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During World War I British code breaking was performed by Naval Intelligence
in "Room 40" at the Admiralty. In 1920 this function was transferred from
the Navy to the Foreign Office. At first, without an Enigma machine, there
wasn't much hope of breaking the German codes, and the British had almost
given up trying. In particular, the rotor wiring was unknown until, on
July 25, 1939, the Poles handed an Enigma replica over to the dumbfounded
British during a secret meeting near Pyry.
In August, codebreaking operations were physically moved to Bletchley Park (known familiarly as "BP"), an estate 40 miles from London, although its official designation was "Station X", as the tenth station. It was renamed Government Code and Cypher School (GC&CS, often irreverently referred to as the Golf, Cheese and Chess Society) under the direction of Naval Commander Alastair Denniston. A number of "huts" (small frame houses) were built to house the various divisions. Chess masters, mathematicians, professors and linguists were recruited from all over Great Britain, most from Cambridge University. Ian Fleming, one member of the team, went on to write the James Bond novels. A coding machine was featured in the James Bond movie "From Russia with Love." Not surprisingly, the brief view of the machine reveals it is nothing like an Enigma, since in 1968 the Enigma was still under wraps.
BP and Enigma
When Gordon Welchman reported to BP's Alastair Denniston he was assigned to the man in charge of Enigma operations, Dillwyn "Dilly" Knox. Alan Turing, a brilliant mathematician who developed the "Universal Turing Machine," the conceptual forerunner of the modern digital computer, was also working in the same department. Although Welchman was supposed to be working on traffic analysis, he idly started to think about how Enigma traffic might be decoded. From consideration of the doubly encoded "sub-keys" used by the Germans (see Chapter I), he devised a method of eliminating some of the many possible keys. A number of sheets were to be perforated with holes at specified locations, and stacked. Light shining through the holes in one or more locations could indicate possible keys. When he submitted his idea to Knox, he was surprised to find that the method was already in use! He had independently invented the Poles' Zygalski sheets, now known as Jeffreys sheets, named after the man in charge of the operation.
The possible keys suggested by the Jeffreys sheets were then tested on "bombes," machines descended from the Polish bomby. During the "phony war," with the French hunkered down on the Maginot line for seven months, BP was starting to decode some German messages. When Hitler invaded Denmark and Norway on April 9, 1940, much of the German traffic was being decoded.
The British codebreakers at Bletchley
Park received an Enigma machine and rotors I to V from the Polish Cipher
Bureau in August 1939. Marian Rejewski, an outstanding Polish cryptanalyst,
had reconstructed the wiring of rotors I to III at the end of 1932 using
mathematical techniques, and the wiring of rotors IV and V before the war
began. The British recovered rotors VI and VII from the crew of U-33 on
12 February 1940, while rotor VIII was captured in August 1940.
In May and June 1940, using cleartext and cipher text captured from Schiff 26, Hut 8 at Bletchley Park had solved some April naval Enigma traffic with the aid of the first bombe (a high speed key-finding aid). That bombe, which was Alan Turing's brainchild, was much slower than the bombes with the 'diagonal board' invented by Gordon Welchman. The improved bombe, with the board, entered service in mid-August.
On average more than 3,000 coded messages arrived at Bletchley Park each day from the 'Y' Stations. Messages were taken to different 'Huts', depending on whether they had come from the German army, air force, navy or another source. A message from a U-boat would go
to Hut 8. Sometimes messages began with the same words, such as a weather report. This gave clues (called a CRIB) about how the rest of the message had been encoded. When the code-breakers eventually worked out what the CRIB letters might be, they tested them on the machine shown on the left, called a BOMBE. Bombes were huge, noisy electro-mechanical machines which could check through combinations of letters far quicker than a human being could. When the Bombe stopped, this meant that the code-breakers' guesses were right. All that days U-boat messages could then be decoded. The decoded messages were in German in blocks of five letters. They had to be
carefully translated into English.
On the 9th of May, 1941, U-110 had been about to attack an Allied convoy when it was forced to surface by British ships protecting the convoy. The German crew surrendered. The British were anxious to make sure that the Germans did not find out that U-110 and its codebooks had been captured. All the sailors who took part in the operation were sworn to secrecy. If the Germans had found out, they would almost certainly have changed their codes. This would have made the code-breakers' job far more difficult - but by 1943 they had the help of Colossus - the world's first programmable electronic digital computer.
Colossus was built for the code-breakers at Bletchley Park by post office engineers in 1943. The computer was as big as a room - 5 metres long, 3 metres deep and 2.5 metres high - and was made mainly from parts used for post office telephone and telegraph systems. It was a development from the mechanical Bombes. Colossus worked by 'reading', through a photoelectric system, a teleprinter tape
containing the letters of the coded message. It read 5,000 letters a second.
All possible combinations of the coded message were checked with the cypher key generated by Colossus. A teleprinter typed out the results of Colossus's search, revealing the settings which had been used by the Germans to send their messages. Ten Colossus Mark 2s were eventually built. They cracked the Enigma codes and even more complicated ones devised by the Germans on their 12 -rotor cipher machines. A complete Mark 2 Colossus machine has recently been rebuilt and is on display at Bletchley Park.
The information revealed by the code-breakers at Bletchley Park was called ULTRA. ULTRA was so secret that only those who needed to know about it - like the British Prime Minister, Winston Churchill - were told of its existence.
Bletchley Park today