In addition, a decisive advantage of the British concept and another important difference to the Polish approach was the ability of the bomb to completely effect the plug board by linking several (usually twelve) Enigma roller sets in a ring and with the help of cribs ( probable words assumed in the text ) to be able to shed. In contrast to the bomba , which became less and less effective as the number of plugs increased, the British bomb would still have been able to identify keys even if the Germans had plugged in all 26 letters (with the help of 13 double cords) - which they never did incorrectly and not a single letter would have been left unplugged.

Without the Pyry meeting and the willingness of the Poles to pass on all their knowledge to their allies, the British might not have succeeded in deciphering the German Enigma machine, which is essential for the war effort. One can only speculate about the possible consequences of such an "unbreakable" or at least unbroken Enigma on the course of the war or even world history, because history does not reveal its alternatives (see also: Historical Consequences in the Enigma article ).

See also

literature

  • Friedrich L. Bauer : Deciphered Secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, ISBN 3-540-67931-6 .
  • Gustave Bertrand : Énigma ou la plus grande enigme de la guerre 1939–1945 . Librairie Plon, Paris 1973.
  • Chris Christensen: Review of the IEEE Milestone Award to the Polish Cipher Bureau for “The First Breaking of Enigma Code” . Cryptologia . Rose-Hulman Institute of Technology. Taylor & Francis, Philadelphia PA 39.2015,2, pp. 178-193. ISSN  0161-1194 .
  • Ralph Erskine : The Poles Reveal their Secrets - Alastair Dennistons's Account of the July 1939 Meeting at Pyry . Cryptologia. Rose-Hulman Institute of Technology. Taylor & Francis, Philadelphia PA 30.2006,4, pp. 294-395. ISSN  0161-1194 .
  • John Gallehawk: Third Person Singular (Warsaw, 1939) . Cryptologia. Rose-Hulman Institute of Technology. Taylor & Francis, Philadelphia PA 3.2006,3, pp. 193-198. ISSN  0161-1194 .
  • Marek Grajek : An Inventory of Early Inter-Allied Enigma Cooperation. Proceedings of the 1st International Conference on Historical Cryptology, PDF; 12.5 MB 2018, pp. 89–94.
  • Francis Harry Hinsley , Alan Stripp: Codebreakers - The inside story of Bletchley Park . Oxford University Press, Reading, Berkshire 1993, ISBN 0-19-280132-5 .
  • David Kahn : Seizing the Enigma - The Race to Break the German U-Boat Codes, 1939-1943 . Naval Institute Press, Annapolis, MD, USA, 2012, pp. 92f. ISBN 978-1-59114-807-4 .
  • Marian Rejewski and Henryk Zygalski : Brief description of the dissolution methods. Bertrand Archive SHD DE 2016 ZB 25/6, Dossiers Nos. 281 and 282, ca 1940.
  • Gordon Welchman : The Hut Six Story - Breaking the Enigma Codes . Allen Lane, London 1982; Cleobury Mortimer M&M, Baldwin Shropshire 2000, ISBN 0-947712-34-8 .

Web links

Individual evidence

  1. Simon Singh: Secret Messages . Carl Hanser Verlag, Munich 2000, p. 199. ISBN 3-446-19873-3 .
  2. ^ Marian Rejewski: An Application of the Theory of Permutations in Breaking the Enigma Cipher . Applicationes Mathematicae, 16 (4), 1980, pp. 543-559. Accessed: April 15, 2015. PDF; 1.6 MB
  3. a b Friedrich L. Bauer: Deciphered secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, p. 412.
  4. ^ Gordon Welchman: The Hut Six Story - Breaking the Enigma Codes . Allen Lane, London 1982; Cleobury Mortimer M&M, Baldwin Shropshire 2000, p. 207. ISBN 0-947712-34-8 .
  5. ^ Gordon Welchman: The Hut Six Story - Breaking the Enigma Codes . Allen Lane, London 1982; Cleobury Mortimer M&M, Baldwin Shropshire 2000, p. 16. ISBN 0-947712-34-8 .
  6. ^ Hugh Sebag-Montefiore: Enigma - The battle for the code . Cassell Military Paperbacks, London 2004, p. 49. ISBN 0-304-36662-5 .
  7. Marek Grajek: An Inventory of Early Inter-Allied Enigma Cooperation. Proceedings of the 1st International Conference on Historical Cryptology, PDF; 12.5 MB 2018, p. 89.
  8. a b c Friedrich L. Bauer: Deciphered secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, p. 421.
  9. ^ A b David Kahn: Seizing the Enigma - The Race to Break the German U-Boat Codes, 1939-1943 . Naval Institute Press, Annapolis, MD, USA, 2012, p. 92. ISBN 978-1-59114-807-4 .
  10. National Archives : Name Sandwith, Humphrey Robert Date of Birth: 19 June 1894 Rank: Captain ... (English). Retrieved June 9, 2015.
  11. John Gallehawk: Third person singular (Warsaw, 1939) . Cryptologia. Rose-Hulman Institute of Technology. Taylor & Francis, Philadelphia PA 3.2006,3, p. 198.
  12. Gustave Bertrand: Énigma ou la plus grande enigme de la guerre 1939-1945 . Librairie Plon, Paris, 1973, p. 59.
  13. ^ Marian Rejewski and Władysław Kozaczuk : Summary of Our Methods for Reconstructing Enigma and Reconstructing Daily Keys and of German Efforts to Frustrate Those Methods. In Enigma - How the German Machine Cipher Was Broken and How It Was Read by the Allies in World War Two . University Publications of America, Frederick, MD and Arms and Armor Press, London, 1984, p. 269.
  14. David Kahn: Seizing the Enigma - The Race to Break the German U-Boat codes 1939 -1943 . Naval Institute Press, Annapolis, MD, USA, 2012, p. 93. ISBN 978-1-59114-807-4 .
  15. ^ Marian Rejewski: 'Enigma' 1930–1940 Metoda i historia rozwizania niemieckiego szyfru maszynowego (w zarysie) . Accessed: April 15, 2015. Rejewski's report on the Enigma (1940) (Polish)
  16. ^ Kris Gaj, Arkadiusz Orłowski: Facts and myths of Enigma: breaking stereotypes. Eurocrypt, 2003, p. 9.
  17. ^ Francis Harry Hinsley, Alan Stripp: Codebreakers - The inside story of Bletchley Park . Oxford University Press, Reading, Berkshire 1993, plate 9. ISBN 0-19-280132-5 .
  18. Ralph Erskine: The Poles Reveal their Secrets - Alastair Dennistons's Account of the July 1939 Meeting at Pyry . Cryptologia. Rose-Hulman Institute of Technology. Taylor & Francis, Philadelphia PA 30.2006,4, p. 294.
  19. ^ Mavis Batey: Dilly Knox - A Reminiscence of this Pioneer Enigma Cryptanalyst. Cryptologia, Rose-Hulman Institute of Technology. Taylor & Francis, Philadelphia PA 32.2008,2, p. 116.
  20. ^ AG Denniston: How News was Brought from Warsaw at the end of July 1939 . PRO 25/12, 1948.
  21. a b Ralph Erskine: The Poles Reveal their Secrets - Alastair Dennistons's Account of the July 1939 Meeting at Pyry . Cryptologia. Rose-Hulman Institute of Technology. Taylor & Francis, Philadelphia PA 30.2006,4, p. 300.
  22. ^ Krzysztof Gaj: Polish Cipher Machine -Lacida . Cryptologia . Rose-Hulman Institute of Technology. Taylor & Francis, Philadelphia PA 16.1992,1, ISSN  0161-1194 , p. 74.
  23. ^ Alastair Denniston: How News was Brought from Warsaw at the end of July 1939 . PRO 25/12, 1948.
  24. Ralph Erskine: The Poles Reveal their Secrets - Alastair Dennistons's Account of the July 1939 Meeting at Pyry . Cryptologia. Rose-Hulman Institute of Technology. Taylor & Francis, Philadelphia PA 30.2006,4, pp. 297-301.
  25. Tony Sale: The Bletchley Park 1944 Cryptographic Dictionary . Publication, Bletchley Park, 2001, p. 28. Accessed: April 15, 2015. PDF; 0.4 MB
  26. a b Peter Twinn: The Defense Enigma in Francis Harry Hinsley, Alan Stripp: Codebreakers - The inside story of Bletchley Park . Oxford University Press, Reading, Berkshire 1993, p. 127. ISBN 0-19-280132-5 .
  27. ^ Mavis Batey: Dilly Knox - A Reminiscence of this Pioneer Enigma Cryptanalyst. Cryptologia, Rose-Hulman Institute of Technology. Taylor & Francis, Philadelphia PA 32.2008,2, pp. 104-130.
  28. Peter Twinn: The Abwehr Enigma in Francis Harry Hinsley, Alan Stripp: Codebreakers - The inside story of Bletchley Park . Oxford University Press, Reading, Berkshire 1993, p. 126. ISBN 0-19-280132-5 .
  29. ^ Hugh Sebag-Montefiore: Enigma - The battle for the code . Cassell Military Paperbacks, London 2004, p. 42. ISBN 0-304-36662-5 .
  30. Michael Smith: Enigma decrypted - The "Codebreakers" from Bletchley Park . Heyne, 2000, p. 40. ISBN 3-453-17285-X .
  31. Michael Smith: Enigma decrypted - The "Codebreakers" from Bletchley Park . Heyne, 2000, p. 39. ISBN 3-453-17285-X .
  32. ^ Gordon Welchman: The Hut Six Story - Breaking the Enigma Codes . Allen Lane, London 1982; Cleobury Mortimer M&M, Baldwin Shropshire 2000, p. 219. ISBN 0-947712-34-8 .
  33. ^ Kris Gaj, Arkadiusz Orłowski: Facts and myths of Enigma: breaking stereotypes. Eurocrypt, 2003, p. 11.
  34. ^ Hugh Sebag-Montefiore: Enigma - The battle for the code . Cassell Military Paperbacks, London 2004, pp. 381f. ISBN 0-304-36662-5 .
  35. ^ Polish contributions to the break of the Enigma. Retrieved April 24, 2015.
  36. ^ Gordon Welchman: The Hut Six Story - Breaking the Enigma Codes . Allen Lane, London 1982; Cleobury Mortimer M&M, Baldwin Shropshire 2000, p. 204. ISBN 0-947712-34-8 .
  37. ^ Francis Harry Hinsley, Alan Stripp: Codebreakers - The inside story of Bletchley Park . Oxford University Press, Reading, Berkshire 1993, pp. 11ff. ISBN 0-19-280132-5 .