Bletchley Park 1

Bob: Alrighty, Hanne. So, you know about Hans-Thilo Schmidt, the achievements of the Poles, and how the Germans used the Enigma machine. I think we're ready to jump into the Enigma codebreaking efforts of World War II.

Hanne: Yippee!

Bob: We'll begin at Bletchley Park—a place in Britain closely associated with the Enigma codebreaking story.

Alice: Ah, yes, Bletchley Park. Nestled in the English countryside, in the town of Bletchley, Bletchley Park was a large estate. As a quick glimpse into its history, around 1883, Sir Herbert Leon, an English financier and politician, purchased the estate as a country retreat for himself and his family. He significantly expanded the existing house on the estate, transforming it into a mansion, which ended up becoming his primary residence.

Bob: The Leons' estate was always buzzing with activity, as hundreds of workers were employed to maintain the mansion and its grounds. These workers ranged from maids and cooks, who worked inside the mansion, to gardeners, who tended to the flowers, lawns, and fruit trees around the estate, to farm workers, who looked after the animals on the farm. There never seemed to be a dull moment at Bletchley Park.

Alice: This constant activity was amplified by the Leons' involvement in the local community. They hosted a variety of events at Bletchley Park—athletic competitions, funfairs, flower shows, and cake-baking contests. In fact, these events became so popular that special trains were arranged to run to Bletchley Railway Station so people from all over the country could attend.

Hanne: Wow! It sounds like Bletchley Park was the place to be.

Alice: It certainly was. The events at Bletchley Park became the highlight of the year for many who attended.

Bob: Sir Herbert Leon lived at Bletchley Park until his death in 1926. After, his wife, Fanny Leon, continued to manage the estate and stay active in the community until her death in 1937. Then, a builder, Captain Hubert Faulkner, purchased the property with plans to develop it into a housing estate. However, in May of 1938, a high-ranking British intelligence officer, Admiral Sir Hugh Sinclair, bought the mansion and part of the estate to set up Britain's codebreaking centre.

Hanne: Interesting. Why did British intelligence choose Bletchley Park to set up their codebreaking centre?

Bob: Well, several factors likely influenced their decision. First, Bletchley Park was a secluded country estate, which provided the privacy and isolation needed for the mentally demanding work of codebreaking. Then, there was the fact that Bletchley Park had plenty of space for expansion—not only was there the mansion, but there were also vast amounts of empty land. This meant that British intelligence could quickly and easily build new facilities as the codebreaking operations grew. But perhaps the determining factor was its location. Bletchley Park was far enough from Britain's capital, London, to avoid the brunt of any expected military action, yet it was close enough to facilitate easy communication with key military units. It was also close to Bletchley Railway Station, which provided quick and easy transport in and out of Bletchley Park.

Alice: On top of that, Bletchley Park was strategically located between two of Britain's top universities, where many of the codebreakers were expected to come from. This made it easier to recruit key personnel and quickly get operations up and running.

Bob: Right. British intelligence had already begun contacting academics from these two prestigious universities, informing them of the kind of codebreaking work they might be needed for if war broke out. And some even began working part-time for British intelligence before the war.

Alice: Indeed. It was this early communication that allowed the first set of codebreakers to report to Bletchley Park just days after World War II officially began. And in the years that followed, thousands more were brought on board to support the codebreaking efforts.

Hanne: Wow! Thousands of people were involved in the codebreaking operations at Bletchley Park! That's… a lot.

Alice: Yes. And it's worth highlighting that the workforce wasn't just large but also remarkably diverse. There were mathematicians, engineers, chess champions, linguists, historians, and experts in German studies, all of whom brought their unique skills to the task of breaking Enigma. Additionally, about 75 percent of the workforce at Bletchley Park were women, and their contributions were crucial to the overall success of the codebreaking operations.

Hanne: Amazing.

Alice: It really was. Never in the field of cryptography had such a diverse group of people come together in such staggering numbers to work towards a common goal.

Hanne: To break the Enigma machine.

Alice: Precisely.

(Hanne thinks.)

Hanne: I find it captivating how Bletchley Park was transformed from a peaceful country estate into a hub of codebreaking activity. Exactly how different was the estate after British intelligence took over?

Bob: Oh, Bletchley Park was very different—it seemed almost a completely different estate from when the Leons owned it. There were no longer the scores of flowers, lawns, and fruit trees like before, nor was there the variety of animals. Instead, several single-storey buildings—or "huts", as they were referred to—were constructed around the estate. Captain Hubert Faulkner, the builder who briefly owned Bletchley Park after the Leons, had begun construction of the huts, and when British intelligence acquired the estate, they hired him to continue building these huts around the grounds.

Alice: These huts were crucial for managing the growth of the codebreaking operations, which quickly outgrew the mansion. They were also used to separate the different codebreaking departments, with each team usually identified by their hut number. And as the war progressed, more huts were built to keep up with the increasing demands.

Hanne: Huh! It was almost as if Bletchley Park was being transformed into a little town with all these huts popping up around the estate. It's kind of cool to think how the same spaces once used for funfairs and flower shows were now filled with teams of codebreakers working tirelessly to crack codes. But… I suppose that's the nature of land, isn't it? Stories buried beneath stories.

Alice: Absolutely. Bletchley Park—a festive countryside estate transformed into a secret codebreaking centre.

Hanne: Magical.

Bob: So, Hanne, to give you some valuable background, we'll quickly go through some pre-war British codebreaking developments and then move on to the work done at Bletchley Park. In fact, let's go all the way back to Room 40—the heart of the British codebreaking efforts of World War I.

Hanne: Okay.

Bob: We won't say too much about World War I itself—just that it began in 1914 and ended in 1918. However, what we would highlight is that the intelligence produced by Room 40 had a huge impact on many key events during the war. The success of the Room 40 operations helped convince many high-ranking British officials of the vital role that codebreaking could play in national defence. This appreciation for codebreaking only grew in the years after the war, so when the time came to tackle the Enigma machine, it was not as if British intelligence suddenly decided to buy a countryside estate and hire thousands of people. Instead, their understanding of the importance of codebreaking was a natural progression from World War I, leading them to expand their codebreaking centre from a single room to an entire estate.

Hanne: I see.

Bob: Now, that said, you might find it surprising to learn that the British weren't very interested when French intelligence approached them in the early 1930s with documents from Hans-Thilo Schmidt. Although they looked at the initial documents and concluded that they were insufficient to recreate a military Enigma, at the time, there was a British policy against paying for intelligence material. So, while the British likely made copies of the initial documents before returning them to the French, they did not have access to all the subsequent documents that Hans-Thilo Schmidt sold to the French throughout the 1930s. On the other hand, the Poles were willing to pay for these documents and received a steady flow of crucial information that greatly aided their codebreaking efforts.

Hanne: Hmm. Why weren't the British as interested as the Poles were in the intelligence material provided by the French? And what was this British policy of not paying for intelligence material all about?

Bob: Well… understanding the historical context can help answer these questions. You see, while we have the benefit of hindsight, in the early 1930s, it was not obvious that a major war would break out at the decade's end. So, although the British codebreakers were working on the Enigma machine, it was not a top priority for their government.

Hanne: Ah, I see.

Bob: Additionally, in the 1930s, the world was in the midst of a severe economic depression. As a result, the British government would have decided to focus its spending on supporting its economy rather than on foreign intelligence. In fact, when Admiral Sir Hugh Sinclair bought Bletchley Park for British intelligence in 1938, he had to use his own money to do so.

Hanne: Huh.

Bob: As for why the Poles were willing to pay for French intelligence despite the global economic depression, there are a few key factors to consider. First, unlike Britain, Poland shared a border with Germany, making them more vulnerable to a German attack. Second, Poland's military was smaller than Britain's, so they needed every advantage they could get from intelligence material. And third, and probably most importantly, after World War I, Germany lost territory that became part of the Republic of Poland—the Republic of Poland was re-established in 1918 from lands previously controlled by Germany, Austria-Hungary, and Russia. Therefore, any political instability in Germany could lead to attempts to reclaim its lost territory.

Hanne: Okay, I get it. Basically, Poland was in a much more precarious position than Britain was, which made it more important for them to monitor Germany and break the Enigma machine.

Bob: Spot on.

Alice: However, it's worth noting that even though it wasn't a top priority for their government at the time, the British were still able to make some great progress on the Enigma problem. Although they were unable to solve the internal wiring of the German military Enigma, they did succeed in breaking the Spanish and Italian versions of Enigma.

Bob: Ah, right. While we've been talking about Enigma as if it were a single machine, recall that Enigma was actually a family of machines with different models. We're not going to delve into the specifics of each model, but they're essentially variations of the one we described earlier—with the three rotors, the reflector, and the plugboard.

Hanne: Okay, so the British succeeded in breaking the Spanish and Italian models of Enigma but struggled with the German military model.

Bob: Well, I wouldn't say that they struggled with the German military model since they were actually very close to solving its internal wiring. They made significant strides using some of their own techniques, but their final obstacle was the change in the wiring connections inside the entry disk.

Hanne: Oh, the QWERTZ… in the commercial version that the Germans changed to ABCDEF… in the military version?

Bob: Exactly. If the British had correctly guessed, like Marian Rejewski did, that the Germans had changed the wiring to alphabetical order, they would have been able to solve the internal wiring of the military Enigma as well.

Hanne: Interesting. I did not realise that the British were that close.

Bob: Yes. They almost had it. … So, that was the extent of the British success in the 1930s leading up to World War II—they managed to break the Italian and Spanish models of Enigma but were unsuccessful with the German military model.

Hanne: Got it.

Bob: Awesome.

Alice: Now, Hanne, let's quickly revisit some of the pre-war diplomatic events that we discussed earlier, but this time provide you with a bit more detail. After that, we'll transition to the work done at Bletchley Park.

Hanne: Okay.

Alice: So, in November of 1938, as the German political situation worsened, British and French intelligence met with the hopes of forming a partnership. To show what each side had to offer, the French shared documents they had acquired from Hans-Thilo Schmidt over the years, while the British revealed many of the breakthroughs they had made on the Enigma machine. The French were impressed with all the progress that the British had made and, knowing that the Poles were also working on breaking Enigma, decided to set up a three-country conference in Paris, France, for January of 1939.

Hanne: Right. This was the conference where Britain, France, and Poland agreed to share all that they knew about the Enigma machine but where no one shared anything of real value.

Alice: Correct. One of the British attendees at this conference was the codebreaker Alfred Dillwyn Knox, or Dilly Knox. He had played a key role in Room 40 during World War I and later made significant contributions to breaking Enigma in the 1930s. During the January 1939 conference, Dilly Knox became frustrated by the lack of information provided by the Poles. He had asked how the QWERTZ… wiring in the entry disk of the commercial version had been altered in the military version, but the Polish delegation gave him such vague answers that Dilly quickly realised they had little understanding of the machine's workings.

Hanne: Huh?

Alice: This was because, while the British delegation included their top intelligence officials and codebreakers, the Polish delegation brought only their top intelligence officials, leaving behind their codebreakers, who had the deep technical knowledge of the Enigma machine.

Hanne: Oh.

Alice: The Poles deliberately kept their codebreakers out of the conference to avoid revealing too much to the British and French. However, the British codebreakers could tell that the Polish contingent lacked detailed knowledge of the Enigma machine, and Knox, especially, was annoyed by this. In fact, the British team had to keep him calm during the conference, fearing that he would lose his cool and disrupt the emerging collaboration. Fortunately, there was no major outburst, but Knox was furious afterwards, feeling that the whole thing had been a waste of time.

Hanne: Ooooh, inter-country relations… fun stuff.

Alice: Ha! Yep, always a thrill!

(Hanne thinks.)

Hanne: And I suppose we again need to consider the historical context to understand why the Poles were hesitant to share their breakthroughs with the British and the French during the January 1939 conference.

Alice: Correct. At the time, Britain and France were trying to appease Germany in hopes of avoiding another major war. Without going into too much detail, in September of 1938, they allowed Germany to annex part of Czechoslovakia in exchange for a promise not to invade the rest of the country. However, despite being allies of Britain and France, Czechoslovakia wasn't even invited to the conference where this deal about their territory was made. And without support from the British and French militaries, Czechoslovakia, whose soldiers were willing and ready to fight to defend their territory, was forced to surrender part of its land to Germany. So, the Poles would have been aware of these developments and had good reason to be cautious in trusting the British and the French with their Enigma insights.

Hanne: I see. Dilly Knox's frustrations would have been the least of their concerns.

Alice: Exactly. But just to reiterate, this overview of the annexation of Czechoslovakia was a super basic summary, and we recommend looking into something called the "Munich Agreement"—also known as the "Munich Betrayal"—to get a deeper insight into the historical context.

Bob: Right. As a starting point, we strongly encourage you to check out the life of Sir Nicholas Winton as well as the 2023 film One Life. It's a powerful representation of what life was like in Czechoslovakia during that period and shows the extraordinary impact one person can have on the lives of others.

Hanne: Okay, One Life, I'll check it out.

Bob: Great.

Alice: So, following the January 1939 conference, the German political situation significantly worsened. In March, Germany broke its so-called promise and invaded the rest of Czechoslovakia. This finally made it clear to Britain and France that negotiating with Germany was not going to be an option. The Poles were also not making any further progress on Enigma and, realising the growing threat posed by the German military, reached out to the British and French in July, indicating that they had new developments related to Enigma.

Hanne: Right. British and French personnel then travelled to Poland, and over a two-day conference, the Poles revealed everything they knew about the Enigma machine.

Alice: Correct. However, for some reason, on the first day of the conference, it was only the Polish intelligence officials who were present, not the codebreakers themselves. This frustrated Dilly Knox yet again, as his questions about the QWERTZ… wiring change were met with vague responses—again! He was even further annoyed when he learnt that the Poles had not only managed to recreate the German military Enigma but had also developed several methods to break it. The Polish officials were explaining some of their incredible codebreaking techniques, but their lack of technical knowledge only made Dilly Knox furious. At the end of the first day, he told his British colleagues that the Poles must have bought their information from some intelligence source. He simply could not fathom that they had cracked the Enigma machine on their own.

Bob: However, everything changed on the second day. Dilly's mood completely shifted as the British were finally introduced to the Polish codebreakers. At last, Knox got the intellectually stimulating discussion he'd been craving and learnt that the QWERTZ… wiring in the commercial Enigma had been changed to ABCDEF… in the military Enigma—a configuration that the British thought to be so simple, they hadn't even bothered to try it!

Alice: Knox was also impressed by the brilliance of the three young mathematicians—Marian Rejewski, Henryk Zygalski, and Jerzy Różycki. He admired the methods that they had developed to break Enigma and was especially captivated by their Bomba machine. However, he learnt of the improvements that the Germans had made to Enigma over the years and how these changes affected the Polish methods. He also learnt that the Polish Bomba heavily relied on the Germans' repeated message key procedure and realised that the Germans could abandon this practice at any time. But despite these challenges, the Poles' use of electromechanical devices to break Enigma sparked new ideas in Knox's mind. He returned from the July 1939 conference far more excited and inspired than he had been after the January meeting.

Bob: When Knox returned to Britain, he was eager to share the details of the conference with his British counterparts. In particular, he wanted to talk to Alan Turing, a young mathematician who had been working part-time for British intelligence at the time. Knox explained to Turing how Rejewski had simply guessed that the QWERTZ… wiring had been changed to ABCDEF… in the military Enigma. However, Knox believed that there had to be a mathematical approach to arrive at the solution. Turing, who had a much stronger mathematical background, reassured him that, given the limited information available, it was highly unlikely that they could have solved it mathematically. They were extremely fortunate that the Germans had used alphabetical order and even more so that Rejewski had uncovered it.

Alice: Knox and Turing then discussed all the methods that the Poles had developed to break Enigma and began exploring how they might design their own electromechanical device to tackle the problem. Turing began working on a design.

Bob: When World War II began, Alan Turing was among the first set of codebreakers who reported to Bletchley Park in early September to begin working full-time for British intelligence. And thanks to his previous discussions with Dilly Knox, he had a strong understanding of the history of the British and Polish work on the Enigma problem. This allowed him to hit the ground running with little need for further orientation.

Hanne: Fascinating. And I take it this is where we transition to the work done at Bletchley Park?

Bob: Indeed it is. So, by November of 1939, Turing had completed his initial design for the British Enigma-breaking machine. It became known as the "British Bombe", and the first Bombe, codenamed "Victory", was installed at Bletchley Park in March of 1940. However, it was very inefficient.

Alice: Luckily, Gordan Welchman, another exceptional British mathematician who reported to Bletchley Park at the start of World War II, introduced a major improvement. Without going into the technical details, he invented something called the "diagonal board", which significantly increased the Bombe's efficiency. This contribution was so significant that the British Bombe is often referred to as the "Turing-Welchman Bombe".

Hanne: Interesting. From the Polish Bomba to the British Bombe. Any reason for the similarity in names?

Bob: Yes. It turns out that the common language among the British, French, and Polish intelligence groups was, oddly enough, German. As a result, the 1939 three-country conferences were conducted in German! So, although the Poles referred to their machine as "Bomba"—the Polish word for "bomb"—in the conference, they used the word "Bombe"—the German word for "bomb". And it seems as if the British just stuck with this name. Though, it should be noted that the Polish Bomba and the British Bombe had entirely different designs.

Hanne: Right. I assume that the British Bombe did not rely on the repeated message key at the start of each Enigma message.

Bob: Correct. Instead, the British Bombe used something called a "known-plaintext attack", which allowed it to make logical deductions and reduce the number of possibilities for the daily key. In a known-plaintext attack, a cryptanalyst knows some of the plaintext, and this helps them to break the ciphertext.

Alice: For instance, Hanne, imagine that we have some ciphertext produced from an Enigma machine. As codebreakers trying to discover the key, our best option would be brute force. But… what if we told you that part of the plaintext contains the words "attack the"? How could this help us in breaking the ciphertext?

(Hanne thinks.)

Hanne: Hmm… I don't see how knowing a few words in the plaintext would make much difference.

Alice: Well, actually, it's a huge help. This is because of the one major weakness in the Enigma machine. It turns out that despite all the magic that happens between pressing a key and a lamp lighting up—where electricity flows through the plugboard, then through the three rotors right to left, then through the reflector, then back through the three rotors left to right, then through the plugboard again—a letter would never—never ever—be encrypted to itself!

Hanne: Oh!

Alice: And, as to why this is the case, basically, there is a unique path for every letter from the rightmost rotor to the reflector. And since the letter is always changed at the reflector, it means that the letter is bound to be different when it gets back to the rightmost rotor. In other words, after the seven swaps of going through the three rotors, then the reflector, then through the three rotors again, the resulting letter would always be different. Then, when we add the plugboard into the equation, the property would still hold—the letter that lights up on the lampboard would never be the same letter that was pressed on the keyboard.

Bob: Admittedly, it does take some time to wrap your head around it all, so we encourage you to think more deeply about all the swaps made in the Enigma machine and convince yourself why a letter would never be encrypted to itself.

Hanne: Alright. I'll give it some thought.

Bob: Sweet. So… knowing that a letter can never be encrypted to itself in Enigma, and knowing a bit of the plaintext, we can line up the letters of the known plaintext at every position in the ciphertext. Then, wherever the same letter shows up in both plaintext and ciphertext, we know for sure that's not the correct position. For example, in this case:

Ciphertext:… j r k a x k b t f q e z o b x q f s f q v …

Plaintext:… a t t a c k t h e * * * * * * * * * * * * …

We know that this isn't the correct position for "attack the" since two letters of "attack" line up with the same letter in the ciphertext—namely, the second 'a' and the 'k'.

Hanne: Right.

Bob: Instead, we focus only on where none of the letters match, and we use these positions as the starting points for our codebreaking efforts. So, one possible position for "attack the" would be:

Ciphertext:… j r k a x k b t f q e z o b x q f s f q v …

Plaintext:… * * * * a t t a c k t h e * * * * * * * * …

Hanne: Got it.

Alice: Now of course, recognising these positions is just the first step—it takes some brilliance to be able to effectively use them to break Enigma. This is where the genius of Alan Turing came in. He focused on these cases where none of the letters matched and designed the Bombe to take in educated guesses and make logical deductions to significantly reduce the number of possible keys. Again, we won't delve into the technical details, but essentially, the Bombe ruled out many possible starting configurations, leaving the codebreakers with a much smaller set to test manually. However, its speed and efficiency depended heavily on the quality of the crib that the codebreakers had to work with.

Hanne: Quality of the crib?

Alice: Oh, right. The codebreakers referred to the known plaintext as the "crib", and the process of lining up the plaintext and ciphertext was called "cribbing". The term stemmed from the informal use of the word "crib", which meant "to cheat", and in a sense, this is what the codebreakers were doing. They were not breaking Enigma from scratch; they were relying on having some known plaintext to help them uncover the ciphertext.

Hanne: I see.

Bob: And to underscore just how important the crib was, in August of 1940, a few Bletchley Park codebreakers wrote a letter of protest to top British officials when they learnt that the locations and staff at the intercept stations were being changed. These British intercept stations were crucial to the overall success of the codebreaking operations, as they intercepted German communications, recorded the ciphertext, and noted as many additional details as possible—things like the source and destination of messages. However, the success of these tasks depended heavily on the experience and skill of the staff involved. In their letter, the codebreakers pointed out previous cases where incorrectly recorded ciphertext had prevented them from breaking Enigma on certain days. They also mentioned cribbing and explained that finding a useful crib often involved knowing specific details like the sender and receiver of messages, which was not always easy to obtain. The codebreakers relied heavily on the expertise of the personnel at the intercept stations to provide this vital information. They said that the new staff, who had already begun working during this transitionary period, were not consistently providing the necessary details, thereby undermining the effectiveness of the crib method. Perhaps the line in their letter that best summed up their message was:

"The actual margin between victory and defeat on a single day is often so small that the slightest reduction in the standards of interception might be fatal."

Hanne: I see. So, the quality of the crib had a big impact on the codebreaking efforts, and knowing details like the sender and receiver helped the codebreakers come up with more useful cribs.

Alice: Correct. And it's worth mentioning that there were some—interesting ways—that the British came up with cribs during the war. For instance, one approach involved attacking certain locations just so they could intercept the German communications afterwards. Naturally, they expected words related to the attack to appear in the messages going out from these locations—things like the name of the location attacked, the nature of the attack, the details of the damage, and so on. In this way, they were actively trying to inject words into the Germans' plaintext.

Hanne: Interesting.

Alice: However, they were also able to passively find cribs as well, thanks to some recurring patterns in German messages. For example, every morning, the Germans sent out an encrypted weather report for the day—there was no real reason to encrypt these reports, but they did it anyway. And these messages always contained the word "wetterbericht", the German word for "weather report".

Hanne: A top-secret weather report. I guess the Germans didn't want the British to find out when it was going to rain.

Alice: It seems so.

Bob: And in another case, there was a German Enigma operator stationed in a remote location who sent the same message at the same time every day, simply stating that he had nothing to report. It was predictable elements like these that greatly assisted the British codebreakers in finding useful cribs.

Hanne: I see.

Bob: Also, since Enigma only had 26 keys—one for each letter of the alphabet—it meant that the Germans had to spell out numbers in their messages. This was advantageous for the codebreakers since numeric geographic coordinates were frequently used in military communication and could be used as cribs.

Alice: Well, on that note, different branches of the German military used different methods for communicating numbers—some spelt them out, while others used letters to represent them. So, the British were not always able to exploit the fact that the Enigma machine lacked numeric keys, but definitely, in some cases, it was helpful.

Hanne: Okay. So, the British got cribs from targeted attacks, some predictable German messages, and, to an extent, spelt-out numbers in Enigma messages.

Alice: Spot on. And while the Poles did use cribbing to some extent as well, it was the British whose codebreaking techniques relied heavily on finding useful cribs.

Hanne: Got it.

Bob: Now, in addition to cribbing, another technique worth mentioning is traffic analysis. Traffic analysis is a cryptanalytical technique that studies patterns in communication, such as frequency, volume, and routing of messages. Even if the ciphertext cannot be decrypted, traffic analysis could still yield useful information. For example, if the British noticed a sudden spike in the volume of messages from a certain German military unit on a certain day, they could infer that an imminent attack from that unit was likely. They could then put measures in place to defend against the possible attack.

Hanne: Right.

Bob: But the Germans were aware of traffic analysis and countered it by keeping their daily message volume relatively consistent—though on some days, major spikes were still noticeable. However, on the days when the message volume was low, the Germans would send dummy messages to maintain a consistent amount. For the most part, these messages were just random gibberish.

Hanne: I see.

Bob: However, the possibility that a message could be a dummy message made things tricky for the British since decrypting a ciphertext whose plaintext was meaningless was both time-consuming and frustrating.

Hanne: Why did it matter whether a message was a dummy or not when decrypting?

Bob: It was because the effectiveness of many of the British codebreaking techniques relied, in part, on common patterns found in the German language. But if the plaintext was just a random string of letters, these patterns would not appear, and the British techniques would not be as effective. Thus, an extra effort had to be made on the British side to ensure that they were not trying to break a dummy message.

Hanne: Oh.

Bob: Of course, once the daily key was discovered, it did not matter whether the message was a dummy or not; decryption of the ciphertext would be easy. But when still searching for the key, choosing the right ciphertext to work with was crucial. This again highlights why the personnel at the intercept stations needed to be highly skilled, not just in accurately recording the ciphertext but also in providing valuable context about the message.

Hanne: Right.

Alice: Now, it turns out that while the German operators were supposed to type random characters for the dummy messages, every now and again, an operator would just press a single key repeatedly to form the message. For instance, let's say that a German operator repeatedly typed 'X' as the plaintext for a dummy message. What would this mean?

(Hanne thinks.)

Hanne: The ciphertext would not contain any 'X's, since a letter is never encrypted to itself in Enigma.

Alice: Exactly. A cryptanalyst might notice this and realise that there is a good chance that the ciphertext maps to a dummy message composed entirely of 'X's. They could then use this as a crib to assist in breaking Enigma.

Hanne: Oh! So, while dummy messages were supposed to protect against traffic analysis—which I assume they did—they also introduced security vulnerabilities because of the carelessness of some German Enigma operators.

Alice: That's right. And on that note of security vulnerabilities introduced by the German Enigma operators, there was another incident—related to the message keys—where this took place. Recall that to protect against the advanced statistical attacks, the German Enigma operators had to choose a random three-letter message key to encrypt every message.

Hanne: Yes.

Alice: Well, the effectiveness of this procedure greatly relied on the randomness of the message keys. However, many operators did not take it seriously. Instead, some always picked three letters that were next to each other on the keyboard, while others used the same three letters for all their message keys—often their own initials or those of a loved one. These predictable message keys made the British codebreakers' jobs a lot easier.

Hanne: Interesting. So, once again, something that the Germans introduced to improve security—the message keys—ended up creating security vulnerabilities because many operators failed to properly randomise them.

Alice: That's right. Dilly Knox is often credited with identifying this vulnerability, though it was likely noticed by several others as well. Regardless, one of the first predictable message keys that he and his team came across was "CIL". So, as a play on the word "silly", they called this German mistake "cilly". After that, they were always on the lookout for more cillies.

Hanne: A hunt for cillies.

Alice: Indeed.

Bob: So, that's the gist of some of the main Enigma codebreaking developments that took place at Bletchley Park. There are some other developments that we'll touch on in a bit, but for now, here are the key takeaways from our discussion:

• The Turing-Welchman Bombe was a codebreaking machine that used cribbing to break Enigma.
• Cribbing was useful because of Enigma's major flaw—a letter could never be encrypted to itself.
• The British obtained cribs both actively and passively, assisted by the expertise of the staff at their intercept stations.
• Additional weaknesses introduced by the German Enigma operators further assisted the British codebreakers—things like repeating a single letter to form dummy messages and using poorly chosen message keys to encrypt their messages.

Hanne: Got it.

Alice: And to put things in perspective, here's a timeline of how the main Enigma codebreaking developments unfolded during the first year of the war:

And here's a timeline of some of the key military operations that took place during the same period:

Hanne: Wow.

Bob: So… we encourage you, Hanne, to reflect on how the codebreaking operations developed alongside the military battles in the early phases of World War II, since breaking Enigma wasn't some isolated cryptographic challenge—it was a crucial part of British military strategy.

Hanne: Right. The decrypted Enigma messages would have provided valuable intelligence that would have assisted the British war effort.

Bob: Exactly. However, that said, the extent to which the codebreaking operations influenced certain military campaigns—like Operation Dynamo and the Battle of Britain—is still debated. Some historians argue that breaking Enigma played a major role, while others believe its impact was limited. And since the specifics of which Enigma messages were actually broken aren't widely known, this debate is likely to continue. Therefore, we can only just vaguely state that breaking Enigma had some impact on Operation Dynamo and the Battle of Britain. However, its impact on other military campaigns—like in the Battle of the Atlantic—is better known and documented.

Alice: Indeed. To better understand the broader context in which the British codebreakers were working, let's step away from the codebreaking for a moment and explore these military campaigns. We'll start with Operation Dynamo, move on to the Battle of Britain, and finally, talk a bit about the Battle of the Atlantic.

Hanne: Alrighty.