Ultra
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- This article is about WW II intelligence material codenamed "Ultra." For other usages, see Ultra (disambiguation)
Ultra (sometimes capitalised ULTRA) was the name used by the British for intelligence resulting from decryption of German communications in World War II. The term eventually became the standard designation in both Britain and the United States for all intelligence from high-level cryptanalytic sources. The name arose because the code-breaking success was considered more important than the highest security classification available at the time (Most Secret) and so was regarded as being Ultra Secret.
Much of the German cipher traffic was encrypted on the Enigma machine, hence the term "Ultra" has often been used almost synonymously with "Enigma decrypts."
Until the name "Ultra" was adopted, there were several cryptonyms 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 for their most important traffic, to which the British gave the generic code-name FISH. Several distinct systems were used, principally the Lorenz SZ 40/42 (initially code-named TUNNY) and Geheimfernschreiber (code-named 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.
F.W. Winterbotham, in The Ultra Secret (1974), quotes the western Supreme Allied Commander, Dwight D. Eisenhower, as at war's end describing Ultra as having been "decisive" to Allied victory in World War II.
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Sources and history
Encrypted messages
The Ultra material largely came from the German cypher traffic. These messages were generated on several variants of an electro-mechanical rotor machine, called Enigma. The Enigma machine was widely thought to be in practice unbreakable in the 1920s, when a variant of the commercial Model D was first used by the German Navy. The German Army, Navy, Air Force, Nazi party, Gestapo, and German diplomats all used Enigma machines, but there were several variants (eg, the Abwehr used a four-rotor machine without a plugboard, and Naval Enigma used different key management from that of the Army or Air Force, making its traffic far more difficult to cryptanalyze). Each variant required different cryptanalytic treatment. The commercial versions were not so secure. Dilly Knox, of GC&CS, is said to have broken it during the 1920s.
Breaking the cipher
- Main article: Cryptanalysis of the Enigma
The fundamental break into the Enigma systems that were to be used by Nazi Germany was made in Poland in 1932, just on the eve of Adolf Hitler's accession to power, by Marian Rejewski. The 27-year-old mathematician used advanced mathematics (permutation theory) for the first time 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. They 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, bomba, and perforated sheets). Well before 1938, much German Enigma traffic was being routinely decrypted by the Poles; but accelerating changes in German operations (encipherment procedures, frequency of key changes, greater rotor choice) and looming war led the Poles to share their achievements in Enigma decryption with France and England. This happened during the famous meeting at Pyry, in the Kabaty Woods south of Warsaw, on July 25, 1939. Since neither the French nor the British had succeeded in breaking Enigma traffic, this was a major windfall for Poland's western allies.
Armed with this Polish assistance, the British began work on German Enigma traffic. (Work that would be done after the outbreak of World War II in France, at PC Bruno outside Paris, would be strictly the domain of the Polish Cipher Bureau cryptologists who had escaped Poland.) Early in 1939 Britain's secret service had installed its Government Code and Cypher School (GC&CS) at Bletchley Park, 50 miles (80 km) north of London, to work on enemy message traffic. They also set up a large interception network to collect enciphered messages for the cryptologists at Bletchley. Eventually there would be a very large organization controlling the distribution of the resulting – secret – decrypted information, which in time came to be called "Ultra." Strict rules were established to restrict the number of people who knew about the existence of Ultra in the hope of ensuring that nothing (e.g., leaks, actions) would alert the Axis powers that the Allies were reading their messages. Earlier in the war, the product from Bletchley Park was 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 group at Bletchley Park working on breaking messages was a mix of crossword enthusiasts, chess mavens, mathematicians, and pioneer computer scientists. The last group overlapped some of the others and included Alan Turing, one of the fathers of modern computing. By 1943, large portions of the intercepted signals (more than 2,000 daily at the maximum) were routinely read, including some from Hitler himself. Such information enabled the Allies to routinely develop an accurate picture of enemy plans and orders of battle, and when used sensibly were of great value in forming the basis of Allied strategic and tactical war plans.
Methods of attack
British attacks on the Enigmas were similar to the original Polish methods, but naturally continued evolving to keep pace with the growing complexity of German equipment and procedures. (For a discussion of the many identical techniques used by the Poles and the British, see Kozaczuk 1984, appendix F.) A particular challenge would be German Naval Enigma. Even before the war, it had been a challenge to the Poles; only a portion of Naval Enigma had been read at B.S.-4 (the Cipher Bureau's German section) due to limited Bureau personnel and resources and because knowledge of army and air force traffic had been deemed more important to Poland's defense. (Kozaczuk, pp. 31, 58.)
One mode of attack on the Enigma 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 various common German phrases, like "Heil Hitler" or "please respond," which were found to likely be in this or that plaintext; successful guesses as to the plaintext were known at Bletchley as cribs. With a probable plaintext fragment and the knowledge that no letter could be enciphered as itself, it wasn't uncommon that a corresponding ciphertext fragment could be identified. This often provided a large hint as to the message settings.
German operators themselves also gave the cryptanalysts immense help on a number of occasions. In one instance an operator was asked to send a test message, so he simply hit the T key repeatedly and sent it. A British analyst received a long message without a single T in it from the interceptor stations, 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 codes, often their own initials or those of their girlfriends (one apparently had the initials "C.I.L.", so Bletchley Park named such hints 'cillies'). Analysts were set to finding these messages in the sea of intercepted traffic every day, which winnowed out enough possibilities to allow Bletchley to use other original Polish techniques as well to find the initial settings for the day. Other German operators used "form letters" for daily reports, notably weather reports, so the same crib could be used every day.
Had the Germans ever replaced every rotor at the same time, it is possible that the British would not have been able to break back into the system. And had German operational practice been better, things would have been much more difficult. However, because of both the expense and the difficulty of getting all those new rotors to all the necessary ships and units, it was never done. Instead the Germans simply added new rotors to the mix every so often, allowing the wirings of the newest ones to be worked out by the British after a short period.
Use of Ultra
Usable Ultra information came too late to be of great help during the Battle of Britain.
The Allies were seriously concerned to conceal from the Axis command that they had made breaks into any of the Enigma traffic. This was taken to the extreme that, for instance, though they had intercepted and knew the whereabouts of U-boats lying in wait in mid-Atlantic, these were not generally hunted unless a "cover story" could be arranged — often a search plane was "fortunate enough" to sight the U-boat, thus explaining an attack. Ultra information was used to attack and sink many of the Afrika Korps supply ships traveling to North Africa; but, as in the North Atlantic, every time such information was used, some "innocent" explanation had to be provided: often scout planes were sent on otherwise unnecessary missions, to ensure they were spotted by the German military. The British were, it is said, more careful about this than the Americans, and the difference in caution on this point was a source of friction between them.
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 German Air Force Enigma-cypher variants, were able to give Churchill information about the issuing of maps of England and Ireland to the Sealion 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 plaintext to use, either. Different and far more difficult methods had to be used to break into Naval Enigma traffic, and with the U-boats running freely in the Atlantic after the 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; and 2 days later U-110 was captured, together with an Enigma machine, code book, operating manual and other information that enabled Bletchley Park to break submarine messages until the end of June. And it was done again shortly afterwards.
Naval Enigma machines or settings books were captured from a total of 7 U-boats and 8 German surface ships. These included U-boats U-505 (1944) and U-559 (1942) and a number of German weather boats and converted trawlers such as the Krebs, captured during a raid on the Lofoten Islands off Norway. More fantastic scenarios were contemplated, such as Ian Fleming's James Bondian suggestion to "crash" captured German bombers into the sea near German shipping, hoping they would be "rescued" by a ship's crew, which would be taken captive by commandos concealed in the plane who would capture the cryptographic material intact.
In other cases the Allies forced the Germans to provide them with a crib. To do this they would drop mines (or take some other action), and then listen for messages thus provoked. In the case of mining this or that channel, they expected the word "Minen" would be in some of them. This technique was called gardening at Bletchley.
Even these brief periods were enough to have dramatic effects on the progress of the War. Charting the amount of traffic decoded against the British shipping losses for that month shows a strong pattern of increased loss when Naval Enigma was blacked out, and vice versa. But, by 1943, so much traffic had been decrypted that the code breakers had an excellent understanding of the messages coming from various locations and times. For instance, a brief message sent from the west at 6am was likely to be sent by a weather reporting boat in the Atlantic, and that meant the message would almost certainly contain these cribs, and similarly for other traffic. From this point on, Naval Enigma messages were being 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 due to the new rotors the Germans had added from time to time, that number was much larger than the Poles had been left with. In order to solve this problem the Allies, especially the US, "went industrial", and produced much larger versions of the Polish bomba that could test thousands of possible key settings very rapidly indeed.
Some Germans had suspicions that all was not right with Enigma. Karl Dönitz received reports of "impossible" encounters between U-boats and enemy vessels which made him suspect some compromise of his communications. In one instance, three U-boats met at a tiny island in the Caribbean, and a British destroyer promptly showed up. They all escaped and reported what had happened. Dönitz immediately asked for a review of Enigma's security. The analysis suggested that the signals problem, if there was one, wasn't due to the Enigma itself. Dönitz had the settings book changed anyway, blacking out Bletchley Park for a period. However the evidence was never enough to truly convince him that Naval Enigma was being read by the Allies. The more so, since his counterintelligence B-Dienst group, who had partially broken Royal Navy traffic (including its convoy codes during the early part of the War), supplied enough information to support the idea that the Allies were unable to read Naval Enigma. 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 4 wheels rather than 3. Fortunately, for the Allies, in December, a U boat mistakenly transmitted a message using the four rotor machine before it was due to be implemented. Realising the error, they re-transmitted the same message using the 3 rotor Enigma, giving the British sufficient clues to break the new machine very shortly after it became operational on February 1 1942. The U-boat network which used the four rotor machine was known as Triton, codenamed Shark by the Allies. Its traffic was routinely readable.
It is commonly claimed that the breaks into Naval enigma resulted in the war being a year shorter, but given its effects on the Battle of the Atlantic (1940) alone, that might be an underestimate.
A break of some messages (not in German Enigma, however) led to the defeat of the Italian Navy at Capa Matapan, and was preceded by another 'fortunate' search plane sighting. British Admiral Cunningham also did some fancy footwork at a hotel in Egypt to prevent Axis agents from taking note of his movements and deducing that some major operation was planned. Ultra information was of considerable assistance to the British (Montgomery being "in the know" about Ultra) at El Alamein in Western Egypt in the long running battle with the Afrika Korps under Rommel. Intelligence from signals between Adolf Hitler and General Günther von Kluge was of considerable help during the campaign in France just after the Allied landings on D-Day, particularly in regard to estimations of when German reserves would be committed to battle.
By 1945 almost all German Enigma traffic (Wehrmacht, Navy, Luftwaffe, Abwehr, SD, etc.) could be decoded within a day or two, yet the Germans remained confident of its security. Had they been better informed, they simply could have, and surely would have, changed systems, forcing Allied code-breakers to start over. The Germans considered Enigma traffic so secure that they openly discussed their plans and movements, handing the Allies a huge amount of very useful information. However, Ultra information was also misused or ignored at times. For instance, Rommel's intentions just prior to the Battle of the Kasserine Pass in North Africa had been suggested by Ultra, but this was not taken into account by the Americans. Likewise, Ultra traffic suggested an attack in the Ardennes in 1944, but the Battle of the Bulge was a surprise to the Allies because the information was disregarded.
After the War, the American TICOM project teams found and detained a considerable number of German crypto personnel. Among the things they learned was that German cryptographers, at least, understood very well that Enigma messages might be read; they knew Enigma was not unbreakable. They just found it impossible to imagine anyone going to the immense effort required. (See Bamford's Body of Secrets in regard to the TICOM missions immediately after the War.)
A most intriguing alleged, and still open, use of Ultra information may have been in 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. The allegation is that it was, in major part, a way for the British to get Ultra information to the Soviets in a way which appeared to have come from highly placed espionage, and not from cryptanalysis of German radio traffic. The Lucy ring was operated by, apparently, one man, Rudolf Roessler, and was initially treated with considerable suspicion by the Soviets when it began to operate. The information it provided was accurate and timely, and Soviet agents in Switzerland (including Alexander Rado, the director) eventually took it quite seriously.
Magic and Purple in Europe
In the Pacific Theatre, the Japanese cypher machine called "Purple" was unrelated to the Enigmas, but was used for the highest level Japanese diplomatic traffic. It was also cracked, though, by the US Army's Signal Intelligence Service. Some of the Purple traffic was very useful elsewhere during the War, for instance the very detailed reports by the Japanese Ambassador to Germany which were sent to Japan using the Purple machine. Some of these included reviews of German strategy and intentions, some were reports of direct inspections (in one case of the Normandy beach defenses!) by the Ambassador, and some were reports of long interviews with Hitler.
The Japanese are said to have obtained an Enigma machine as early as 1937, although whether they were given it by their German allies, or bought a commercial version which, except for the plugboard and the actual rotor wirings, was essentially the German Army / Air Force machine, is disputed.
Postwar public disclosure of Ultra
While it is obvious why Britain and the United States went to considerable pains to keep Ultra a secret until the end of the war, it has been a matter of some conjecture why Ultra was kept officially secret for 29 years thereafter, 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 versions exist as to why Ultra was kept secret so long. Each has plausibility. All may be true. First, as David Kahn pointed out in his 1974 New York Times review of F.W. Winterbotham's The Ultra Secret, after World War II the British gathered up 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. A second explanation relates to a misadventure of Winston Churchill's between the World Wars, when he publicly disclosed information obtained by decrypting Russian secret communications; this had prompted the Russians to change their cryptography, leading to a cryptologicial blackout. The third explanation is given by Winterbotham (The Ultra Secret, introduction), who recounts that two weeks after V-E 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, American message-breaking which had been the most extensive, this meant that the importance of Enigma decrypts to the prosecution of the war remained unknown. Discussion by either the Polish or the French of Enigma breaks carried out early in the war would have been uninformed regarding breaks carried out during the balance of the war. Nevertheless it was the public disclosure of Enigma decryption, in the book Enigma (1973) by French Intelligence officer Gustave Bertrand, that generated pressure to discuss the rest of the Enigma/Ultra story.
The British ban was finally lifted in 1974, the year that a key participant on the distribution side of the Ultra project, F.W. Winterbotham, published The Ultra Secret. Wintherbotham's book is very interesting, but is in error on many points. He worked on the operation to distribute Ultra to end consumers and, based on the evidence of his book, did not understand much about cryptology. Peter Calvocorressi's book, Top Secret Ultra (1980), is in this regard better written and more responsible. He was involved in Bletchley Park's intelligence analysis of decrypts, working between the cryptological operation and Winterbotham's distribution operation.
The official history of British intelligence operations during WW II was published in five volumes from 1979 to 1988. It was mostly written by Sir Harry Hinsley, with one volume by Michael Howard. There is also a one-volume collection of reminiscences by Ultra veterans, edited by Hinsley.
As mentioned, after the war, surplus Enigmas and Enigma-like machines were sold to many countries around the world, which remained convinced of the security of the remarkable cipher machines. Their traffic was not so secure as they believed, however, which is of course one reason the British and Americans made the machines available. Switzerland even developed its own version of the Enigma, the NEMA, and used it for decades (at least into the late '70s).
Some information about Enigma decryption did get out earlier, however. In 1967 the Polish military historian Władysław Kozaczuk in his book Bitwa o tajemnice (Secret War) first revealed that the German Enigma had been broken by Polish cryptologists before World War II. The same year, David Kahn in The Codebreakers described the 1945 capture of a Naval Enigma machine from U-505 and mentioned, somewhat in passing, that Enigma messages were already being read by that time, requiring "machines that filled several buildings." In 1971 Ladislas Farago's The Game of the Foxes gave an early published version of the myth of the purloined Enigma that enabled the British (according to Farago, Alfred Dillwyn Knox) to crack the cipher (Farago mentions an Abwehr Enigma). By 1970 newer, computer-based ciphers were becoming popular as the world increasingly turned to computerised communications, and the usefulness of Enigma copies (and rotor machines generally) rapidly decreased. It was decided at this point to "let the cat out of the bag," and revelations about some of Bletchley Park's operations were permitted in 1974.
The National Security Agency retired the last of its rotor-based encryption systems in the 1980s.
Difficulties with some disclosures
Many accounts of the Enigma-decryption story, and of other World War II cryptological happenings, have been published. Several are unreliable in many respects. This can be traced to a number of causes:
- First, not all of the authors were in a position to know. Several books have been published by those on the Ultra distribution side at Bletchley Park, but work there was very seriously compartmentalised, making it difficult to credit some episodes when they are due only to such a source. The story about Churchill deliberately not interfering with a Luftwaffe bombing of Coventry which was known through Enigma decrypts is one such. Peter Calvocoressi's book, Top Secret Ultra, contains a sounder account of the episode.
- Second, the cryptanalytic work was tricky and quite technical. It requires someone with a considerable understanding of cryptanalysis, and of Enigma, to adequately comprehend -- or explain -- how either worked.
- Third, documents have been 'lost' in secret archives. Those not actually lost have taken decades to be released to the public, and some are, presumably, still to be released. In any case, none of them were originally written, nor made available later, with historical clarity in mind; considerable perspective is required to make reasonable use of them.
- Fourth, governments have chosen to keep secret or release information to serve their own purposes, not historical accuracy or completeness.
- Fifth, several authors have had agendas which took precedence over accuracy in their reports. At least one incident is known of whole-cloth fabrication regarding British cryptanalytic progress on a particular World War II Japanese Navy cryptosystem. The account was claimed to have been written from the unpublished memoirs of an Australian cryptanalyst, but substantive parts of the published version appear to have been simply invented.
- Sixth, many writers have not done their research. The exact fate of the Enigma spy "Asch" was not publicly known till Hugh Sebag-Montefiore tracked his daughter down, circa 1999. Her account can be found in his book.
As with other history, but more than for most, the history of cryptology, especially its recent history, should be read carefully, due to its complexity and to possibly confusing or misleading agendas.
Ultra's strategic consequences
There has been controversy about the influence of Allied Enigma decryption on the course of World War II. Probably the question should be broadened to include Ultra's influence not only on the war itself, but on the postwar period as well.
Wartime consequences
An exhibit in 2003 on "Secret War" at the Imperial War Museum, in London, quoted British Prime Minister Winston Churchill telling King George VI: "It was thanks to Ultra that we won the war." Churchill's greatest fear, even after Hitler had suspended Operation Sealion and invaded the Soviet Union, was that the German submarine wolfpacks would succeed in strangling sea-locked Britain. He would later write, in Their Finest Hour (1949): "The only thing that ever really frightened me during the war was the U-boat peril." The major factor that averted Britain's defeat in the Battle of the Atlantic was her regained mastery of Naval-Enigma decryption.
Had Britain lost the Battle of the Atlantic, it is difficult to see how the United States, deprived of the British Isles as a forward military base, could have entered action in the European and North African theaters. The war in Europe would have been played out essentially between Germany and the USSR. It is a serious question whether the Soviets would have prevailed, bereft of American aid and of Ultra intelligence provided by British sources or Soviet agents such as John Cairncross working at Bletchley Park. The Soviets' position would have become still less tenable had Japan, as Stalin feared, opened a second front against the Soviets, as part of a "strike-north" strategy (which Japan, in the event, abandoned for a "strike-south" strategy).
If one accepts conservative estimates that Ultra speeded the end of the war by a year or two, then in the absence of Ultra and of Allied aerial campaigns against Germany such a grace period might have sufficed for her to gain a crushing advantage in jet aircraft, missiles, perhaps atom bombs.
Whichever of the two had won out — Germany or the Soviet Union — Europe and the world surely would, at least in the short term, have looked very differently in the absence of moderating powers.
Postwar consequences
F.W. Winterbotham, the first author to limn, in his 1974 book The Ultra Secret, the influence of Enigma decryption on the course of World War II, likewise made the earliest contribution to an appreciation of Ultra's postwar influence, which now continues into the 21st century — and not only in the postwar establishment of Britain's GCHQ (Government Communication Headquarters) and the United States' NSA (National Security Agency). "Let no one be fooled," Winterbotham admonishes in chapter 3, "by the spate of television films and propaganda which has made the war seem like some great triumphant epic. It was, in fact, a very narrow shave, and the reader may like to ponder [...] whether [...] we might have won [without] Ultra."
Had the postwar governments of major powers realized how Allied victory in World War II hung by a slender thread that had first been spun by three mathematicians working on Engima decryption for the general staff of a middling central European power, they might perhaps have been more cautious in choosing their own wars.
Further reading
A fictional version of this story is told in the novel Enigma by Robert Harris (ISBN 0099992000), the movie made from the novel—see "Enigma (2001 film)"—and is somewhat covered, also fictionally, in Neal Stephenson's Cryptonomicon (ISBN 0099410672).
A short account of World War II cryptology is Battle of Wits (2000) by Stephen Budiansky; it covers more than just the Enigma story. Hugh Sebag-Montefiore's Enigma: the Battle for the Code (2000), which focuses largely on Naval Enigma, includes some previously unknown information—and many photographs of individuals involved; Bletchley Park had been his grandfather's house before it was purchased for GC&CS. David Kahn's Seizing the Enigma (1991) is essentially about the solution of Naval Enigma, based on seizures of German naval vessels; British success in the endeavor almost certainly saved Britain from defeat in the crucial Battle of the Atlantic and thereby made the United States' entry into the war's European theater possible. A brief description of the Enigma, as well as other codes/ciphers, can be found in Simon Singh's The Code Book (1999). Information on British cryptology appears in the official history of British intelligence in World War II, edited by Sir Harry Hinsley; he also co-edited, with Alan Stripp, a volume of memoirs by participants in the British cryptological effort, Codebreakers: the Inside Story of Bletchley Park (1993). Marian Rejewski wrote a number of papers on his 1932 break into Enigma and his subsequent work on the cipher, well into World War II, with his fellow mathematician-cryptologists, Jerzy Różycki and Henryk Zygalski; most of Rejewski's papers appear in Władysław Kozaczuk's 1984 Enigma: How the German Machine Cipher Was Broken, and How It Was Read by the Allies in World War Two (edited and translated by Christopher Kasparek), which remains the standard reference on the crucial foundations laid by the Poles for World War II Enigma decryption.
Broken Enigma messages are still extremely valuable today, as they provide some of the best surviving direct accounts of the Nazi war effort.
External links
- Literature on ULTRA (http://clark.cam.muskingum.edu/uk_folder/ukwwii_folder/ukwwiiultra_folder/ukwwiiultratoc.html)nl:Ultra