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Enigma - the silent struggle of the code breakers

When the world fire begins, the inconspicuous country estate Bletchley Park becomes one of the most important bases of the Allied secret services. An eccentric group of ingenious thinkers is trying to decipher the Germans' secret radio communications. In the center: the Enigma encryption machine

Winston Churchill's column reached the small town of Bletchley, 80 kilometers north-west of London, on the morning of September 6, 1941. The wagons pass a gatehouse, behind which a park opens, part of a spacious country estate. Rose beds, on the right a pond, in front of it, on a well-tended meadow, a sequoia tree. The wire of an antenna stretches from its top to the thin chimneys of an almost 60-year-old mansion.

Bletchley Park is a curious mix of Victorian styles with its pointed gable bay windows, squat turret, battlements and stone griffins in front of the entrance.

And the perfect camouflage. Because the property hides the best-kept secret of the Second World War. Bletchley Park is "Station X", the tenth station of the British Secret Intelligence Service. The international secret service MI6 has housed its affiliated "School for Coding and Encryption" there.

Another deception: The aim of this department is not to encode and encrypt, but to decode hostile messages. The Bletchley Park property is the center of a worldwide network of listening posts that intercept all available German radio messages - messages that are sent by government agencies, the NSDAP leadership, but above all by the Wehrmacht: encrypted and as Morse code. Station X is supposed to crack these codes.

Imagination and mathematics as Weapons of Station X

On September 6, 1941, almost 1,000 men and women were working on the 400 by 400 meter park area. On both sides of the old main house with its two storeys and the facade of 40 meters length, craftsmen have built several wooden barracks to make space for the numerous departments.

In peacetime, the people of Bletchley Park were university teachers or military personnel, translators or bankers, chess masters, mathematicians, students or artists. Now they are code breakers and are bound to the highest level of secrecy.

They fight one of the most important battles of World War II - in a very unique, very quiet way: their battlefield is communication, their weapons are logic, math, imagination and intuition.

And their opponent is dangerous and full of cunning: a machine that its inventors, the Germans, consider invincible. Their internal structure enables their users to transform every message into an apparently inextricable gibberish made up of letters, which can only be transformed back into meaningful sentences by those who have both the device and its secret settings.

The inventor of this machine, the German engineer Arthur Scherbius, named it after the Greek word for puzzle: "Enigma".

And the project that MI6 in Bletchley Park is trying to solve this mystery is called "Ultra". It is short for ultra secret: very secret.

Churchill is amazed at the assembled eccentrics

Stewart Menzies, the chief of the secret service, personally leads Churchill over the plant that day. Show him barracks in which there are large, loudly rattling machines, women type on typewriter-like devices, mentally-minded men crouch in front of large stacks of paper.

After the tour, the premier climbs a pile of building materials outdoors. He wants to give a speech - and is amazed at the gathering of eccentrics that faces him on this sunny September day. “When you look at you like that, it's hard to believe that you know any secrets,” he begins.

There is Dillwyn Knox, Greek professor and Egyptologist at King’s College in Cambridge and now head of Enigma decryption. Knox is considered a cranky tinkerer. Sometimes he shows up to work in a dressing gown because he had an idea about the toilet and forgot to get fully dressed. Naval intelligence hired Knox as a code breaker back in World War I, and his analytical skills are legendary. The story circulates in Bletchley Park of a Hungarian text that he deciphered without knowing the language: he treated the whole thing as a purely abstract problem.

Or Frank Birch, also a decryption veteran of the First World War. The small, bald man first made a name for himself as a historian in Cambridge, then as an actor and pantomime. Now, in Bletchley Park, he is head of the department responsible for the German Navy.

Or Stuart Milner-Barry, the Times chess correspondent, who was recruited for the deciphering service by a former fellow student in Cambridge.

Or Geoffrey Tandy, the former director of the Natural History Museum in London. He owes his presence in Bletchley Park to a mistake: He is an expert on "cryptogams", on spore plants - mosses, ferns and algae. The recruiters of the military intelligence service have obviously misunderstood this specialization as "cryptograms": encrypted messages. Tandy stayed with it anyway.

Alan Turing also listens to Churchill, a mathematician who, like Knox, previously lectured at Cambridge. The shy researcher in the always unironed suit is considered a mathematical genius and greatest thinker at Bletchley Park.

Unusual ways

Everyone who gathers here is awesome in their own way - Bletchley Park is a place of concentrated mind. This is exactly how the prime minister imagined it. Even if a little less eccentric. Before leaving, he therefore turns to the head of the secret service one more time. Of course, he remembers instructing him to "turn every stone" when recruiting staff - "but I didn't think you'd take me so literally".

But isn't the Enigma an adversary that intelligence agents have never seen before? If you want to defeat this machine, you have to go unusual ways.

Wanted secure encryption methods

In the spring of 1918 Arthur Scherbius applied for a patent for his cipher machine. At trade fairs, he mainly offered them to companies that wanted to encrypt their communications - the German military were not interested in the expensive machine, which was marketed as the "Enigma" from 1923 onwards.

That changed in 1925 when the Reichswehr realized that their coding procedures, which they believed to be secure, had been exposed during the First World War. At that time, the transmission of messages by radio was still in its infancy, and so many messages were encrypted and then telegraphed directly to the recipient via cable.

Since then, however, the use of radio waves had also become the rule in military communications - with the disadvantage that radio messages could now be received by everyone. Technical progress therefore forced the military to look for new encryption methods that were also secure for wireless communication.

It would take 14,000 years to decipher the Enigma

And didn't Scherbius advertise that no encryption technology is as secure as that of the Enigma? Wasn't he calculating that if they tried a different machine setting every minute, it would take those who tried to overcome it alone would take 14,000 years?

In 1926 the German Navy switched to the Enigma system. A year later, Scherbius secured a similar patent from the Dutchman Hugo Koch and improved the Enigma again. In 1928 the army acquired this version and modified it with additional elements.

And after the National Socialists came to power, the air force, military defense, SS, Reich Security Service, Reichsbahn and several ministries also took over the apparently secure encryption technology.

Because it had obvious advantages: the devices were hardly larger than an electric typewriter and battery-operated. In the simplest version, they weighed twelve kilograms, but with their wooden casing they were easy to transport and could therefore be used anywhere, including at the front.

The halo of invincibility

The way in which the Enigma works differs from the encryption methods that were common up until then, because it combines several of these techniques in one device. And it does not work according to an externally recognizable, logical principle that is repeated over and over again during encryption - such as the one whose invention is attributed to Gaius Julius Caesar. The Roman ruler is said to have written ciphertexts by shifting the alphabet by a fixed number of places: A two-key turns every A into a C, a four-key always results in an E. The Enigma is different. It also turns an A into a C, sometimes an E. However, in the same text. Or even in the same word.

The heart of the Enigma are three rotating rollers, so-called rotors, on each of which the 26 letters of the alphabet can be seen. Each character is passed on electrically from cylinder to cylinder and is always re-encrypted. In addition, at least one rotor automatically advances one position after each letter.

In this way, the machine converts what is typed into the encrypted text. It appears, letter for letter, above the keyboard on a field of illuminated letters.

On top of that, the rotors are interchangeable and are electrically wired together inside in a special way. While in the pre-war years only three reels were used in alternating order, from 1938 onwards the radio operators were able to create their three-way combinations from five and later even from eight different reels.

In this way, the Enigma encrypts each individual character with an individual key and theoretically enables more than 2 × 1023 coding variants in the simplest Reichswehr version - a two with 23 zeros, more than a fifth of a quadrillion. A technique that gives the Enigma the aura of invincibility.

Enigma code books are among the most secret documents of the war

Four people are normally involved in the encryption and decryption of a message: a radio operator enters the plain text on the keyboard and dictates the encrypted letters that appear on the light field to a comrade; he translates it into Morse Code and sends it out. The recipients do the opposite.

But the system only works if both machines are set exactly the same. That is why every network of Enigma users has special “key books” which indicate how the machines are to be set up, which rollers to use and which starting position they are to be brought into.

The keys are set for a month in advance and change every 24, later even every eight hours. The code books are among the most secret documents of the war. The Navy, for example, prints them on easily water-soluble paper - so that no diver can retrieve such a book from a sunken ship.

Security measure turns out to be a weak point

The Germans know that the risk of breaking into the code increases when many messages are encrypted with the same basic machine setting. In addition to the daily encryption provided by the code book, each individual message is therefore given its own three-letter code, the “saying key”.

The sending radio operator thinks up this himself and sends it, coded with the basic setting, twice in succession to his counterpart to be on the safe side. Then he adjusts the reels again according to the letters of the key and writes his radio message. Because the recipient uses the same code book and thus the same basic setting, he receives the slogan key in plain text, and can now move the three reels of his own Enigma into the position of these three letters - without changing other parameters of the basic setting - and read the message .

A complex process that aims to make radio communications safer. In fact, it is a weak point.

Enigma lands in the Warsaw customs office

Because in 1928 a stranger arrived at the customs office in Warsaw: a heavy wooden box from Germany. It arouses the suspicion of a customs officer because the addressee in Warsaw emphatically demands that the consignment be returned to the sender immediately and unchecked. Customs decide to inspect the box - and find an Enigma.

The officers inform the Polish General Staff, which sends two encryption experts to the customs office. They're investigating the mysterious machine. Then they repack them and send them back to Germany. Nobody notices anything there.

The Poles now know the construction of the Enigma and its working principle - but not its internal wiring. Because the machine at the Warsaw customs office was the commercial version of which the Polish secret service will soon get its own copy. The German Wehrmacht, however, uses a model that is wired differently. The Poles lack the technical details for this model.

Patterns are hidden in the encrypted messages

They received them three years later from the French secret service, with which they had a cooperation agreement. The French have an agent in the encryption station of the German Reichswehr Ministry and forward the spied Enigma material to the Poles. They can now have replicas of the machine made.

The cryptologists also learn about the three-letter key that has to be sent twice in succession at the beginning of each message from the service regulation for the Enigma deployment, which has also been leaked.

This means: In the sequence of the first six encrypted letters of a message, the first corresponds to the fourth character, the second to the fifth, and the third to the sixth. A pattern that makes deciphering a lot easier.

In addition, the Poles find certain regularities in the character sequence in the encrypted texts.

The invention of the cryptological bomb

At the beginning of 1933, a group around the young mathematician Marian Rejewski succeeded for the first time in decoding tapped German messages. Five years later, Rejewski designed a system with 18 rotors that could automatically simulate all conceivable roller combinations and basic positions of the Enigma. The Poles call their invention "Bomba Kryptologiczna" (cryptological bomb).

The bomba needs 100 to 120 minutes to determine the position and position of the rollers for a coded message. If a possible setting is found, an Enigma replica is used to test whether a meaningful German text comes out.

But by the end of 1938 the bomba was largely useless. The Wehrmacht is upgrading the Enigma of the Army and Air Force with two more rollers, so to a total of five. This increases the number of possible roller combinations tenfold. Rejewski and his people now need much larger machines - but they don't have the means for that.

Poles tell the French and British about their successes

Because Poland also fears that a German attack is imminent, the General Staff decides to initiate the French and British.

At the end of July 1939, a memorable meeting took place under the highest secrecy. The Poles sent Marian Rejewski. Dillwyn Knox and Alastair Denniston, the head of the school for coding and encryption, have come from England.

The English do not yet know about the successes of the Poles. For several months now, Knox has been looking in vain for a way to break into the Enigma. The British are all the more astonished when they hear Rejewski's reports.

Bomba is coming to Bletchley Park

The Poles also give their guests the blueprint for the Bomba - and two of their Enigma replicas.

Just over a month after the meeting, the German army invades Poland. Rejewski's department is hastily disbanded and the bomba is destroyed before it can fall into the hands of the Germans. The men who were closest to the enigma of the Enigma must go into hiding.

Marian Rejewski and the other cryptologists moved to Romania, then to France. When the Germans invade there, the group flees to Great Britain via Spain, Portugal and Gibraltar.

Rejewski is taken over as decipherer by MI6, but has nothing more to do with the Enigma. But his knowledge and the replica of the cipher machine have long since reached Bletchley Park.

Secret Service recruits new staff with a crossword puzzle competition

Late summer 1939.The country estate is noticeably populated, station X is set up. Around 140 people belong to the first crew. Since the mansion quickly proves to be too small, craftsmen start building barracks for workplaces.

On September 3rd, Great Britain declares war on Germany. In Bletchley Park, the pressure is now growing to reveal the enemy's radio communications as quickly as possible. 30 people initially work in the team that, under the direction of Dillwyn Knox, is supposed to break the codes, including veterans who already deciphered secret messages from the enemy during the First World War, but also many newcomers that Knox and his staff had toured in the months before the elite universities of Oxford and Cambridge have recruited: linguists, mathematicians, historians - as well as Egyptologists, who are well versed in exploring mysterious systems of symbols through their use of hieroglyphics.

When it comes to recruiting staff, the secret service takes unusual approaches. The “Daily Telegraph” calls on its readers to take part in a speed competition in solving crossword puzzles. Shortly afterwards, the MI6 contacted the fastest participants: with job offers for Bletchley Park.

Alan Turing enriches Station X

On September 4, 1939, the 27-year-old mathematician Alan Turing arrives. Four years ago he designed the theoretical basis for a (never realized) machine that can do every conceivable calculation by reading and labeling a band with marked and empty rectangles - the basic principle of the computer.

Turing is homosexual, a criminal offense in Britain at the time, which could make him vulnerable to blackmail. In addition, he is an eccentric, his way of working chaotic. In summer he cycles to work with his gas mask on - for fear of hay fever.

"Only I have just enough authority and skill to keep him and his ideas in a certain order and discipline," writes Knox in a note to Director Denniston.

In any case, Turing is an absolutely necessary reinforcement for Knox's group. Because it is still far from breaking the German code. The Polish replica of the Enigma helps the team, but is already out of date; The other insights of the Poles are also insufficient.

The only thing left for the code breakers to do is to retreat to the barracks with the concrete floors, bare lightbulbs and poorly heating coke ovens. To try again and again, to combine, to consider.

Cryptologists work with paper templates and guesswork

So there are probabilities that make it easier to decipher. Of the theoretically possible 17576 key words, only 20 to 30 may have to be checked.

Another weak point are phrases that keep appearing in German radio communications - "OBERKOMMANDO" for example, or "NO SPECIAL INCIDENTS". And a combination of words that can almost always be found at the end of a message: "HEIL HITLER".

Because the Enigma never encrypts a letter with itself, grid templates can be made from paper using the fixed word combinations. The cryptologists push these over the text at those positions in the intercepted message where they suspect these expressions, until, for example, no O as O, no M as M and no S as S is encrypted in the template with the word "Incidents" is. Then you have an entry point.

Difficult but successful work

In order to draw conclusions about the rollers used and the exact settings of the Enigma from these clues, the tinkerers have to manually test all possible variants on the replicas. If you have found a machine position in which the encrypted phrase appears at the corresponding position, you can test whether a meaningful German text also appears on the light field of the Enigma for the rest of the message with this setting. If not, all in vain, the code breakers have to start over.

It is arduous work that often takes days, weeks, months for a single report - but it is ultimately successful.

Bletchley Park is struggling with a flood of reports

In January 1940, the cryptologists in Barrack 4 deciphered the first Enigma report of the war. It comes from a radio network that Knox has marked with a green colored pencil in his organization plans and that is therefore known as "Green Key Circle" in Bletchley Park. In Station X, the Germans' radio networks were marked in the first years of the war as if from an ink box: In addition to green, there are red, yellow, brown, blue and various mixed colors.

The first green message is disappointing in terms of content - a sequence of irrelevant things such as the weather report - but it is a start.

On April 9, 1940, the Wehrmacht marched into Denmark and Norway