'Ultra' was the British-coined Allied designation for intelligence resulting from decryption largely of German communications (June 1941/August 1945).
The name, which became virtually the standard British and US designation for all intelligence derived from high-level cryptanalysis, resulted from the fact that the associated code-breaking success was considered more important than the highest security classification available at the time (Most Secret), and was therefore regarded as Ultra Secret.
Much of the German cipher traffic was encrypted on the Enigma machine, and as a result the term 'Ultra' has often been used almost synonymously with Enigma decrypts. Until the name 'Ultra' was adopted, there were several codenames for intelligence from this source, including 'Boniface'. For some time thereafter, 'Ultra' was used only for intelligence from this channel. Later the Germans began to use several stream cipher teleprinter systems, to which the British allocated the generic codename 'Fish', for their most important traffic. Of the several systems used, the most importantly were the Lorenz SZ 40/42 (initially codenamed 'Tunny') and Geheimsfernschreiber (codenamed 'Sturgeon'). These also were broken, particularly 'Tunny', which the British thoroughly penetrated. It was eventually attacked using the Colossus, considered to be the forerunner of the electronic programmable digital computer. Although the volume of messages read from this system was much smaller than that from the Enigma, they more than made up for it in their importance.
At the end of World War II, the Supreme Allied Commander in Europe, General Dwight D. Eisenhower, said that 'Ultra' had been 'decisive' to the Allied victory.
'Ultra' material was derived largely from German cipher traffic. These messages were generated on several variants of the Enigma electro-mechanical rotor machine. The Enigma machine was widely thought to be, in practical terms, unbreakable during the 1920s, when a variant of the commercial Model D was first used by the German navy. The German armed forces, Nazi party, SS in all its forms, and German foreign ministry all used Enigma machines, but there were several variants (the Abwehr used a four-rotor machine without a plugboard, for example, and the naval Enigma machine used different key management from that of the army and air force, making its traffic far more difficult to cryptanalyse). Each variant required different cryptanalytic treatment.
The fundamental break into the Enigma system was made in Poland during 1932 by Marian Rejewski. This 27-year-old mathematician used advanced mathematics (group theory and in particular permutation theory) to crack the Enigma system. Together with two colleagues at the Polish general staff’s Biuro Szyfrów (cipher bureau), he went on to develop practical methods of decrypting Enigma traffic. The team designed working 'doubles' of the Enigmas and developed equipment and techniques which helped in finding the keys needed for decryption (including the 'grill', 'clock', cyclometer, cryptologic 'bomb', and perforated sheets). Well before 1938, the Poles were routinely decrypting much Enigma traffic, and then the acceleration of the rate of changes in German operations (encipherment procedures, frequency of key changes, greater rotor choice) and the imminence of war led the Poles to share their achievements in Enigma decryption with France and the UK. This happened at a meeting at Pyry, in the Kabaty woods, to the south of Warsaw on 25 July 1939.
Neither the French nor the British had yet succeeded in breaking Enigma traffic, so this was a major cryptanalytic windfall for Poland’s western allies. Armed with this Polish assistance, the British began work on German Enigma traffic. Work on Enigma after the outbreak of World War II was carried out in France, at PC Bruno outside Paris, by Polish cipher bureau cryptologists who had escaped from the German 'Weiss' (i) invasion.
Early in 1939 the UK’s secret service had installed its Government Code and Cypher School (GC&CS) at Bletchley Park, some 50 miles (80 km) to the north of London, to work on German message traffic, and also created a major radio traffic interception network to collect enciphered messages for the cryptologists at Bletchley and at five nearby off-site outstations at Adstock, Gayhurst, Wavendon, Stanmore and Eastcote.
Eventually there was a very large organisation controlling the distribution of the resulting secret decrypted information. There were strict rules to restrict the number of people who knew of 'Ultra' and its origins in the hope of ensuring that nothing, such as leaks or actions, would alert the Axis powers that the Allies were reading any of their messages.
Before the term 'Ultra' came into use, the product from Bletchley Park was for a time codenamed 'Boniface' to give the impression to the uninitiated that the source was a secret agent. Such was the secrecy surrounding reports from 'Boniface' that 'his' reports were taken directly to Prime Minister Winston Churchill in a locked box to which he personally held the key.
The Bletchley Park workers included a mix of crossword enthusiasts, chess experts, mathematicians, and pioneer computer scientists. Among the last was Alan Turing, one of the founders of modern computing.
By 1943 a large proportion of intercepts (more than 2,000 per day at the height of operations) was routinely read, including some from Adolf Hitler himself. Such information enabled the Allies to maintain an often remarkably accurate picture of enemy plans and orders of battle and, when appropriately used, this was of inestimable value in formulating Allied strategy and tactics.
British attacks on the Enigma system were similar to the original Polish concepts, but their evolution was continued in order to keep pace with the growing complexity of the Germans' equipment and procedures. A particular challenge was the German navy’s Enigma machine. Even before the war, this had been a problem to the Poles, and only a portion of the Naval Enigma had been read at BS-4 (the cipher bureau’s German section) as a result of personnel and resources limitations, and because knowledge of army and air force traffic had been deemed more important to Poland’s defence requirements. One mode of attack on the Enigma machine relied on the fact that the reflector (a patented feature of the Enigma machines) guaranteed that no letter could be enciphered as itself, so an A could not be sent as an A. Another technique counted on common German phrases, such as 'Heil Hitler' or 'please respond', which were likely to occur in a given plain text: a successful guess as to a plain text was known at Bletchley as a 'crib'. With a probable plain text fragment and the knowledge that no letter could be enciphered as itself, a corresponding cipher text could often be identified, and this provided a clue to message keys.
On some occasions, German cipher clerks inadvertently helped Allied cryptanalysts: in one instance, for example, a clerk was asked to send a test message, and hit the 'T' key repeatedly and transmitted the resulting letters. A British analyst received from an intercept station a long message containing not a single 'T' and immediately realised what had happened. In other cases, as they had before the war, Enigma operators would constantly use the same settings for their message keys, often their own initials or those of a girlfriend: one apparently had the initials 'C.I.L.', so Bletchley Park named such hints 'cillies'. Analysts were set to finding such messages in the sea of daily intercepts, which winnowed out enough possibilities to allow Bletchley to use other original Polish techniques as well to find the initial daily keys. Other German operators used 'form letters' for daily reports, notably weather reports, so the same crib might be used every day.
Had the Germans ever replaced every rotor at the same time, the British might not have been able to break back into the system. And had German operating practices been more secure, things would have been much more difficult for the British cryptologists. However, because of the expense and difficulty of getting new rotors to all ships and units, this was never done. Instead the Germans every so often added new rotors to the mix, thereby allowing the British to work out the wirings of the newest rotors.
Usable 'Ultra' information came too late to be of great help during the Battle of Britain in the summer and autumn of 1940.
The Allies were always seriously concerned to conceal from the Axis powers the fact that they had broken into Enigma traffic. This was taken to the extreme that, for instance, though they knew from intercepts the whereabouts of U-boats lying in wait in mid-Atlantic, the U-boats often were not hunted unless a 'cover story' could be arranged: a search aeroplane might be 'fortunate enough' to sight the U-boat, thus explaining the Allied attack.
'Ultra' information was used to attack and sink many of the vessels delivery supplies and equipment to the Axis forces in North Africa but, as in the North Atlantic, every time such information was used an 'innocent' explanation had to be provided: reconnaissance aircraft were often sent on otherwise unnecessary missions to ensure they were spotted by the Germans.
The British seem to have been somewhat more disciplined about such measures than the Americans, and this difference was a source of friction between the two Allied powers. The distribution of 'Ultra' information to Allied commanders and units in the field involved considerable risk of discovery by the Germans, and great care was taken to control both the information and knowledge of how it was obtained. Liaison officers were appointed for each field command to manage and control dissemination.
In the summer of 1940 British cryptanalysts, who were successfully breaking Luftwaffe Enigma cipher variants, were able to give Churchill information about the issue of maps of England and Ireland to the 'Seelöwe' invasion forces.
From the beginning, the naval version of Enigma used a larger selection of rotors than did the army or air force versions, as well as operating procedures that made it much more secure than other Enigma variants. There was no hint at all to the initial settings for the machines, and there was little probable plain text to use. Different and far more difficult methods had to be used to break into the German naval Enigma traffic, and with the U-boats running freely in the Atlantic after the June 1940 fall of France, a more direct approach recommended itself. On 7 May 1941 the Royal Navy deliberately captured a German weather ship, together with cipher equipment and codes. Two days later U-110 was captured, together with its Enigma machine, code book, operating manual and other information that enabled Bletchley Park workers to break U-boat arm messages until the end of June. And it was done again shortly afterward this. Naval Enigma machines or settings books were captured from seven U-boats and eight German surface ships. These included U-559 and U-505 in 1942 and 1944 respectively, and a number of German weather boats and converted trawlers such as Krebs, captured during a commando raid on the Lofoten islands off Norway.
In other cases the Allies induced the Germans to provide them with cribs. To do this they would drop mines (or take some other action), then listen for messages thus provoked. In the case of mining, they expected the word 'Minen' to occur in some of the messages. This technique was, at Bletchley, called 'gardening'. Even these brief periods were enough to exercise a marked effect on the course of the war.
The charting decrypted Enigma traffic against British shipping losses for a given month showed a strong pattern of increased losses when the naval Enigma was blacked out, and vice versa. But by 1943 so much traffic had been decrypted that Allied cryptologists had an excellent understanding of the messages coming from various locations at various times. Thus a brief message sent from the west at 06.00 was likely to have been broadcast by a weather-reporting boat in the Atlantic, and that meant the message that would almost certainly contain these cribs; and similarly for other traffic. From this point on, the naval Enigma messages were read constantly, even after changes to the ground settings.
However, the new tricks only reduced the number of possible settings for a message. The number remaining was still huge, and as a result of the new rotors which the Germans periodically added, that number was much larger than the Poles had faced. In order to solve this problem the Allies, especially the USA, 'went industrial' and produced much larger versions of the Polish 'bombs' that could rapidly test thousands of possible key settings.
Some Germans had suspicions that all was not right with the Enigma system. Admiral Karl Dönitz received reports of 'impossible' encounters between U-boats and Allied vessels, and this made him suspect some of his communications had been compromised. In one instance, three U-boats met at a tiny island in the Caribbean, and a British destroyer promptly arrived. The U-boats all escaped and reported what had happened. Dönitz immediately asked for a review of the security of the Enigma system. The analysis suggested that the signals problem, if there was one, was not due to the Enigma machine itself. Dönitz had the settings book changed, blacking out Bletchley Park for a period. However, the evidence was never enough to truly convince him that the naval Enigma traffic was being read by the Allies, the more so as his B-Dienst signals intelligence group, which had partially broken Royal Navy traffic (including its convoy codes early in the war), supplied enough information to support the idea that the Allies were unable to read the naval Enigma traffic. Coincidentally, German success in this respect almost exactly matched in time an Allied blackout from naval Enigma.
In 1941 British intelligence learned that the German navy was about to introduce M4, a new version of Enigma with four rather than three rotors. Fortunately for the Allies, in December a U-boat mistakenly transmitted a message using the four-rotor machine before it was scheduled for activation. Realising the error, the U-boat retransmitted the same message using the three-rotor Enigma, giving the British sufficient clues to break the new machine soon after it became operational on 1 February 1942. The U-boat network which used the four-rotor machine was known as 'Triton', codenamed 'Shark' by the Allies, and its traffic was routinely readable.
It is commonly claimed that the breaks into the naval Enigma system resulted in a one-year shortening of World War II but, given its effects on the 2nd Battle of the Atlantic alone, that might be an underestimate. Breaking of some messages (not in the German Enigma system) led to the defeat of the Italian navy in the Battle of Cape Matapan, and was preceded by another 'fortuitous' sighting by a reconnaissance aeroplane. Admiral Sir Andrew Cunningham, commanding the Mediterranean Fleet, also undertook a neat little physical deception measure in Egypt to prevent Axis agents from taking note of his movements and deducing that a major operation was planned.
'Ultra' information was of considerable assistance to the British at the 2nd Battle of El Alamein in western Egypt during the long-running battle with the Axis forces commanded by Generalfeldmarschall Erwin Rommel. Intelligence derived from signals between Hitler and Generalfeldmarschall Günther von Kluge was of considerable help during the campaign in France just after the 'Neptune' (iii) landings for 'Overlord', particularly in regard to estimates of when German reserves might be committed to battle.
By 1945 almost all German Enigma traffic could be decrypted within a day or two, yet the Germans remained confident of its security. Had they been better informed, they could have changed systems, forcing Allied cryptologists to start from the beginning. The Germans considered Enigma traffic so secure that they openly discussed their plans and movements, handing the Allies huge amounts of information.
However, 'Ultra' information was also at times misused or ignored. Rommel’s intentions just before the Battle of the Kasserine Pass in North Africa late in 1942 had been suggested by 'Ultra', but this was not taken into account by the Americans. Likewise, Ultra' traffic suggested an attack in the Ardennes late in 1944, but 'Wacht am Rhein' came as a total surprise to the Allies because the information was disregarded.
After the war, the US Target Intelligence Committee (TICOM) project teams found and detained a considerable number of German cryptographic personnel. Among the things they learned was that German cryptographers, at least, understood very well that Enigma messages might be read and therefore that Enigma traffic was not unbreakable, but also found it impossible to imagine anyone going to the immense effort required.
An intriguing question concerns the alleged use of 'Ultra' information by the 'Lucy' spy ring. This was an extremely well informed and rapidly responsive ring which was able to get information 'directly from the German General Staff Headquarters', often on specific request. It has been alleged that 'Lucy' was, in major part, a way for the British to feed 'Ultra' intelligence to the Soviets in a way that made it appear to have come from highly placed espionage and not from cryptanalysis of German radio traffic. The 'Lucy' ring was operated, apparently, solely by Rudolf Rössler, and its product was initially treated with considerable suspicion by the Soviets. The information it provided was accurate and timely, however, and Soviet agents in Switzerland eventually took it quite seriously.
In the Pacific theatre the Japanese cipher machine dubbed 'Purple' by the Americans, and unrelated to the Enigma system, was used for the highest level of Japanese diplomatic traffic. It was also cracked by the US Army’s Signal Intelligence Service. Some 'Purple' decrypts proved useful elsewhere, such as detailed reports by Japan’s ambassador to Germany, which were encrypted on the 'Purple' machine. These reports included reviews of German strategy and intentions, reports on direct inspections (in one case, of Normandy beach defences) by the ambassador, and reports of long interviews with Hitler. The Japanese are said to have obtained an Enigma machine as early as 1937, although it is debated whether they were given it by their German ally or bought a commercial version which, except for plugboard and actual rotor wirings, was essentially the machine used by the German army and air force.
While it is obvious why the UK and USA went to very considerable pains to keep 'Ultra' a secret until the end of the war, it has been a matter of some conjecture why it was kept officially secret for 29 years after the war’s end, until 1974. During that period the important contributions to the war effort of a great many people remained unknown, and they were unable to share in the glory of what is likely one of the chief reasons the Allies won the war, or at least as quickly as they did. At least three possibilities as to why 'Ultra' was kept secret so long: each is plausible and all may be true. First, after World War II the British gathered all the Enigma machines they could find and sold them to Third World countries, confident that they could continue reading the messages of the machines' new owners. Second, between the world wars Churchill mistakenly publicly disclosed information obtained by decrypting of Soviet secret communications, and this prompted the Soviets to change their cryptography, leading to a cryptological blackout. The Allies had no wish to see any repeat of such as episode. Third, two weeks after VE-Day Churchill requested that former recipients of 'Ultra' intelligence be asked not to divulge the source or the information they had received from it, in order that there might be neither damage to the future operations of the Secret Service nor any cause for the Allies' enemies to blame it for their defeat. Since it was British and, later, US message-breaking which had been the most extensive, this meant that the importance of Enigma decrypts to the prosecution of the war remained unknown.