Breaking Enigma

The Poznań course

At the turn of 1928 and 1929, Professor Zdzisław Krygowski of the University of Poznań, in agreement with the Second Department of the General Staff, organised a secret cryptology course for mathematics students. Classes were held in the evenings in the City Command building on Święty Marcin Street; the lecturers included Captain Maksymilian Ciężki.⁴

The participants included Marian Rejewski, Jerzy Różycki and Henryk Zygalski. According to Łukomski, eight graduates were later employed at the Poznań branch of the Cipher Bureau.^2,4,5

The Saxon Palace, December 1932

On 1 September 1932 the Poznań branch was closed, and the three mathematicians moved to Warsaw, to section BS-4, which dealt with German ciphers and was housed in the Saxon Palace.^1,4,5

On 8 December 1932 Captain Gustave Bertrand of French intelligence brought material on Enigma to Warsaw. Bertrand had earlier recruited an informant in Germany.^2,6

Still in December 1932, the three cryptologists broke the Enigma cipher. Between Christmas and New Year, Rejewski read the first messages.^1,2

Tools: from replica machines to the bomb

From 1934 the Cipher Bureau used a dozen or so reconstructed copies of Enigma; Section II had commissioned their construction from the AVA radio engineering works in Warsaw.²

Rejewski devised the card catalogue and the cyclometer, a device that made it possible to find the current key within a dozen or so minutes. Różycki devised the so-called clock method, which made it possible to determine the choice and setting of the rotor, and Zygalski the perforated sheets named after him, which made it possible to determine the order of the rotors.^1,2,5

According to the AW text on the decryption, when in September 1938 the Germans introduced further changes to the construction and operation of Enigma, Rejewski and his colleagues created the cryptologic bomb to break the cipher automatically. The ABW publication likewise links the bomb to a change in the way Enigma encoded messages and dates it to 1938.^1,2

Pyry, July 1939

The first meeting with the intelligence services of France and Great Britain took place in January 1939 in Paris. The second was held in Warsaw in July 1939.²

On 25 July 1939 the guests came to the Cipher Bureau centre in the Kabaty Forest near Pyry, code-named “Wicher” (Gale). On the Polish side those present included Gwido Langer, Maksymilian Ciężki, and Rejewski, Różycki and Zygalski; on the French side Gustave Bertrand; on the British side Alastair Denniston, head of the Government Code and Cypher School, and Dilly Knox.^2,3,7

The Poles demonstrated a copy of Enigma, the Zygalski sheets and the bombs, and the allies received reconstructed copies of the machine from the AVA works. The British and French realised that the Poles were far ahead both in the volume of material they had already read and in the construction of the bomb.^2,3,5

What happened next

According to GCHQ, the knowledge gained in Poland changed the British centre's approach to breaking Enigma. In January 1940 in Paris, Alan Turing met the Polish cryptologists. Discussions with Rejewski underpinned the design of the first British bombe, which was working at Bletchley Park before the German offensive in the West.³

After September 1939 the Polish cryptologists made their way through Romania to France. They worked at the French-Polish centre “P.C. Bruno” in Gretz-Armainvilliers and, after the fall of France, from October 1940 to November 1942, at the clandestine centre “P.C. Cadix” in Uzès.¹

The Cipher Bureau's archive was largely destroyed during the evacuation in September 1939.⁵

What is disputed

The sources differ on these details. We give both versions.

When was the Poznań branch set up: in 1929 or in September 1930? Łukomski and the Enigma Cipher Centre give 1929; the ABW publication gives September 1930.^1,4,8
When was the cyclometer built: in 1934 or in 1936? The ABW publication gives 1934; the AW text on the decryption gives 1936.^1,2
When was the cryptologic bomb built: in 1936 or in 1938? The ABW publication and the AW text on the decryption give 1938; the biography of Rejewski on the AW website gives 1936.^1,2,5
How many participants completed the cryptology course? The Foreign Intelligence Agency writes of 23 selected students, of whom three completed the course, and in its text on the decryption of 20 students; Łukomski writes of eight graduates employed at the Poznań branch.^2,4,5
How many days did the July 1939 meeting last? GCHQ gives 25–26 July, the Foreign Intelligence Agency 25–27 July. The visit to Pyry took place on 25 July.^2,3,7

Sources

Internal Security Agency, Marian Rejewski: in the service of Polish cryptology (ABW Publishing House, Wiązowna 2022, ISBN 978-83-964225-4-5) (in Polish)
Foreign Intelligence Agency, Enigma decryption
GCHQ, The Pyry Forest meeting
Grzegorz Łukomski, The Poznań beginnings of ‘Enigma’, ‘Mars. Problematyka i Historia Wojskowości’ vol. 10, Warsaw–London 2001 (reprint: Adam Mickiewicz University) (in Polish)
Foreign Intelligence Agency, They broke Enigma (in Polish, with English versions of the biographies)
GCHQ, Director GCHQ commemorates crucial pre-war Enigma information sharing meeting in Poland (2014)
dzieje.pl (Polish Press Agency and Polish History Museum), Sharing a secret (in Polish)
dzieje.pl (Polish Press Agency and Polish History Museum), P. Bojarski, head of the Enigma Cipher Centre: here, where we are now, the cryptologists Różycki, Zygalski and Rejewski worked (in Polish)

Checked: 4 November 2026