by Alastair Denniston- Code-breaking From Room 40 to Berkeley Street
GC&CS was changing from two groups working on diplomatic cryptographic systems as a sideline to their Service work, to one which developed Service sections as an adjunct to its diplomatic work. Traffic Section was responsible for the intake and sorting of intercepts. Distribution and Reference Section edited and distributed decrypts translated by cryptanalysts and kept in contact with users to assess their needs. The Civil side in 1939 outnumbered all of the Service sections combined and consisted of a large number of cryptanalytic units, ranging from one to fifteen staff. The Head of each continued to report directly to AGD. Some of the units carried out cryptanalytic research and others operated as a centre for the production of decrypt intelligence and unit boundaries were flexible. A Commercial Section with six staff was set up in 1938 to scan and select from a mass of intercepted correspondence, mainly in plain language or public commercial codes. This was passed to a centre in London which eventually became the Ministry of Economic Welfare. However, the civil side of GC&CS was functionally far more limited than the military side. Its sole functions were cryptanalysis and translation. It had little involvement with traffic analysis or intelligence and even interception presented few problems, as the most reliable source of supply were the copies of cable traffic that had been handed in for transmission and held by the Post Offices and Cable Companies after they had been sent.
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On 7 December 1931, a representative of the Polish cipher department met a French intelligence officer at the Central Station in Warsaw. The officer, Captain Gustave Bertrand, carried with him operational instructions for the Enigma machine being used by the German military. The Gebrauchanleitung für die Chiffriermaschine Enigma also contained four drawings of the machine. A further document in his possession, the Schlüsselanleitung, provided vital information about the various settings of the machine. His source for these documents was a German by the name of Hans-Thilo Schmidt, who had first contacted the intelligence representative at the French Embassy in Berlin on 8 June 1931. Through his brother Rudolf, Schmidt had been given a job in the German Defence Ministry’s Cipher Centre, the Chiffrierstelle, known as the ChiStelle. Rudolf Schmidt had been the previous head of the ChiStelle and, ironically, had approved the Enigma machine for use by the German Army. This was the very machine which his brother had now betrayed for no other reason than money to fund his lavish lifestyle. The Deuxième Bureau’s Service de Renseignements had assigned one of its operatives, Rudolf Lemoine, to make contact with Hans-Thilo and they duly met on 1 November 1931 at the Grand Hotel in Verviers, a town in eastern Belgium around 15 miles from the German border. Bertrand attended a subsequent meeting on 8 November, along with a photographer. As well as the two Enigma documents, Schmidt produced an organisation chart of the ChiStelle, an army hand cipher and a memorandum on poison gas. Schmidt was duly paid 10,000 marks (about £41,000 today) and Bertrand returned to Paris. He handed them to Colonel Bassières, a top French cryptanalyst who, after analysing them for two weeks, told Bertrand that they were of little use to French intelligence. Bertrand now turned to British intelligence and on 23 November, handed copies to their Paris representative, Wilfred (Bill) Dunderdale. Three days later, Dunderdale also rejected the documents, as they would be of little use in decrypting Enigma messages. Thus it was that Bertrand got permission to offer the material to Polish intelligence and was subsequently greeted by them with open arms.73
Polish intelligence’s interest in the material from Hans-Thilo Schmidt was understandable, given the difficult historical relationship between Poland and Germany. At the end of WW1, the newly-reborn Polish state had taken part of Silesia, Pomerania and territory around Poznan, as it had been part of Poland before the partition by Frederick the Great. This was a cause of considerable anger in Germany, and an atmosphere of enmity and continual tension persisted, fuelled by the long history of numerous and bloody wars between Poland and Germany. The re-born Polish state felt seriously threatened by its neighbour to the west. On 15 July 1928, Polish radio-monitoring stations in Starogard, Gdanski, Poznan and Krakow-Krzeslawice had intercepted the first German messages which were in a machine-generated cipher. The following year a course in cryptology was started at the University of Poznan for twenty of its most advanced mathematics students who could also speak German. The course was set up at the initiative of the radio intelligence department and some of their specialist officers would give lectures to the students, the most gifted of whom would be asked to volunteer to continue their studies within Military Intelligence. Throughout 1931, a cipher bureau was created which was an amalgamation of the radio intelligence and cryptography sections. The new bureau was headed by Major (later Lieutenant-Colonel) Gwido Langer. As the new bureau was being organised in 1931, the course at Poznan was coming to an end, and three students had frequently managed to solve the German ciphers that had been set for them on the course. It was decided to set up a small section of the department for them in Poznan. Their names were Marian Rejewski, Hendryk Zygalski and Jerzy Różycki.
The German military had introduced a new version of an Enigma machine with a plugboard at the beginning of June 1930 and, the following year, the Poles purchased a commercial model on the open market in Germany. At the beginning of September 1931, Rejewski, Zygalski and Różycki were employed by Section BS 4 (German intelligence) of the Cipher Office of Department II of the General Staff in Warsaw. This section was under the command of Captain (later Major) Maksymilian Ciez’ki. Rejewski was the most advanced academically of the three young students, having just been awarded his degree in mathematics. He had also spent a further year studying the subject in depth at the University of Gottingen in Germany. He began work on his own in October 1932 with the commercial Enigma machine which had been purchased earlier. He was also receiving several dozen messages daily which had been encrypted on the military version of the Enigma machine. By December, he had a photograph of the military machine, user instructions for it and a schedule of daily keys for September and October 1932. Remarkably, by the end of December 1932, Rejewski had reconstructed the internal connections within the Enigma machine and identified the indicator system74 currently being used by the German Army. He had done it entirely using a mathematical technique called permutation theory. Along with significant contributions from Zygalski and Różycki, the first breakthrough in solving the German military Enigma machine had been achieved. The three young Polish cryptanalysts handed to their superior the first completely decrypted Reichswehr75 signal which had been encrypted using the military Enigma.
In early 1933, Rejewski, Zygalski and Różycki began working together and were successful for a number of years in reading encrypted German Army messages. After the GAF introduced Enigma at the end of 1934, they grouped radio operators who worked alongside Enigma operators into different radio networks. The number of these grew rapidly and the Poles had to monitor an increasing amount of traffic. By 1 February 1936, the Germans had introduced significant changes which improved the security of the Enigma machine. Each radio network had its own setting of the machine which changed every twenty-four hours. The following year, the Polish General Staff transferred their German cryptographic section BS4 to a camouflaged and high-security new headquarters in the Kabacki Woods near Pyry outside Warsaw. They developed technologies such as the Zygalski sheets, the cyclometer and the bomba to help work out the Enigma settings being used by operators on different networks. Up until December 1938, GAF and Army Enigma operators had three wheels for their Enigma machines. They would change the order of the wheels in the machine each day, and as each wheel was wired uniquely, this would in effect change the wiring in the machine. This meant that there were six possible wheel configurations and the Poles had built six bombas to speed up their work. On 15 December, the Enigma operators were given two additional wheels and could now choose three from a set of five uniquely wired. This increased the number of possible wheel configurations to sixty and the Poles would need a bomba for each additional configuration. They
simply did not have the resources to build fifty-four more machines. To make matters worse, the Germans had introduced a plugboard at the front of the Enigma machine with twenty-six sockets and a number of letters were plugged together. When they also increased the number of plugboard connections to ten, working out the daily settings of the Enigma machine became a daunting task indeed!
The deployment of the Enigma machine across all of its services was becoming an increasing concern to AGD. From 1937 onward, it was obviously desirable that British naval, military and air intelligence should get in very close touch with their French colleagues for political and military reasons. Sinclair had always been keen to maintain a close liaison between SIS and GC&CS and had established links with the French intelligence agency the Deuxième Bureau de l’État-major général, directed by Colonel Maurice-Henri Gauché. Sinclair’s deputy, Menzies, had a close relationship with Colonel Louis Rivet, the head of the French Army’s ‘2ième Bureau’ (the Services de Renseignements et de Contre-Espionnage militaire). Bertrand worked under Rivet but AGD believed that it was Dunderdale, although he had little knowledge of cryptography, who urged the British to liaise with the French on a technical level.76
AGD was concerned about how far he could go in collaborating with the French. He wrote to Sinclair on 2 November 1938, asking for guidance.
Liaison with the French
I should like to have your guidance as to the limits to which this liaison should go.
We have received about 100 documents of varying types and varying value:-
Three photographs of codes of which one might well have had great value in the event of war with Germany last month.
Photographs of documents relating to the use of the Enigma machine which did increase our knowledge of the machine and have greatly aided our researches.
Studies of the German Military and Air services.
A full description of the German Y Service.
Reconstructed German and Italian codes (unrecyphered).
Considerable amount of German and Italian intercepted telegrams.
The results of French D/F work.
We have given them purely cryptographic assistance in the shape of :-
Our reconstruction to date of the Italian recyphered and unrecyphered books.
Copies of two Italian recyphered books on which they set great store.
We propose to give them results of our Y work in the shape of:-
German Naval Call Signs.
A complete study of the German Air Force.
A complete study of the German Military.
It appears to us that their cryptographic work is less ambitious than ours. They have worked on the German and Italian unrecyphered codes with success and on the German Military hand cypher (double transposition).
As to their Y Service it appears:-
That the Navy works separately. I was informed by a Naval Officer liaison officer with ? that they relied entirely on their station at Dunkerque for interception and D/F.
We have received no Naval intercepts but the French version of situation report of the German Navy from time to time through? We have reciprocated with our version through Section III.
That the army, Y and D/F service is considerable; M/F interception is more productive that ours.
The Military Section report that the organisation is not so complete as here, no night watch being kept and no work done on Sundays.
That the Y watch on German Air Force is very poor (vide Air Section report).
That they have an organisation for watching N. Italy, Libya and to a certain extent Eritrea (vide reports from Italian Diplomatic Section and Military Section).
It is very remarkable that we have received no example of any interception in Spain, either German, Italian or Spanish, work which occupies a large part of our staff. It occurs to me that this may have been a special study for the Navy who will no doubt watch the Mediterranean and Bay of Biscay.
This may also account for the entire absence of reference to Italian Naval Cyphers. On my own work on French Naval Cyphers during the last two years I have come across references to interception, etc. which lead me to suppose that it is the French Navy and the Bureau de Chiffre of the Marine who carry out this side of the work.
Our main reason for seeking this liaison in the first place was the desire to leave no stone unturned which might lead to a solution of the Enigma Machine as used by the various German services. This is of vital importance for us and the French have furnished us with documents which have assisted us but we are still doubtful if success can be obtained without further documents. During the coming meetings we hope to show Mr. X. the lines on which we are working and make clear to him what other evidence we need in the hope that his agents may produce it.
We also hope to obtain from him more material prior to September 15th, 1938, as there was a slight prospect of a break-in on the military machine before their mobilisation change.
We shall continue to work on the machine and will naturally give Mr. X. any results we obtain.
An interchange of current intercepted material may be necessary, unless the French prefer to put the onus of research on us.
We can also continue to cooperate on the reconstruction of the two Italian codes. As yet there has been no discussion on Italian Naval and Military cyphers and on this question I should like to receive orders in view of the fact (a) that I suspect that the Italian Naval is being worked in another branch in Paris and (b) that the Foreign Office considered that cooperation on the Italian Military telegrams between Spain and Rome was, at that time, undesirable.77
In early January, Bertrand asked AGD to come to Paris to meet some Polish experts in cryptography. In the hope that such a meeting would help GC&CS with work on German and Soviet ciphers, AGD attended meetings on 9 and 10 January along with Tiltman, Knox and Hugh Foss.78 In attendance on the Polish side were Langer and Ciez’ki; on the French side Bertrand and a French cryptanalyst, Henri Braquenie. The meeting was shrouded in secrecy and mystery and began with a presentation by Ciez’ki on the results of the Polish research into the Enigma machine as used by the German services. The British delegation was not impressed, as Ciez’ki gave a long and pedantic account of results which Tiltman had already achieved in an hour’s work. AGD and his colleagues felt that the Poles had little to contribute and Knox went further, claiming that the Poles’ knowledge of the Wehrmacht Enigma was ‘nil’.79 Writing of the meeting in May 1948 (see Appendix 5), AGD soon realised that in fact the Poles had been vetting the British, and had instructions not to disclose any of their real work on Enigma unless the British and French could demonstrate that they had made comparable progress and were prepared to share it with them.80
While nothing more was heard from the Poles for six months, tensions between Poland and Germany were close to breaking point. In the middle of July, Langer sent an invitation, through Bertrand, for AGD and his colleagues to come to Warsaw. The invitation specifically included Knox, as both the Poles and the French knew that he was working on the Enigma machine as used by the German military. AGD was very reluctant to take Knox with him, given his temperamental nature. However, Sinclair instructed him to do so and after discussions with the DNI, AGD was also told to include Commander Humphrey Sandwith, Head of the Admiralty’s interception service, in his group.81 He wanted to talk to the Poles about placing a site in Poland to help with British direction finding efforts. Bertrand proposed that the second tri-lateral meeting be held in Warsaw from 24–27 July. Bertrand and Sandwith travelled by air to Warsaw but AGD and Knox travelled by train as they wished to see Germany, possibly for the last time for quite some while. The latter arrived on the 24th and were met by Bertrand and the Poles. They were lodged at the Hotel Bristol while the French stayed at the Hotel Polonia.
The Poles entertained their visitors to lunch at the Hotel Bristol and ironically the fairly banal conversation was conducted in German as it was the only common language of all in attendance. On
Wednesday, the 27th, the Poles called for AGD and Knox at 7.00 am and drove them to a clearing in the Kabacki woods about 20 kilometres from Warsaw. About a kilometre from the small town of Pyry, a clearing had been made and here lay the Polish Biuro Szyfrow’s secret headquarters which was partly underground. It was in the shape of a square with sides of about 200m surrounded by a high wall enclosing two brick buildings. The smaller of the two served as an air-raid shelter and radio station which was serviced by aerials which could be lowered to avoid detection.
The Poles had exploited a weakness in the Germans’ procedures for Enigma in the 1930s. Hendryk Zygalski had devised a method which exploited this weakness to help work out some of the settings used in the Enigma machine for any given day. It was based upon a catalogue of perforated sheets and it had the huge advantage that it was not compromised by the plug connections on the Enigma machine. As there were only six wheel orders in operational use at this time (the operator could mix up his three wheels in six ways), a complete catalogue contained twenty-six perforated sheets, one for each of the possible ring settings82 on the right-hand wheel. Thus, 6 x 26 = 156 sheets had to be manufactured. When the Germans introduced two additional wheels in December 1938, the number of wheel orders had increased from six to sixty and 1,560 sheets were required. The Poles had also developed two machines to help with their work. The cyclometer helped with the task of constructing a card index system containing information about all possible Enigma start positions that could have been used. The bomba had three pairs of Enigma wheel systems driven by an electric motor and exploited the same weakness as the Zygalski sheets. However, the increase in possible wheel orders and lack of resources meant that the bomba could no longer be used.