Ultra

v • d • e The Enigma cipher machine

Enigma machine Enigma rotor details Cryptanalysis of the Enigma Cipher Bureau Cyclometer Perforated sheets Bomba Bombe Hut 6 Hut 8 Banburismus Ultra

Ultra (sometimes capitalised ULTRA) was the name used by the British for intelligence resulting from decryption of encrypted German radio 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.

Group Captain 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.

Sources and history

Although Ultra included mainly material derived from Enigma decrypts, it also included material from other sources.

Later in the war the Germans introduced on-line 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.

Encrypted messages

Ultra material largely came from German cipher 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 (e.g., 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 cryptanalyse). 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 (group theory, particularly permutation theory) and cracked the Enigma system, with some guidance from instruction manuals from a German source (Hans-Thilo Schmidt) via French Intelligence. It has been characterised as the most momentous advance in cryptanalysis in a millennium. Together with two colleagues at the Polish General Staff's Cipher Bureau (Polish: Biuro Szyfrów), 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, cryptologic bomb, and perforated sheets). Well before 1938, much German Enigma traffic was being routinely decrypted by the Poles; but accelerating changes in German operations (operational procedures, frequency of key changes, greater rotor choice) and looming war led the Poles to share their achievements in Enigma decryption with France and Britain. This happened during a famous meeting at Pyry, in the Kabaty Woods south of Warsaw, on July 25, 1939. Since neither the French nor the British had broken into Enigma traffic, this was a major cryptanalytic windfall for Poland's Western Allies.

Armed with this Polish assistance, the British and French began work on German Enigma traffic. Work on Enigma after the outbreak of World War II in France, at PC Bruno outside Paris, was done in part by Polish Cipher Bureau cryptanalysts 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. For some time, Bletchley Park and PC Bruno cooperated on work against the Enigma messages. The British had set up a large interception network to collect enciphered messages for the cryptanalysts at five near-by off-site outstations at Adstock, Gayhurst, Wavendon, Stanmore, and Eastcote. Eventually there was a very large organization centered at Bletchely controlling the distribution of the resulting—secret—intelligence, which came to be called "Ultra." Strict rules were established to limit the number of people who knew of Ultra (and its source) in the hope of ensuring that nothing (e.g., leaks or actions) would alert the Axis Powers that the Allies were reading their messages. Prior to use of the term Ultra, the product from Bletchley Park was for a time codenamed "Boniface" to give the impression to the uninitiated its 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 mavens, mathematicians and pioneer computer scientists. Amongst the latter was Alan Turing, one of the founders of modern computing. By 1943, a large proportion of intercepts (over 2,000 daily at the height of operations) were routinely read, including some from 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, was of great value in formulating Allied strategy and tactics.

Methods of cryptanalytic attack

British attacks on the Enigmas were similar to the original Polish methods, but continued evolving to keep pace with the German equipment and procedure changes. (For a discussion of the many techniques shared between the Polish and British approaches, see Kozaczuk 1984, appendix F.) A particular challenge was 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 Enigmas) guaranteed no letter could be enciphered as itself, so 'A' could not be sent as 'A'. Another technique counted on common German phrases, such as "Heil Hitler" or "please respond," which were likely to occur in a given plaintext; a successful guess as to a plaintext was known at Bletchley as a crib. With a probable plaintext fragment and the knowledge that no letter could be enciphered as itself, a corresponding ciphertext fragment could often be identified. This provided a clue to message keys.

On some occasions, German cipher clerks helped Allied cryptanalysts unknowningly. In one instance, a clerk was asked to send a test message, hit the T key repeatedly, and transmitted the resulting ciphertext. 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 constantly used the same settings for their message keys, often their own initials or those of a girlfriend (one clerk is said to have had a girlfriend named Cillie, and would continuously use CIL as the rotor setup; alternatively, their choices were merely foolish, as ABC, hence 'silly'. Bletchley Park named such hints "cillies" or "sillies"). Analysts were set to finding such messages in the sea of daily intercepts, which winnowed out enough possibilities to permit Bletchley to use other techniques to find the initial daily key. 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 cryptanalysts, according to the head of Hut Six at Bletchley, Gordon Welchman. However, due to the expense and difficulty of getting new rotors to all ships and units, rotors were never replaced simultaneously. 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. Human operational errors were never eradicated either.

Capturing of German naval cryptographic material

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. ^[1]

On 7 May 1941 the Royal Navy deliberately captured a German weather ship, together with cipher equipment and codes; and two 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.

Naval Enigma machines or settings books were captured from a total of seven U-boats and eight German surface ships. These included U-559 (1942), U-505 (1944), 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 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 this or that channel, 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 markedly affect the course of the war. Charting decrypted Enigma traffic against British shipping losses for a given month shows a strong pattern of increased losses when 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 6 a.m. was likely to have been broadcast 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 that the Germans had added from time to time, that number was much larger than the Poles had faced. In order to solve this problem the Allies, especially the US, "went industrial" and produced much larger versions of the Polish bomba that could rapidly test thousands of possible key settings.

In 1941 British intelligence learned that the German Navy was about to introduce M4, a new version of Enigma with 4 rotors 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 inaugurated. Realizing the error, the U-boat retransmitted the same message using the 3-rotor Enigma, giving the British sufficient clues to break the new machine soon 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.

German suspicions about Enigma cryptanalysis

The Allies were seriously concerned with the prospect of the Axis command finding out that they had broken into the 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 plane might be "fortunate enough" to sight the U-boat, thus explaining the Allied attack. Ultra information was used to attack and sink many Afrika Korps supply ships bound for North Africa; but, as in the North Atlantic, every time such information was used, an "innocent" explanation had to be provided: often scout planes were sent on otherwise unnecessary missions, to ensure they were spotted by the Germans. The British were, it is said, more disciplined about such measures than the Americans, and this difference was a source of friction between them.

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, was not 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 early in the war),^[2] 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 another case, the Germans became suspicious of Ultra when five ships from Naples headed for North Africa with essential supplies for Rommel's campaign were all mysteriously attacked and sunk by an Allied airforce. As there was no time to have the ships all spotted by the airforce beforehand and then sunk accordingly, the decision went directly to Churchill whether or not to act solely on Ultra intelligence. He gave the simple order "Sink them". Afterwards, a message was sent by the Allies to Naples congratulating a fictitious spy and informing him of his bonus. The Germans decrypted this message and believed it.^[3]

There were however scenarios in which Ultra intelligence could be taken advantage of with little or no risk of the Germans expecting a compromise. One example would be the military deception preparations for the D-day landings. These involved use of dummy tanks, fake ships and notional armies to fool the Germans into thinking that the Allied invasion would take place at the Pas de Calais, as opposed to Normandy. Ultra intelligence confirmed to the Allies that these deceptions were working and gave all senior decision makers involved greater confidence of a successful invasion.

By 1945 almost all German Enigma traffic (Wehrmacht, Kriegsmarine, Luftwaffe, Abwehr, SD, etc.) 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 cryptanalysts to start over. 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 prior to the Battle of the Kasserine Pass 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 most Allied commanders because the information was disregarded. On yet another account, during the North African Campaign, an overly pessimistic report by German officer Friedrich Paulus led the British to believe that the German Afrika Korps was much weaker than it actually was, prompting them to launch the disastrous Operation Battleaxe.

After the War, American TICOM project teams found and detained a considerable number of German cryptographic personnel. Among the things they learned was German cryptanalysts, at least, understood full well Enigma messages might be read; they just found it impossible to imagine anyone going to the immense effort and investment required.^[4]

Purple decrypts in Europe

In the Pacific theater, the Japanese cipher machine dubbed "Purple" by the Americans, and unrelated to the Enigmas, was used for highest-level Japanese diplomatic traffic. It was also cracked, by the US Army's Signal Intelligence Service. Some Purple decrypts proved useful elsewhere, for instance 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 defenses) 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 German Army / Air Force machine.

Dissemination of Ultra intelligence

Preparation of Ultra summaries

Initially, army and air force related intelligence derived from SIGINT sources (mainly Enigma decrypts) was compiled in summaries at GCHQ (Bletchley Park) Hut 3. The summaries were subsequently distributed under the codeword "BONIFACE", presumably to imply that they were the result of human intelligence operations. The Admiralty (Royal Navy) produced its own intelligence summaries at the RN Operational Intelligence Centre (OIC),^[5] which were distributed under the codeword "HYDRO".^[6]

In June 1941 new arrangements were made for distribution of Boniface bulletins and from this point on the term "ULTRA SECRET" was used".^[7] The term "Ultra" was reportedly suggested by Commander Geoffrey Colpoys, RN, who served in the RN OIC.

Distribution of Ultra to Army and Air Force

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.

Dissemination of Ultra intelligence to field commanders was achieved by the MI6, which operated Special Liaison Units (SLU) attached to major army and air force commands. The activity was organized and supervised on behalf of MI6 by Group Captain Frederick William Winterbotham.^[8][9] The SLU included intelligence, communications and cryptographic elements. Each SLU was headed by a British Army officer, usually a major, known as "Special Liaison Officer". The main function of the Liaison Officer or his deputy was to pass Ultra intelligence bulletins to the commander of the command he was attached to, or to other indoctrinated staff officers. In order to safeguard Ultra, special precautions were taken. The standard procedure was for the Liaison Officer to present the intelligence summary to the recipient, stay with him while he studied it and then take it back and destroy it.

Fixed SLU's existed at the Admiralty, the War Office, the Air Ministry and RAF Fighter Command. These units had permanent teleprinter links to Bletchley Park.

Mobile SLUs were attached to field army and air force headquarters. These SLUs depended on radio communications.

The first mobile SLUs appeared during the French campaign of 1940. An SLU supported the British Expeditionary Force (BEF) headed by General Lord Gort. The first liaison officers were Robert Gore-Browne and Humphrey Plowden.^[10] A second SLU of the 1940 period was attached to the RAF Advanced Air Striking Force at Meaux commanded by Air Vice-Marshal P H Lyon Playfair. This SLU was commanded by Squadron Leader F.W. (Tubby) Long.

Distribution of Ultra to intelligence agencies

In 1940 special arrangements were made within the British intelligence services for handling BONIFACE and later Ultra intelligence. The Security Service started "Special Research Unit B1(b)" under Herbert Hart. In the SIS this intelligence was handled by "Section V" based at St. Albans.^[11]

Radio communications and cryptography

The communications system was founded by Brigadier Sir Richard Gambier-Parry, who was Head of MI6 Section VIII from 1938 to 1946 and was based at Whaddon Hall in Buckinghamshire, UK.^[12]. Ultra summaries from Bletchley Park were sent over landline to the radio transmitter site of Section VIII at Windy Ridge. From there they were transmitted over radio to the destination SLU.

The communications element of each SLU was called "Special Communications Unit" or SCU. Radio transmitters were constructed at Waddon Hall workshops, while receivers were the National HRO, made in the USA. The SCU's were highly mobile and the first such units used civilian Packard cars. The following SCUs are listed:^[12] SCU1 (Whaddon Hall), SCU2 (France before 1940, India), SCU5, SCU6 (possibly Algiers and Italy), SCU7 (training unit in the UK), SCU8 (Europe after D-day), SCU9 (Europe after D-day), SCU11 (Palestine and India), SCU12 (India), SCU13 and SCU14. ^[13]

The cryptographic element of each SLU was supplied by the RAF and was based on the TYPEX cryptographic machine and one time pad systems.

The RN Ultra messages from the RN OIC to ships at sea were necessarily transmitted over normal naval radio circuits and were protected by one time pad encryption. ^[14]

Use of Ultra

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.

It is commonly claimed that the breaks into Naval Enigma resulted in the war being a year shorter, but given its effects on the Second Battle of the Atlantic alone, that might be an underestimate.

Breaking of some messages (not in German Enigma) led to the defeat of the Italian Navy at Cape Matapan, and was preceded by another "fortuitous" 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 a major operation was planned.

Ultra intelligence 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 D-Day landings, particularly in regard to estimates of when German reserves might be committed to battle.

On the other hand, the Red Army was well aware of the German buildup, locations and attack time precisely, prior to the battle of Kursk even without the Ultra information provided to them.

Lucy

An intriguing question concerns 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 "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; this is in some doubt, because the Soviets (via an agent in Bletchley, John Cairncross) knew Britain had broken Enigma. The Lucy Ring was operated, apparently, by one man, Rudolf Roessler, and was initially treated with considerable suspicion by the Soviets. The information it provided was accurate and timely, and Soviet agents in Switzerland (including Alexander Rado, the director) eventually took it quite seriously.

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 cryptological 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.

The official history of British intelligence in World War II was published in five volumes from 1979 to 1988. It was chiefly edited by Harry Hinsley, with one volume by Michael Howard. There is also a one-volume collection of reminiscences by Ultra veterans, Codebreakers (1993), edited by Hinsley and Alan Stripp.

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 (Battle for Secrets) first revealed that the German Enigma had been broken by Polish cryptanalysts 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 also 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 shortly after this (1974) that a decision was taken to permit some revelations about some Bletchley Park operations.

The United States National Security Agency retired the last of its rotor-based encryption systems, the KL-7 series, in the 1980s.

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 wolf packs 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." A major factor that averted Britain's defeat in the Battle of the Atlantic was her regained mastery of Naval-Enigma decryption.

There were, however, also other technologies, equipments, and tactics which moved the Battle of the Atlantic in the Allies' favour. As the air gap over the North Atlantic closed and convoys received escort carrier protection, anti-submarine aircraft became extremely efficient hunter-killers with the use of centimetric radar and airborne depth charges. Improvements to Huff-Duff (radio triangulation equipment used as part of ELINT) meant a U-boat's location could be found even if the messages they were sending could not be read (and simply avoiding a known submarine was often sufficient). Improvements to ASDIC (SONAR), coupled with Hedgehog, improved the likelihood of sinking a U-boat.

From February 1942, when Air Marshal Arthur Harris became Air Officer Commanding of Bomber Command, the RAF implemented large-scale night area bombardment of German cities. The destruction of city centres not only killed civilians and destroyed factories, houses and railways, but damaged and degraded the telephone and telex network. Usually the long-distance cable network was rapidly repaired, so that the still running telex network was closed down by Allied order only on 8th May, 1945. However, a surge in communications demand by German command and control and the effect of unrepaired destructions forced the Germans, as the war progressed, to rely ever more heavily on encrypted radio traffic, which of course the Allies were able to read.

After D-Day, with the resumption of the strategic bomber campaign over Germany, Harris remained wedded to area bombardment. Historian Frederick Taylor argues, as Harris was not cleared for access to ULTRA, he was given some information gleaned from Enigma but not the information's source. This affected his attitude about post-D-Day directives (orders) to target oil installations, since he did not know senior Allied commanders were using high-level German sources to assess just how much this was hurting the German war effort, so Harris tended to see the directives to bomb specific oil and munitions targets as a "panacea" (his word), and as a distraction from the real task of breaking German morale (i.e. killing civilians and stopping the war effort).^[15]

Postwar consequences

F.W. Winterbotham, the first author to outline, 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."

See also

• Military intelligence
• Hut 6
• Hut 8
• Magic (cryptography)

Notes and References

Further reading

• Winterbotham, F.W. (2000 [1974]). The ULTRA Secret. Orion Books Ltd. ISBN 0-75283-751-6. 

• United Kingdom WWII Ultra. Retrieved on 2008-02-15. Extensive literature list on ULTRA.

• The story is also somewhat covered, fictionally, in Neal Stephenson's Cryptonomicon (ISBN 0-09-941067-2).

• A short account of World War II cryptology is Stephen Budiansky's Battle of Wits (2000). It covers more than just the Enigma story.

• Hugh Sebag-Montefiore (2000). Enigma: the Battle for the Code. Cassel Military Paperbacks. ISBN 0-304-36662-5.  This book 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.

• Thomas Parrish. The American Codebreakers.  This book, earlier published as The Ultra Americans, concentrates on the U.S. contribution to the codebreaking effort.

• Welchman, G. (1997). The Hut Six Story. M&M Baldwin, 158. ISBN 0-947712-34-8.  Describes briefly production of intelligence in BP Hut 3.

• A 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.

• Ronald Lewin (1978). Ultra goes to War. 

• John Winton (1988). Ultra at Sea. 

• Nigel West (1986). The SIGINT Secrets The Signals Intelligence War 1900 to Today. 

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