Zygalski sheets: Polish codebreaking and the role of reconstruction in the Top Secret exhibition at the Science Museum
The Top Secret: From ciphers to cyber security exhibition at various Science Museum Group sites from 2019 to 2022 explored the remarkable, little-known world of codebreaking, ciphers and secret communications from the trenches of the First World War to cyber security today. At the heart of the exhibition was the personal and technological story of codebreaking at Bletchley Park, the British centre for codebreaking and cryptanalysis during the Second World War, as well an acknowledgement of the vital pre-war contribution of the Polish Cipher Bureau.
Zygalski sheets, developed by Polish codebreaker and mathematician Henryk Zygalski in 1938, were a manual grid-based cardboard system used by the Polish Cipher Bureau and Bletchley Park to aid the decryption of German Enigma machine cipher messages. Lacking original artefacts and visual historical representation thereof, the Top Secret curator Dr Elizabeth Bruton turned to external experts on the Polish cipher bureau and codebreaking in the Second World War, Dr Dermot Turing (writer) and Jeremy McCarthy (volunteer at The National Museum of Computing (TNMOC)). The ensuing email conversation explored how the design and use of the Zygalski sheets could be reconstructed from the existing and sometimes contradictory evidence and sources. This conversation offers a rare insight into the research process and expert peer review behind an exhibition object and label. Secondly, we offer a reflection on museum practice, exploring how the resulting reconstruction was interpreted and displayed within the Top Secret exhibition alongside authentic historic artefacts from the Science Museum Group’s and GCHQ’s historic collections.
1. Introduction and context: about the Top Secret exhibitionhttps://dx.doi.org/10.15180/221702/002
The Top Secret: From ciphers to cyber security exhibition at various Science Museum Group (SMG) sites from 2019 to 2022 explored the remarkable and little-known world of codebreaking, ciphers and secret communications from the trenches of the First World War to cyber security today. It did this using hand-written documents, declassified files and previously unseen artefacts from the Science Museum Group’s and GCHQ’s historic collections, as well as those of other generous lenders (Science Museum, 2021).
The exhibition first opened at the Science Museum, London, in July 2019 to coincide with the centenary of GCHQ, which was initially founded in 1919 as the UK’s first professional peacetime signals intelligence agency, the Government Code & Cypher School (GC&CS). The remarkable 100-year history of GCHQ was used as a lens through which to look at the often life-and-death importance of codes, ciphers and secret communications and the dichotomy at the heart of signals intelligence: the importance of concealing messages while revealing those of adversaries.
At the heart of the exhibition was the rich personal and technological story of Bletchley Park, the British centre for codebreaking and cryptanalysis during the Second World War. But the opening showcase in this section focused on the vital pre-Second World War successes by mathematicians and codebreakers at the Polish Cipher Bureau – in particular Henryk Zygalski, Marian Rejewski and Jerzy Różycki – in breaking messages sent using the German military Enigma cipher machine. In July 1939, with the risk of German invasion seemingly imminent, the Poles shared this knowledge with their British and French allies for the first time. The methods developed by the Polish Cipher Bureau were a vital contribution to later codebreaking and cryptanalysis work at Bletchley Park, especially related to the German military Enigma cipher.
When the Top Secret exhibition first opened at the Science Museum, the display included an ‘Enigma Double’ machine, a reverse engineered German military Enigma cipher machine developed by mathematicians and codebreakers Henryk Zygalski, Marian Rejewski and Jerzy Różycki at the Polish Cipher Bureau. The ‘Enigma Double’ represented the vital pre-Second World War work of the Polish Cipher Bureau as well as the ‘Enigma Relay’, the Polish-Anglo-French collaboration in breaking Enigma during the Second World War. Generously lent by the Józef Piłsudski Institute, London, the ‘Enigma Double’ is one of only two known surviving examples and one of only four Polish Enigma copies that were produced in France after key staff from the famous Polish Cypher Bureau left Poland after the German invasion in September 1939 (Dowell, 2020). Shortly after the Top Secret exhibition closed at the Science Museum, in February 2019, the Piłsudski Institute donated the machine to the Museum of Polish History currently (as of early 2022) under construction at the Citadel in Warsaw, Poland (Dowell, 2020).
Thus, for the Top Secret exhibitions at the Science & Industry Museum (SIM) at Manchester and the National Science & Media Museum (NSMM) at Bradford, we needed to replace the ‘Enigma Double’ with interpretation that represented the pre-war contribution of the Polish Cipher Bureau as well as the Polish-Anglo-French collaboration in breaking Enigma. For this, we selected two pieces of interpretation: a Second World War military cap worn by Polish mathematician and codebreaker Henryk Zygalski, generously lent by the Bletchley Park Trust, and a reconstruction of the Zygalski sheet, a manual grid-based cardboard system used by the Polish Cipher Bureau and Bletchley Park to aid the decryption of German Enigma machine cipher messages and developed by Zygalski in 1938. As discussed in more detail in the conversation below, there are no surviving original Zygalski sheets nor contemporary representations thereof. Descriptions and reconstructions – sometimes contradictory – of the Zygalski sheets have been produced previously with, as of early 2022, two notable examples on display in museums or forming part of a museum collection: decryption sheets similar to Zygalski sheets used to decipher Enigma in the Computer History Museum collections and a demonstration of two perforated sheets on display at Bletchley Park Museum around 2004.
The publication of F W Winterbotham’s The Ultra Secret (1974) led to increased public interest and awareness of codebreaking activities before and during the Second World War, particularly efforts to break the German Enigma and Lorenz cipher systems (Winterbotham, 1974). However, Winterbotham’s text had a number of serious errors including, as cryptographic expert and historian David Kahn pointed out in his review of The Ultra Secret published in the New York Times in December 1974, ‘Winterbotham’s attributing the original Enigma solution to information from a Polish employee of the cipher machine factory cheats the Poles of credit for one of the great cipher solutions of history’ (Kahn, 1974). Details of the Poles’ great cipher solutions of history before the Second World War, including Zygalski’s contribution, began to be published from the mid-1970s onwards, initially in English and Polish. Three of the publications included descriptions of the Zygalski sheets, described in more detail in the next section of this article (Garliński, 1980; Rejewski, 1980; Kozaczuk, 1984).
In the following sections, we will provide a brief context to the Polish Cipher Bureau’s vital successes in breaking the German military Enigma cipher machine in the 1930s including how they shared this information with their British and French allies. Secondly, we will use a lightly edited email conservation between Top Secret curator Dr Elizabeth Bruton and external experts on the Polish cipher bureau and codebreaking in the Second World War, Dr Dermot Turing (writer) and Jeremy McCarthy (volunteer at The National Museum of Computing (TNMOC) to explore how the design and use of the Zygalski sheets could be reconstructed from the existing and sometimes contradictory text-based evidence and sources. Thirdly, we will offer a reflection on museum practice, exploring how the resulting reconstruction could be interpreted and displayed within the Top Secret exhibition alongside authentic artefacts from the Science Museum Group’s and GCHQ’s historic collections and presenting a wider discussion of reconstruction, reproduction and authenticity within museum displays.
2. Reconstructed Zygalski sheets: a conversationhttps://dx.doi.org/10.15180/221702/003
The following conversation began as a series of written email exchanges between the authors of this article: Top Secret curator Dr Elizabeth Bruton and external experts on the Polish cipher bureau and codebreaking in the Second World War, Dr Dermot Turing (writer) and Jeremy McCarthy (volunteer at The National Museum of Computing (TNMOC). The email correspondence has been lightly edited, annotated and reformatted as a conversation which explores how – in the absence of surviving original artefacts or visual historical representations thereof – the design and use of the Zygalski sheets could be reconstructed from the existing (and sometimes contradictory) evidence and sources and the developing scholarship, albeit limited, around Zygalski and the Zygalski sheets.
This conversation offers a rare insight into the collaborative co-production behind the display, interpretation and reconstruction of an artefact in a temporary museum exhibition. Most scholarship relating to museum and exhibition labels focuses on the label format and structure and resulting visitor engagement, treating exhibition labels as a finalised form. Instead, here we consider the co-production process, expert peer review, and research behind an exhibition object and label.
Before we begin, we must note the limits around the conversation. First, the conversation related to static 2D printed versions of the Zygalski sheets. The Top Secret exhibition had three key displays related to the science, technology and basic principles behind codes, ciphers, encryption and decryption and encryption keys: interpretative label text explaining codes, ciphers and encryption next to an audio-visual display showing the operation of an M209-B mechanical cipher machine and therefore looking at encryption, decryption and keys; the Puzzle Zone, where visitors could get ‘hands on’ with mechanical interactive puzzles exploring codes and ciphers and the STEM skills needed to break and create secure communications; and an audio-visual animation demonstrating the mathematics behind and use of public keys in modern communications. We did not want to duplicate these displays and so chose to replace a static historic artefact, the Enigma Double, with another historic artefact, Zygalski’s Second World War military cap, shown alongside the 2D reconstruction and interpretation of the Zygalski sheets. The reconstruction of the Zygalski sheets was thus one of many forms of interpretation – text, historic objects, audio-visual, mechanical interactives, facsimiles, reconstructions and reproductions – displayed throughout the exhibition to engage visitors with the science, technology and people engaged with codes, ciphers and encryption from the First World War to the present day.
Secondly, given the expertise of the peer reviewers, the conversation related to an accurate reconstruction of the Zygalski sheets. As such, the reflection on museum practice and the role of reconstruction, reproduction and authenticity within museum displays was beyond the scope of this conversation and instead has been included in the conclusion of this article. Lastly, the conversation took place in advance of the reconstructed Zygalski sheet going on display and so evaluation of visitor engagement with the display is beyond the scope of this article.
Elizabeth Bruton (EB): Thank you for participating in this discussion. As you know, the Science Museum’s Top Secret temporary exhibition was updated with new objects at the Science & Industry Museum (SIM), Manchester and the National Science & Media Museum (NSMM), Bradford in 2021 and 2022, in particular in the section of the Bletchley Park showcase dealing with Polish codebreaking before and during the Second World War. Dermot, can I start with you, please: can you please tell us something about the role of the Polish codebreakers in the breaking of the Enigma cipher?
Dermot Turing (DT): Of course. I am sure that many visitors to the exhibition will have heard of the German Enigma cipher machine and know that the codebreakers at Bletchley Park found a way of unravelling its secrets so that some of the enciphered messages being sent by the German military forces in the Second World War could be read by the Allies. It’s widely believed that the intelligence gathered from codebreaking, and Enigma in particular, shortened the war and saved many lives.
EB: The importance of the work which took place at Bletchley Park and the vital work delivered by the thousands of men and women who worked there from 1939 to 1945 is well documented and well known, especially since the opening of Bletchley Park Museum in 1993. In a 1993 Cambridge lecture Harry Hinsley, the author of the official history of British intelligence, stated that intelligence obtained from various forms of communications interception at Bletchley Park aka ULTRA shortened the war ‘by not less than two years and probably by four years – that is the war in the Atlantic, the Mediterranean and Europe’ (Hinsley, 1993). This has been much debated since, most notably by fellow Bletchley Park codebreaker Gordon Welchman and more recently by academic, historian and author of the authorised history of GCHQ Professor John Ferris. In this work Ferris both questioned Hinsley’s claims and use of counterfactualism and suggested a more proportional and balanced understanding of the impact of Bletchley Park in the Second World War in the wider context of military campaigns and resources and alternative sources of intelligence (Ferris, 2020; Welchman, 1986).
Another important and perhaps lesser-known aspect to the work at Bletchley Park is the knowledge shared by Polish codebreakers just a few weeks prior to the outbreak of the Second World War. Can you tell us more about this?
DT: Bletchley Park codebreakers were given an enormous boost, only five weeks before the war broke out, in the shape of a handover by a team of Polish codebreakers of everything they knew about Enigma to their British and French allies at the Pyry radio-intelligence centre in the Kabacki Forest near Warsaw in late July 1939. Here, they provided two of their reverse-engineered Enigma machines, also called the ‘Enigma Double’ and mentioned previously. One each to the French and British codebreakers. The ‘Enigma Double’ along with the methods and information provided by the Poles was immensely valuable because the British were – not to put too fine a point on it – fairly clueless about some basic things, like the internal wiring of the wheels of the German military Enigma machine. And they were certainly in no way able to start reading messages encrypted using the German military Enigma machines with a plugboard or ‘Steckerbrett’. The Polish contribution filled this huge hole in British knowledge and provided the groundwork on which Bletchley Park codebreakers built their efforts to decrypt Enigma messages.
EB: Can you be more specific? Who were the Polish codebreakers, and what did they contribute?
DT: At the heart of the team were three mathematicians, Marian Rejewski, Jerzy Różycki and Henryk Zygalski working at the Polish Cipher Bureau from the early 1930s onwards. Rejewski had managed to reverse-engineer the internal wiring of the wheels of the German military Enigma cipher machine back in 1932. But knowing how the machine works was only the start of the problem. There are over 150 million million million ways the machine can be set up for encryption, and if you don’t know which one has been chosen, you’ll never decipher a single message. The main contribution of Różycki and Zygalski was to invent ways to find out what the set-up was.
EB: In the Top Secret exhibition we displayed a reconstruction of a Zygalski sheet, one of the techniques to break German military Enigma messages, developed by Henryk Zygalski in 1938. (The German military generally believing their Enigma cipher machines to be unbreakable due to the millions of millions of possible settings.) However, as late Second World War and post-war Target Intelligence Committee (TICOM) interviews of key German figures in cryptology and SIGINT revealed, the German military was aware of the potential weaknesses and limitations of the German military Enigma machine and so continued to make improvements in the late 1930s and into the Second World War (Rezabek, 2011; 2012).
According to Rejewski, the German military made ‘extensive changes’ between 1936 and August 1938 and ‘the greatest changes’ between September 1938 and 1939 when additional wheels were introduced and so allowing an increased number of possible wheel combinations (Rejewski and Stepenske, 1981). In order to be able to figure out the daily settings for German military Enigma messages in 1938, 1939 and into wartime, new techniques were needed including the Zygalski technique. Additionally, Zygalski sheets provided the first insights into wartime Enigma and were valuable in spanning the gap between the development of the Polish Bomba before the Second World War and the introduction of the Turing-Welchman Bombe at Bletchley Park in 1940 (Christensen, 2017; 2018).
Jeremy, can you describe the technical operation of Zygalski’s technique for us please?
Jeremy McCarthy (JM): Zygalski’s method was to create perforated cardboard sheets. Each sheet represented a single position of the left-hand rotor of the Enigma machine, and the sheets were ruled into a 51 by 51 grid where the positions of the middle and right-hand rotors of the machine were on the vertical and horizontal axes of each sheet. Holes were cut into the sheets to represent rotor orientations where unusual coincidences occur during encipherment. Then the idea was that the sheets are stacked on top of each other, on top of a light-box, and if light shone all the way through the entire stack through a single hole, then that hole through which the light emerged could tell you which rotor orientation was probably being used.
EB: Can you give a bit more detail on how these coincidences could arise?
JM: At the beginning of the war, the Germans began their messages by enciphering a three-letter sequence twice over. These three letters were used to tell the receiving station how to set up the three rotors in their Enigma machine. So, the sender might need to tell the receiver to set their rotors to positions K, R and Y, and he’d encipher KRY twice over on his Enigma machine. This produced an apparently random sequence like JMPLGB, but sometimes a repeat could crop up in positions 1 and 4, or 2 and 5, or 3 and 6, for example ZWVZEH, which showed a repeat in positions 1 and 4. Repeats like this could only happen if the three rotors started off in very specific orientations.
EB: So what do we know about the creation of Zygalski’s sheets?
DT: We know the creation of Zygalski sheets was an immensely tedious exercise, not just to map the rotor combinations which could give rise to these repeats, but then to cut out the holes. Since there were five rotors in use by the time the war started, that meant 1,560 sheets needed to be prepared, each of which had several hundred little holes.
JM: Indeed, and each of those sheets would have had needed somewhere between 900 and 1000 holes to be cut out.
EB: Presumably they used a machine to do it?
DT: Not in the beginning. Bletchley Park used a similar technique called Jeffreys sheets and there are stories of people at Bletchley Park trying to do it with razor blades, which must have been a bit dangerous. But they did get a machine eventually, because cutting them out by hand for that many sheets was too slow.
EB: On 5 September 1939 and just four days after the German Army invaded Poland, Zygalski, Rejewski and Różycki were ordered to leave Warsaw on a special train and destroyed all the evidence that they were working in the Enigma machine so that the German invaders would not learn about their work. They continued their work in France and later Britain but no documented examples of Zygalski sheets or Bletchley Park’s Jeffrey sheets have survived nor indeed any contemporary visual representations of what they looked like.
Despite this, there are pictures of Zygalski sheets in various books and journal articles with circular holes with the most prominent and earliest visual example being Marian Rejewski’s example (shown above) published in the Polish-language journal Applicationes Mathematicae in 1980 and in the English-language journal Cryptologia in 1982, as well as Józef Garliński’s figure of a perforated sheet from The Enigma War published in 1980 (Rejewski, 1980; Rejewski and Stepenske, 1982; Garliński, 1980).
While Rejewski was probably the most prominent Polish codebreaker active before and during the Second World War, his example showing rectangular sheets with round holes was produced about 30 years after the Zygalski sheets were made and had some differences from contemporary written descriptions of the Zygalski sheets and how they were used. Nonetheless, do you think they are accurate?
JM: As they were cutting out the holes using razor blades, I don’t think the holes would be round. Moreover, as I believe the holes to have been square, and as they were arranged in a 51 by 51 grid, I’d expect that the sheets themselves would be close to square.
DT: I agree with Jeremy. When Alan Turing went to Paris in early 1940 to give a complete set of sheets to the Poles – who had been evacuated there after the fall of Poland – Zygalski asked him why the size of the holes was different from what they were used to. He explained that the difference was because the British were using Imperial rather than metric units, and the holes were one-third of an inch on each side. I think that rather tips the scales in favour of square holes and so in favour of Józef Garliński’s perforated sheet from The Enigma War (1980), based on his correspondence with many of the key figures from the Polish Cipher Bureau still alive in the late 1970s, including Rejewski.
EB: Given this contemporary evidence from Alan Turing in 1940 as well as the discussion of past use and methods, I am satisfied that square holes were the method in question. Additionally, it is worth noting that Rejewski describes the figure showing round holes as ‘more or less’ showing how the Zygalski sheets looked rather than claiming complete accuracy and authenticity 30 years after the Zygalski sheets were first produced (Rejewski and Stepenske, 1982). As such, the scale reproduction shown above with square holes is the visual representation we used printed on the wall above the showcase discussing the work of Polish codebreakers in the Top Secret exhibition in 2021 and 2022.
Thank you very much to both for your in-depth knowledge and help with enabling us to put the Zygalski technique into the exhibit.
3. Conclusion including reconstruction, reproduction and ‘authenticity’ in museumshttps://dx.doi.org/10.15180/221702/004
In the conversation above, we discussed how careful and in-depth historic research, peer review, contemporary descriptions, and a deep understanding of the method of production and cryptologic usage of the Zygalski sheets underpinned the reconstruction and interpretation produced for the Science Museum Group Top Secret: From ciphers to cyber security exhibition at the Science & Industry Museum (SIM), Manchester and the National Science & Media Museum (NSMM), Bradford in 2021 and 2022.
This activity of reconstruction acts as an acknowledgement and evidence of how, in the absence of surviving material culture and detailed contemporary visual records, a past object’s design and use can be reconstructed from existing and sometimes contradictory text-based evidence, sources and in-depth discussion of historic methods. This activity also demonstrates the importance of an ‘intellectual agenda’ and research underpinning the display of artefacts and interpretation in Science Museum Group exhibitions and museums more generally (Arnold, 2016).
In this concluding section of the article, we also wanted to reflect on museum practices around reconstruction, reproduction and interpretation alongside authentic historic artefacts on public display. In the case of Top Secret, there were over one hundred historic artefacts on display – some on public display for the first time – predominantly from the Science Museum Group’s and GCHQ’s historic collections, as well as other institutions and private lenders.
The exhibition featured multiple forms of interpretation – artefacts, images, audio-visual content, facsimiles, and a single reconstruction: the Zygalski sheet. Physical artefacts were the most numerous forms of interpretation in the exhibition and were little changed across the three exhibition locations. Some new artefacts about Alan Turing’s post-war career in, and contribution to, early British computing were added to the exhibition for the Science & Industry Museum (SIM) in Manchester and the National Science & Media Museum (NSMM), Bradford (Wilson, 2021). With the exception of the ‘Enigma Double’ being replaced by Zygalski’s military cap and the Zygalski sheet reproduction, artefacts were replaced because of conservation, display and light level limitations for paper material. Where possible, we tried to replace historic documents with equivalent items but, in a few examples, facsimiles were created where no equivalent historic document could be obtained, and this was clearly indicated in the object label and text.
There was a clear visual and interpretative difference between the different forms of exhibit. A few artefacts such as a Second World War dispatch motorbike on loan from the Royal Signals Museum; Christopher Baker’s Murmur Study artwork; and a 2012 GCHQ commemorative quilt made by GCHQ staff were on open display behind barriers. However, most artefacts alongside a few facsimiles (the latter clearly labelled) were on display in museum showcases. In contrast, images and audio-visual content were displayed outside of showcases – in screens and printed on walls and on labels.
We wanted to make it visually and interpretatively clear that the Zygalski sheet reconstruction was not an authentic artefact or facsimile (accurate reproduction) and so chose to display it above rather than within the relevant museum showcase. This was also different to how reproduced (not reconstructed) objects – sometimes historic artefacts in their own rights – are displayed elsewhere in the Science Museum.
The display and interpretation of the Zygalski sheet reproduction also fitted into wider discussions around replicas, reproductions, reconstructions and the ‘contested and controversial concept’ of authenticity in terms of museum practice around museum displays, interpretation and reconstructing experimental and technological practices and experiences. Heering and Everrett respectively have explored how replication can be used to analyse scientific experiments and recover related scientific practices with case studies of solar microscopes from the Deutsches Museum, Munich and Bell and Blake’s 1874 ear phonautograph from the Science Museum collections respectively (Heering, 2017; Everrett, 2019). More generally, Hjalmar et al, Stols-Witlox, and Staubermann have considered how the role of reproducing or reworking historical processes including production processes can be both a beneficial tool for historians of science and technology and play an increasingly prominent role in terms of education and public engagement in classrooms and museums (Hjalmar et al, 2016; Staubermann, 2011; Stols-Witlox, 2020). In contrast, Auslander et al question how or indeed if historic reproduction can be used to fill the gaps in ‘material culture’ (Auslander et al, 2009).
On balance, the scholarship seems to lean towards careful, well-researched and contextualised reconstruction and reproduction having a beneficial role to play. This can assist historic and collections-based research and aid the recovery of material culture and related historic processes and practices; as well as play a pedagogical and public engagement role in museums, public displays and interpretation, and educational spaces. We believe our research, reconstruction, and associated interpretation of the Zygalski sheet has provided the aforementioned general benefits as well as contributing new research insights into the ‘mathematical’ turn in Polish codebreaking before and during the Second World War and highlighting the significance of this codebreaking to the work done at Bletchley Park.
The Top Secret exhibition was supported by principal funder DCMS and principal sponsor Raytheon, with media partner The Telegraph. Thanks go to the Józef Piłsudski Institute, London, especially their curator Dr Olga Topol, for the generous loan of the ‘Enigma Double’ for Top Secret at the Science Museum, London and to the Bletchley Park Trust for their generous loan of Henryk Zygalski’s Second World War military cap for Top Secret at the Science & Industry Museum (SIM), Manchester and the National Science & Media Museum (NSMM), Bradford.
Dr Elizabeth Bruton would like to thank: the two anonymous reviewers and Tony Comer and Dr David Abrutat, former and current Departmental Historians at GCHQ, for their helpful comments and suggestions; her former Science Museum colleagues especially John Liffen and Richard Dunn for discussions around reconstruction and reproduction in museum displays; and her wife, Camen Lei, for her generous support, proofreading, and plentiful cups of tea.