ULTRA was a code word for British communications intelligence (COMINT) in the Second World War, primarily targeted against the German Enigma machine and its derivatives. . This was one of the greatest intelligence triumphs in the history of warfare; the British read many German messages throughout the war.
The COMINT organization was called the Government Code and Cipher School (GCCS), which reported to the Secret Intelligence Service. Eventually, GCCS became a separate organization, the Government Communications Headquarters, with functions similar to the Canadian Communications Security Establishment or the U.S. National Security Agency.
Before the war
The Enigma machine was first built in the 1920s and was adopted by the German military in the late 20s. It used a set of rotors which moved with every character encrypted to give a complex polyalphabetic cipher. Some versions had an additional complication, a plugboard similar to an old telephone switchboard which mixed the signals up before and after the rotors.
The first break was achieved in 1931 by Polish Intelligence. The following year another Pole, Marian Rejewski, broke the more complex version with a plugboard.  The Poles had a novel approach though, unknown to them, Americans were working along similar lines in the same time period. Previously, cryptanalysis had been largely done by linguists, but solving machine ciphers needed sophisticated mathematical techniques so Polish Intelligence recruited mathematicians. They also used machines; the Poles produced the first "bomba", a machine for quickly finding enigma keys, in 1938.
The Poles continued to read Enigma throughout the 30s, improving their techniques as they went along, but they naturally kept their secret, even from their allies. However in 1939, with war looming, they revealed the Enigma attacks to Britain and France. When the war actually started, key people and equipment were evacuated to Romania and thence to France.
Then it was the turn of Britain to keep secrets, even from their allies. They developed a large ULTRA organisation, based at Bletchley, using "bombe" machines (an improved version of the Polish bomba) to attack Enigma and breaking many other ciphers as well. The Polish cryptanalysts were put to work attacking low-level German codes, but they were never brought to Bletchley or told about ULTRA. The secret was revealed to the Americans after they entered the War, but not to other allies such as Russia and China.
Bletchley Park and the early days
Alan Turing, a Cambridge mathematician known at Bletchley as "prof" was one of the key figures. He had done extensive work before the war or the formal theory of computation, including inventing the Turing Machine. At Bletchley, he was a contributor to Bombe and Collosus design and the main player in breaking the U-boats' four-rotor enigma.
Bletchley Park now has a museum and a web site . In 2008, they ran low on money and an appeal was made  for funds to keep it going. Respondents include several major hi-tech companies . There is now a project to digitize their massive collection of files .
Special Liaison system
Early cryptanalytic automation
A 1979 Polish film Sekret Enigmy (The Enigma Secret) dramatises the escape of three Polish mathematicians via Romania to France at the outbreak of war.
Neal Stephenson's novel Cryptonomicon  has a great deal of cryptography in it, competently described. Parts of it take place around the turn of the 21st century, but other parts follow the grandparents of the 21st-century characters through the Second World War. Lawrence Waterhouse is an American who meets Turing at Princeton, learns cryptanalysis from a character apparently based on William Friedman, and later works at Bletchley. Bobby Shaftoe is a Marine Sergeant who does a lot of rather strange missions at Bletchley and Waterhouse's behest.
The 2001 film Enigma , directed by Michael Apted, takes place largely at Bletchley. There is a character clearly based on Turing, a Cambridge mathematician who has solved a major German naval cipher, but unlike Turing he is heterosexual; much of the plot centers on his relationship with a lovely young woman.
U 571  was a film released in 2000 and directed by Jonathan Mostow dealing with a US Navy mission that sets out to capture an Enigma machine and succeeds. It has been heavily criticised for historical inaccuracy; the actual capture of the Enigma was by the Royal Navy, from U 110, and was not so much a planned mission as a case of an officer on the spot taking initiative.
- Winterbotham, F.W. (2000), The Ultra Secret, Orion
- Gilbert Bloch and C. A. Deavours (July 1987). ENIGMA BEFORE ULTRA POLISH WORK AND THE FRENCH CONTRIBUTION.