Technology History

Along with its codebreaking activities, Bletchley Park is associated with the rapid development of technological innovations and particularly related to Enigma and the Bombe, and Lorenz and Colossus.

The Enigma

The Enigma machine is an electro-mechanical device that relies on a series of rotating 'wheels' or 'rotors' to scramble plaintext messages into incoherent ciphertext. Typing in a letter into the machine sent electrical impulses through a series of rotating wheels, electrical contacts and wires to produce the encyphered letter, which lit up on a panel above the keyboard. By typing the resulting code back into a machine with the same settings, the recipient saw the decyphered message light up letter by letter. The rotors and wires of the machine could be configured in many billions of combinations, and each one will generate a completely different ciphertext message. If the set up of the machine is known the ciphertext can be typed back in and it will unscramble the message; otherwise the odds against being able to break Enigma were a staggering 150 million million million to one.

The German military used the Enigma cipher machine during World War to keep their communications secret. Although the machine was available commercially during the 1920s, the military potential of the device was quickly realised and the German army, navy and air force all used a more developed model of the machine to encipher their messages believing that it would make these communications impenetrable to the enemy.

The German authorities believed in the absolute security of the Enigma. However, with the help of Polish mathematicians who had managed to acquire a machine prior to the outbreak of WW2, British code breakers stationed at Bletchley Park managed to exploit weaknesses in the machine and how it was used and were able to crack the Enigma code. A fundamental design flaw meant that no letter could ever be encrypted as itself; an A in the original message, for example, could never appear as an A in the code. Breaking the Enigma ciphers gave the Allies a key advantage, which, according to historians, shortened the war by two years thus saving many lives.


The Lorenz was an even more complex cipher machine than Enigma. Made by the Lorenz company, it was used exclusively for the most important messages passed between the German Army Field marshals and their Central High Command in Berlin. Its size meant that it was not a portable device like Enigma. Bletchley Park code breakers called the machine 'Tunny' and the coded messages 'Fish'.

Lorenz used the 'International Teleprinter Code', in which each letter of the alphabet is represented by a series of five electrical impulses. Messages were enciphered by adding, character by character, a series of apparently randomly generated letters to the original text. Crucially, to decrypt the enciphered message, the receiving Lorenz simply added exactly the same obscuring letters back to the ciphertext. The obscuring letters were generated by Lorenz's 12 rotors, five of which followed a regular pattern, while another five followed a pattern dictated by two pin wheels. Cracking Fish again relied on determining the starting position of the Lorenz machine's rotors.

The great Cryptanalyst, John Tiltman broke the first Fish messages at Bletchley in 1941 using hand-methods that relied on statistical analysis, but by 1944 the Germans had introduced complications which made it virtually impossible to break Tunny by hand alone. Dr Max Newman and his team in the 'Newmanry' were assigned the task of building machines to break Tunny.

The Bombe

Before World War II, Polish crypto-analysts had already designed an electro-mechanical machine to test Enigma rotor settings called a 'Bomba'. However, in December 1938 the German military changed their system slightly thus thwarting the Poles' ability to decrypt Enigma messages. The Poles passed all of their information over to the Britain and France and two mathematicians working at Bletchley Park, Alan Turing and Gordon Welchman, were able to build on this research to develop the 'Bombe' machine. Turing and Welchman exploited the fact that enciphered German messages often contained common words or phrases, such as generals' names or weather reports and so were able to guess short parts of the original message. These guesses were called 'cribs'. The fact that on an Enigma machine no letter can be enciphered as itself made guessing a small part of the text even easier. It also meant that the potential number of settings that the Enigma could be in on that day was greatly reduced.

Before running the Bombe, the wiring at the back of the machine was connected in accordance with a 'menu' drawn up by the code breakers based on cribs. The Bombe found potential Enigma settings not by proving a particular setting, but by disproving every incorrect one in turn. To speed up the codebreaking process, the brilliant mathematician Alan Turing developed an idea originally proposed by the Polish cryptanalysts. The result was the Bombe: an electro-mechanical machine that greatly reduced the odds, and thereby the time required, to break the daily-changing Enigma keys.

Over 200 of the Bombes were built by the British Tabulating Machine company at Letchworth, all of which were destroyed after the war. A Bombe machine has recently been rebuilt at Bletchley Park.


The first machine designed to break the Lorenz cipher was built at the Post Office research department at Dollis Hill and called 'Heath Robinson' after the cartoonist designer of fantastic machines. Although Heath Robinson worked well enough to show that Max Newman's concepts were correct, it was slow and unreliable. Max Newman called in the help of Tommy Flowers, a brilliant Post Office Electronics Engineer. Flowers went on to design and build Colossus, a much faster and more reliable machine that used 1,500 thermionic valves (vacuum tubes). The first Colossus machine arrived at Bletchley in December 1943. This was the world's first practical electronic digital information processing machine - a forerunner of today's computers.

Lorenz had to be cracked by carrying out complex statistical analyses on the intercepted messages. Colossus could read paper tape at 5,000 characters per second and the paper tape in its wheels travelled at 30 mph. This meant that the huge amount of mathematical work that needed to be done could be carried out in hours, rather than weeks. Mark I Colossus was upgraded to a Mark II in June 1944, and was working in time for Eisenhower and Montgomery to be sure that Hitler had swallowed the deception campaigns prior to D-Day on June 6th 1944. There were eventually 10 working Colossus machines at Bletchley Park.

A rebuilt Colossus is on view daily at The National Museum of Computing.