Turing-Welchman Bombe


For centuries some people, organisations and governments have wanted to send information secretly. Different ways of sending a secret message have been developed over time but the most common practice has been to hide information by using a code or a cipher (see box below).

Just as people want to keep their messages to some people secret others want to know what that secret is! That has meant that just as some people will develop codes and ciphers for sending messages secretly, other people will spend their time trying to discover or break them. Sometimes the messages are sent on bits of paper and sometimes people had to remember them. Messages sent in secret can be anything from love letters to new discoveries in technology, but in war time and amongst the military they are usually things that can actually change lives.

For governments during a war keeping their communication secret is highly important and those messages include information between friendly governments, to army leaders giving out orders for an attack, or a request for more supplies. Trying to capture or, intercept, a message being sent in secret can reveal important information to another side. It can be something big such as when a battle is planned or it can be something that seems small such as a shortage of bread, but it all reveals an important aspect of how a side is managing or planning.

Governments established special organisations – intelligence agencies or ‘spies’ to intercept messages, but if the message is encrypted they then have people who need to break that cipher. Once a cipher or code is broken the meaning of the message can be revealed – however, the real significance of that message can only be fully understood if lots of bits of information are gathered together to create a full picture. That means an intelligence group and a codebreaking group has to intercept, break and analyse hundreds of messages to make proper use of the information during a war.

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Code: A way of keeping the meaning of a message hidden by changing whole words e.g.  a series of dots and dashes replace letters and words or at sea flags might be used to replace words and sentences.

Encoded: A message that has been put into code

Cipher: Relies on hiding a message by jumbling up individual letter of the message

Encryption: Sending a whole message with the letters jumbled up using a cipher

Decryption: Knowing what the cipher is to unravel an encrypted message e.g. knowing that each letter has been replaced by the one next to it in the alphabet

Caesar cipher: A way of hiding a message by mixing the letters up by simply moving each one along the alphabet e.g. a = c  b=d c=e etc.

Keyphrase/Keyword/Crib: The way to find out how a substitution cipher has mixed up the letters. The key word gives you the first few letters of the alphabet and then the rest can be worked out.

Decipher: Unravel an encrypted message

Cryptanalyst: Someone who encrypts messages or decrypts encrypted messages.

Intelligence analyst: Someone who has to read through all the information gathered from secret messages and form open sources to try and identify what is true, if there are any patterns in the information and then has to recommend a course of action

Enigma machine: A machine used by the German military in the Second World War to encrypt all their messages that were sent by radio or electric telegraph

Bombe machine: A machine invented to work out how to decrypt a message that was encrypted by an Enigma machine.

Y station: A place in the UK with a large aerial that listened into and wrote down the secret encrypted messages being sent by the Germans and their allies and then sent them to the UK intelligence sites for decryption.

Lorenz machine: A machine that was even more sophisticated than the Enigma machine for sending encrypted messages.

Codes & Ciphers

In the field of secret communications (cryptography), a code refers to a way of keeping the meaning of a message hidden by changing whole words. For example, during the Second World War the word “DYNAMO” was used by the Allies when referring to the operation to rescue troops from Dunkirk beach. In contrast, a cipher relies on hiding a message by jumbling up individual letter of the message. For example the meaning of the word “DUNKIRK” can be hidden by replacing each letter with the next one in the alphabet so that once it has been encrypted is will read “EVOLJSL”.

In both World Wars, all sides relied mostly on the use of ciphers rather than codes as ciphers only required the sender and receiver to share a set of short instructions (e.g. I’ve encrypted this message by shifting each letter to one up in the alphabet) that could be changed easily, rather than a large codebook that included alternatives for all the words that they may need to keep secret.

Secret communications in the First World War

In the First World War or Great War, as well as more traditional methods such as carrier pigeon, millions of messages were sent using the modern technology of radio broadcasts and electric telegraph. Radio waves and electric circuits were used because messages could be sent over long distances in a matter of seconds unlike someone sending a paper message. However, anyone could quite easily listen into radio messages or intercept telegraph communications and understand what the message is. That is why governments started to encrypt their messages– the encryption technique (cipher) had to be agreed in advance so that the person receiving the message could decipher it and understand what they were being sent.

The simplest form of encryption is to use a shift or Caesar cipher in which the original letters are replaced with a letter corresponding to a certain number of letters up or down the alphabet. There are however only 25 distinct shift ciphers possibilities which make them easy to break by simply trying each possibility in turn. A better encryption method is to use a system in which any letter can be represented by any other, in no particular order. These substitution ciphers can be generated by using a keyphrase. For example Royal Air Force as a keyphrase, begin by removing any spaces and repeated letters (ROYALIFCE) and then use this at the beginning of the alphabet. The remainder of the cipher alphabet is the remaining letters, in the correct order, starting where the keyphrase ends.

If someone does not have the keyphrase then trying to work out the cipher can be extremely time consuming as there are many billions of possibilities. How people set about breaking a cipher like this is to use frequency analysis. All languages use some letters more than others, in English for example, the letter E is the most common letter followed by T and then A. By comparing the frequency of letters that appear in the encrypted message to those in a long piece of plain text most of the letters can be identified. A keyword is also called a ‘crib’.

The Government Code and Cipher School

In 1909, the British government created the Secret Intelligence Service (or Bureau as it was known for a while) SIS, was engaged in all areas of intelligence gathering, codebreaking and analysis of information. Also, during the First World War the Army and the Navy both had their own Intelligence units that intercepted and decrypted messages. After the war Hugh Sinclair the Director of Naval Intelligence was given the task of merging the army and navy units and creating The Government Code and Cipher School. Sinclair also became the head of SIS, all of which were based in London.

During the 1920’s and the 1930’s the staff at GC&CS grew and they became aware that new methods of encrypting messages were emerging. Recruitment of codebreakers had usually focussed on linguists, those who were good at languages (especially as many intercepted message once decrypted would still be written in a foreign language). However, technological changes in that period meant that messages were being sent that seemed to have far more complex ciphers than the ones people were used to – it was obvious that some type of machine was also involved to create a cipher. Sinclair decided to start recruiting mathematicians as well as linguists to his team.

In 1938 events across Europe made war look likely again. Sinclair decided that having all his important people in London might be dangerous, as London was likely to be attacked from the air in any war, just as it had been during the First World War. So the government secretly bought, or took over (requisitioned) a number of country houses in the Home Counties around London. One of the key purchases was Bletchley Park in the village of Bletchley, Buckinghamshire, just outside London. The site was ideal as it had plenty of space and was very close to a train station that travelled straight into London. The whole of GC&CS were transferred to Bletchley in 1938 under the leadership of Alistair Denniston.

The Enigma Machine and breaking Enigma

The Enigma machine was invented by a German engineer Arthur Scherbius shortly after WW1

The machine (of which a number of varying types were produced) resembled a typewriter. It had a lamp board above the keys with a lamp for each letter. The operator pressed the key for the plaintext letter of the message and the enciphered letter lit up on the lamp board. It was adopted by the German armed forces between 1926 and 1935. The machine contained a series of interchangeable rotors, which rotated every time a key was pressed to keep the cipher changing continuously. This was combined with a plug board on the front of the machine where pairs of letters were transposed, these two systems combined offered 159 million million million possible settings to choose from, which the Germans believed made Enigma unbreakable.

The Poles had broken Enigma in as early as 1932, but in 1939 with the prospect of war, the Poles decided to inform the British of their successes. Dilly Knox, one of the former British World War One Codebreakers, was convinced he could break the system and set up an Enigma Research Section, comprising himself and Tony Kendrick, later joined by Peter Twinn, Alan Turing and Gordon Welchman. They worked in the stable yard at Bletchley Park and that is where the first wartime Enigma messages were broken in January 1940. Enigma traffic continued to be broken routinely at Bletchley Park for the remainder of the war.

(Extract taken from the Bletchley Park Trust)

The problem for the team at Bletchley was that they needed to break the hundreds of Enigma messages that were coming in each say quickly. That led to the building of the Bombe machine.

Turing-Welchman Bombe

Based on the information presented by the Poles, British mathematician Alan Turing developed a machine that was capable of recovering the key settings. The machine was called Bombe (later: Turing-Welchman Bombe) and was built by the British Tabulating Machine Company (BTM) in Letchworth, Hertfordshire (UK) under supervision of Harold (Doc) Keen.

The name was derived from Bomba, a similar machine developed by the Poles shortly before the outbreak of WWII.

Turing designed the British Bombe in 1939. Compared to the Polish Bomba, it used a completely different approach. It was based on the assumption that a known a crib, is present at a certain position in the message. The Bombes were built by the British Tabulating Company (BTM, later ICL) at Letchworth (UK) under the supervision of Harold 'Doc' Keen [. The first machine, called 'Victory', was delivered at Bletchley Park on 18 March 1940.

The Bombe was further enhanced with the so-called diagonal board, an invention of fellow codebreaker Gordon Welchman, that greatly reduced the number of steps needed for the codebreaking effort. A second Bombe, with Welchman's diagonal board present, was installed on 8 August 1940. It was named 'Agnus Dei', later shortened to 'Agnes' or 'Aggie'. The first machine (Victory) was later modified with a diagonal board as well.

During the course of the war, over 200 Turing-Welchmand Bombes were built. To avoid the risk of losing them in case of a bomb attack, they were spread between Bletchley Park and its so-called Outstations in Wavendon, Adstock, Gayhurst, Eastcote and Stanmore, where they were operated by WRNS, RAF-technicians and civilian personnel.

(Extract taken from the Crypto Museum)

Therefore, the Bombe’s job was to try and find the crib in the message. Once the crib was discovered it was possible to limit how many settings it could possibly be on the Enigma machine to try and decrypt the whole message.

The crib was usually at the start of a message and it helped to indicate to the receiver that they were using the correct settings on their machine.

Cipher books: The German military were issued with cipher books which told them what setting to put their machine on a certain day. The books were changed weekly, except for some parts of the military that could not receive new cipher books easily. Submarines would use their cipher books for weeks as they were often underwater for long lengths of time. Capturing cipher books was extremely useful as it provided the crib for a certain day and for a certain set of communications e.g. all Luftwaffe communiques on the 2 November 1943. If the codebreakers had the crib they could set their enigma machine at the correct settings and they would not need the Bombe team to do its job which could take hours.

From Y Station to action

All across the UK were bases called ‘Y’ stations. The ‘Y’ name came from the shape of the aerial and that was what made it significant. A ‘Y’ station would be listening in to the many radio messages that were sent out across the airways. Some listened to actual voice messages but the majority were there to pick up the encrypted messages being sent between armies and commanders, government departments etc. ‘Y’ station staff (military) would collect hundreds of messages a day which would then be passed on to places such as Bletchley Park either on paper and delivered by a motorcycle rider or using a teleprinter machine – A type of electronic printer that printed out a message that had been sent electronically or down a telephone cable.

At Bletchley: the encrypted message would be given to one of the codebreaking team, they would work with the Bombe machine operators to try and find a breakthrough to the cipher – the crib which would help provide the settings for an enigma machine. If the Bombe hit upon a match and the cipher could be converted into a message it was then handed to the translators. The translators would be attached to a military or intelligence unit. The bombe machines were usually operated by the WRNS (Women’s Royal Naval Service. The RAF team were largely based in Huts 3 and 6 which also contained some of the analysis teams.

Analysis: The decrypted, translated messages would be analysed and compared to other intelligence information. Some of that information came from encrypted messages at Bletchley Park but other material came from further afield and other intelligence sources. The RAF had a site at Medmenham that was its centre for a photographic interpretation unit. All of the reconnaissance and aerial photography that was taken by the RAF on operations was examined there. Information revealed at Bletchley could be passed to Medmenham who might compare discussions of troop movement to the photographs that they had taken.

Alternatively, the analysis might be taken into central London to compare with other information being gathered by SIS and its international network of spies.

Action: Analysis of information provided by decrypted messages could affect military decision making or planning. Sometimes that affect might be immediate e.g. extra aircraft moved to a certain part of the UK in anticipation of an attack. Sometimes the result of the information might be to affect British military planning over a long period of time e.g. how an attack plan for months ahead should be planned and for what date according to what the German were planning. Sometimes the information gathered resulted in no immediate action e.g. during the run up to D Day the Allied invasion of Normandy the decrypted messages and wider intelligence gathering helped to support the British belief that the Germans did not know that the attack was being planned for Normandy. On other occasions decisions to not do anything after messages were decrypted was taken so that the Germans did not suspect that the Enigma messages could be decrypted – that could be a controversial decision as it might mean that British lives would be lost rather than act on intelligence. 

Timeframes: Trying to break the German messages sent by Enigma machines was always a race against time. There was no point finally being able to understand a message if what it was talking about had happened days or weeks before. Therefore, trying to decrypt a message and reveal its meaning was always done with a sense of urgency, ideally within a few hours of the message arriving at Bletchley.

Who worked at Bletchley Park?

When Bletchley was first established in 1938 only a few hundred people were based there, but that number grew as time went on. By January 1945, 10,000 people worked at Bletchley, three quarters of them were women. The whole place operated on a shift pattern so that it was able to operate 24 hours a day. The people there including the women worked as mathematicians, codebreakers, language translators (to translate the message once it was broken), analysis, administration and machine operators.

The role of intelligence during the war

When people think of conflicts and war they usually think of armies going off to war and although gaining information, (intelligence) has always been important it wasn’t always perceived as being as important as the actual fighting. However, the Second World War changed that. The Nazis dominance in Europe, coupled with the rise in technology meant that wars could also be waged through other means – propaganda, strategic planning, deception, and by knowing what your enemy was doing so that you could manipulate them or counter them. The RAF flew thousands of reconnaissance missions in addition to its role in combat. Hundreds of men and women acted as intelligence agents across Europe collecting information and sending it back to the UK. At home in the Britain thousands of men and women served the military as codebreakers, analysts and strategists for deception.

During the planning for D-Day and the invasion of Western Europe the Allies used their intelligence gathering to understand where German troops were, how defences were organised and how quickly they could be reinforced. In addition to preparations for the invasion the Allies spent months sending out false information to convince the Germans that an attack would start at other sites along the French coast or even in Norway. From the intercepted messages that the British decrypted they were also able to tell that the Germans did not expect Normandy to be the invasion zone and were convinced it would be in Calais. Knowing that the Germans were not prepared provided the Allies with a massive advantage when they did successfully attack on the 6 June 1944. It is said that the use of intelligence to deceive as well as to listen in, helped to end the war quicker than just by fighting.