Ultra (cryptography)

The Enigma cipher machine
Enigma machine
Enigma rotors
Breaking Enigma
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Ultra
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Ultra was the designation adopted by

The code name Boniface was used as a cover name for Ultra. In order to ensure that the successful code-breaking did not become apparent to the Germans, British intelligence created a fictional

Much of the German cipher traffic was encrypted on the Enigma machine. Used properly, the German military Enigma would have been virtually unbreakable; in practice, shortcomings in operation allowed it to be broken. The term "Ultra" has often been used almost synonymously with "Enigma decrypts". However, Ultra also encompassed decrypts of the German Lorenz SZ 40/42 machines that were used by the German High Command, and the Hagelin machine.^[a]

Many observers, at the time and later, regarded Ultra as immensely valuable to the Allies.

Most Ultra intelligence was derived from reading radio messages that had been encrypted with cipher machines, complemented by material from radio communications using traffic analysis and direction finding. In the early phases of the war, particularly during the eight-month Phoney War, the Germans could transmit most of their messages using land lines and so had no need to use radio. This meant that those at Bletchley Park had some time to build up experience of collecting and starting to decrypt messages on the various radio networks. German Enigma messages were the main source, with those of the Luftwaffe predominating, as they used radio more and their operators were particularly ill-disciplined.

German

Enigma

"

German military Enigma was first broken in December 1932 by the

Ultra would never have got off the ground if we had not learned from the Poles, in the nick of time, the details both of the German military Enigma machine, and of the operating procedures that were in use.

— Gordon Welchman[22]

At Bletchley Park, some of the key people responsible for success against Enigma included mathematicians Alan Turing and Hugh Alexander and, at the British Tabulating Machine Company, chief engineer Harold Keen.^[16] After the war, interrogation of German cryptographic personnel led to the conclusion that German cryptanalysts understood that cryptanalytic attacks against Enigma were possible but were thought to require impracticable amounts of effort and investment.^[23] The Poles' early start at breaking Enigma and the continuity of their success gave the Allies an advantage when World War II began.^[22]

Lorenz cipher

In June 1941, the Germans started to introduce on-line

Although the volume of intelligence derived from this system was much smaller than that from Enigma, its importance was often far higher because it produced primarily high-level, strategic intelligence that was sent between Wehrmacht High Command (OKW). The eventual bulk decryption of Lorenz-enciphered messages contributed significantly, and perhaps decisively, to the defeat of Nazi Germany.[25]^[26] Nevertheless, the Tunny story has become much less well known among the public than the Enigma one.^[16] At Bletchley Park, some of the key people responsible for success in the Tunny effort included mathematicians W. T. "Bill" Tutte and Max Newman and electrical engineer Tommy Flowers.^[16]

Italian

In June 1940, the Italians were using book codes for most of their military messages, except for the Italian Navy, which in early 1941 had started using a version of the Hagelin rotor-based cipher machine C-38.^[27] This was broken from June 1941 onwards by the Italian subsection of GC&CS at Bletchley Park.^[28]

Japanese

In the

Army- and Air Force-related intelligence derived from signals intelligence (SIGINT) sources—mainly Enigma decrypts in Hut 6—was compiled in summaries at GC&CS (Bletchley Park) Hut 3 and distributed initially under the codeword "BONIFACE",^[30] implying that it was acquired from a well placed agent in Berlin. The volume of the intelligence reports going out to commanders in the field built up gradually.

Naval Enigma decrypted in

The codeword "ULTRA" was adopted in June 1941.[32] This codeword was reportedly suggested by Commander Geoffrey Colpoys, RN, who served in the Royal Navy's OIC.

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 carried out by 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 F. W. Winterbotham. Each SLU included intelligence, communications, and cryptographic elements. It was headed by a British Army or RAF 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, then take it back and destroy it.

By the end of the war, there were about 40 SLUs serving commands around the world. ^[33] Fixed SLUs existed at the Admiralty, the War Office, the Air Ministry, RAF Fighter Command, the US Strategic Air Forces in Europe (Wycombe Abbey) and other fixed headquarters in the UK. An SLU was operating at the War HQ in Valletta, Malta.^[34] These units had permanent teleprinter links to Bletchley Park.

Mobile SLUs were attached to field army and air force headquarters and depended on radio communications to receive intelligence summaries. 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.^[35] 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.

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.^[36]

Radio and cryptography

The communications system was founded by Brigadier Sir Richard Gambier-Parry, who from 1938 to 1946 was head of MI6 Section VIII, based at Whaddon Hall in Buckinghamshire, UK.^[37] Ultra summaries from Bletchley Park were sent over landline to the Section VIII radio transmitter at Windy Ridge. From there they were transmitted to the destination SLUs.

The communications element of each SLU was called a "Special Communications Unit" or SCU. Radio transmitters were constructed at Whaddon Hall workshops, while receivers were the National HRO, made in the USA. The SCUs were highly mobile and the first such units used civilian Packard cars. The following SCUs are listed:^[37] SCU1 (Whaddon Hall), SCU2 (France before 1940, India), SCU3 (RSS Hanslope Park), 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.^[c]

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

RN Ultra messages from the OIC to ships at sea were necessarily transmitted over normal naval radio circuits and were protected by one-time pad encryption.^[38]

Lucy

An intriguing question concerns the alleged use of Ultra information by the "Lucy" spy ring,^[39] headquartered in Switzerland and apparently operated by one man, Rudolf Roessler. This was an extremely well informed, responsive ring that was able to get information "directly from German General Staff Headquarters" – often on specific request. It has been alleged that "Lucy" was in major part a conduit for the British to feed Ultra intelligence to the Soviets in a way that made it appear to have come from highly placed espionage rather than from cryptanalysis of German radio traffic. The Soviets, however, through an agent at Bletchley, John Cairncross, knew that Britain had broken Enigma. The "Lucy" ring was initially treated with suspicion by the Soviets. The information it provided was accurate and timely, however, and Soviet agents in Switzerland (including their chief, Alexander Radó) eventually learned to take it seriously.^[40] However, the theory that the Lucy ring was a cover for Britain to pass Enigma intelligence to the Soviets has not gained traction. Among others who have rejected the theory, Harry Hinsley, the official historian for the British Secret Services in World War II, stated that "there is no truth in the much-publicized claim that the British authorities made use of the ‘Lucy’ ring..to forward intelligence to Moscow".^[41]

Use of intelligence

Most deciphered messages, often about relative trivia, were insufficient as intelligence reports for military strategists or field commanders. The organisation, interpretation and distribution of decrypted Enigma message traffic and other sources into usable intelligence was a subtle task.

At Bletchley Park, extensive indices were kept of the information in the messages decrypted.^[42] For each message the traffic analysis recorded the radio frequency, the date and time of intercept, and the preamble—which contained the network-identifying discriminant, the time of origin of the message, the callsign of the originating and receiving stations, and the indicator setting. This allowed cross referencing of a new message with a previous one.^[43] The indices included message preambles, every person, every ship, every unit, every weapon, every technical term and of repeated phrases such as forms of address and other German military jargon that might be usable as cribs.^[44]

The first decryption of a wartime Enigma message, albeit one that had been transmitted three months earlier, was achieved by the Poles at

Decryption of Enigma traffic built up gradually during 1940, with the first two prototype bombes being delivered in March and August. The traffic was almost entirely limited to Luftwaffe messages. By the peak of the Battle of the Mediterranean in 1941, however, Bletchley Park was deciphering daily 2,000 Italian Hagelin messages. By the second half of 1941 30,000 Enigma messages a month were being deciphered, rising to 90,000 a month of Enigma and Fish decrypts combined later in the war.^[27]

Some of the contributions that Ultra intelligence made to the Allied successes are given below.

• In April 1940, Ultra information provided a detailed picture of the disposition of the German forces, and then their movement orders for the attack on the Low Countries prior to the Battle of France in May.^[45]
• An Ultra decrypt of June 1940 read
  KNICKEBEIN KLEVE IST AUF PUNKT 53 GRAD 24 MINUTEN NORD UND EIN GRAD WEST EINGERICHTET ("The Cleves Knickebein is directed at position 53 degrees 24 minutes north and 1 degree west"). This was the definitive piece of evidence that Dr R V Jones of scientific intelligence in the Air Ministry needed to show that the Germans were developing a radio guidance system for their bombers.^[46] Ultra intelligence then continued to play a vital role in the so-called Battle of the Beams
  .
• During the
  Sir Hugh Dowding, Commander-in-Chief of RAF Fighter Command, had a teleprinter link from Bletchley Park to his headquarters at RAF Bentley Priory, for Ultra reports. Ultra intelligence kept him informed of German strategy,^[47] and of the strength and location of various Luftwaffe units, and often provided advance warning of bombing raids (but not of their specific targets).^[48]
  These contributed to the British success. Dowding was bitterly and sometimes unfairly criticized by others who did not see Ultra, but he did not disclose his source.
• Decryption of traffic from Luftwaffe radio networks provided a great deal of indirect intelligence about the Germans' planned Operation Sea Lion to invade England in 1940.^[49]
• On 17 September 1940 an Ultra message reported that equipment at German airfields in Belgium for loading planes with paratroops and their gear, was to be dismantled. This was taken as a clear signal that Sea Lion had been cancelled.^[50]
• Ultra revealed that a major German air raid was planned for the night of 14 November 1940, and indicated three possible targets, including London and Coventry. However, the specific target was not determined until late on the afternoon of 14 November, by detection of the German radio guidance signals. Unfortunately, countermeasures failed to prevent the devastating
  Ditchley Park, was told that London might be bombed and returned to 10 Downing Street
  so that he could observe the raid from the Air Ministry roof.
• Ultra intelligence considerably aided the British Army's Operation Compass victory over the much larger Italian army in Libya in December 1940 – February 1941.^[56]
• Ultra intelligence greatly aided the Royal Navy's victory over the Italian navy in the Battle of Cape Matapan in March 1941.^[57]
• Although the Allies lost the Battle of Crete in May 1941, the Ultra intelligence that a parachute landing was planned, and the exact day of the invasion, meant that heavy losses were inflicted on the Germans and that fewer British troops were captured.^[58]
• Ultra intelligence fully revealed the preparations for Operation Barbarossa, the German invasion of the USSR. Although this information was passed to the Soviet government, Stalin refused to believe it.^[59] The information did, however, help British planning, knowing that substantial German forces were to be deployed to the East.
• Ultra intelligence made a very significant contribution in the Battle of the Atlantic. Winston Churchill wrote "The only thing that ever really frightened me during the war was the U-boat peril."^[60] The decryption of Enigma signals to the U-boats was much more difficult than those of the Luftwaffe. It was not until June 1941 that Bletchley Park was able to read a significant amount of this traffic currently.^[61] Transatlantic convoys were then diverted away from the U-boat "wolfpacks", and the U-boat supply vessels were sunk. On 1 February 1942, Enigma U-boat traffic became unreadable because of the introduction of a different 4-rotor Enigma machine. This situation persisted until December 1942, although other German naval Enigma messages were still being deciphered, such as those of the U-boat training command at Kiel.^[62] From December 1942 to the end of the war, Ultra allowed Allied convoys to evade U-boat patrol lines, and guided Allied anti-submarine forces to the location of U-boats at sea.
• In the
  Western Desert Campaign, Ultra intelligence helped Wavell and Auchinleck to prevent Rommel's forces from reaching Cairo in the autumn of 1941.^[63]
• Ultra intelligence from Hagelin decrypts, and from Luftwaffe and German naval Enigma decrypts, helped sink about half of the ships supplying the Axis forces in North Africa.[27]^[64]
• Ultra intelligence from Abwehr transmissions confirmed that Britain's Security Service (
  Double Cross System.^[65] This enabled major deception operations.^[66]
• Deciphered JN-25 messages allowed the U.S. to turn back a Japanese offensive in the Battle of the Coral Sea in April 1942 and set up the decisive American victory at the Battle of Midway in June 1942.^[67]
• Ultra contributed very significantly to the monitoring of German developments at
  V-1 and V-2 Intelligence from 1942 onwards.^[68]
• Ultra contributed to
  Montgomery's victory at the Battle of Alam el Halfa by providing warning of Rommel's planned attack.^[64][69]
• Ultra also contributed to the success of Montgomery's offensive in the Second Battle of El Alamein, by providing him (before the battle) with a complete picture of Axis forces, and (during the battle) with Rommel's own action reports to Germany.
• Ultra provided evidence that the Allied landings in French North Africa (Operation Torch) were not anticipated.^[70]
• A JN-25 decrypt of 14 April 1943 provided details of Admiral Yamamoto's forthcoming visit to Balalae Island, and on 18 April, a year to the day following the Doolittle Raid, his aircraft was shot down, killing this man who was regarded as irreplaceable.^[71]
• Ship position reports in the Japanese Army’s "2468" water transport code, decrypted by the SIS starting in July 1943, helped U.S. submarines and aircraft sink two-thirds of the Japanese merchant marine.^{[72]: pp. 226 ff, 242 ff}
• The part played by Ultra intelligence in the preparation for the Allied invasion of Sicily was of unprecedented importance. It provided information as to where the enemy's forces were strongest and that the elaborate strategic deceptions had convinced Hitler and the German high command.^[73]
• The success of the
  Battle of North Cape, in which HMS Duke of York sank the German battleship Scharnhorst, was entirely built on prompt deciphering of German naval signals.^[74]
• US Army Lieutenant Arthur J Levenson who worked on both Enigma and Tunny at Bletchley Park, said in a 1980 interview of intelligence from Tunny

  Rommel was appointed Inspector General of the West, and he inspected all the defences along the Normandy beaches and send a very detailed message that I think was 70,000 characters and we decrypted it as a small pamphlet. It was a report of the whole Western defences. How wide the V shaped trenches were to stop tanks, and how much barbed wire. Oh, it was everything and we decrypted it before D-Day.[75]

• Both Enigma and Tunny decrypts showed Germany had been taken in by Operation Bodyguard, the deception operation to protect Operation Overlord. They revealed the Germans did not anticipate the Normandy landings and even after D-Day still believed Normandy was only a feint, with the main invasion to be in the Pas de Calais.^[76][77]
• Information that there was German Panzergrenadier division in the planned dropping zone for the US 101st Airborne Division in Operation Overlord led to a change of location.^[78]
• It assisted greatly in Operation Cobra.
• It warned of the major German counterattack at Mortain, and allowed the Allies to surround the forces at Falaise.
• During the Allied advance to Germany, Ultra often provided detailed tactical information, and showed how Hitler ignored the advice of his generals and insisted on German troops fighting in place "to the last man".^[79]
• Arthur "Bomber" Harris, officer commanding RAF Bomber Command, was not cleared for Ultra. After D-Day, with the resumption of the strategic bomber campaign over Germany, Harris remained wedded to area bombardment. Historian Frederick Taylor argues that, 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 to target oil installations, since he did not know that senior Allied commanders were using high-level German sources to assess just how much this was hurting the German war effort; thus Harris tended to see the directives to bomb specific oil and munitions targets as a "panacea" (his word) and a distraction from the real task of making the rubble bounce.^[80]

The Allies were seriously concerned with the prospect of the Axis command finding out that they had broken into the Enigma traffic. The British were more disciplined about such measures than the Americans, and this difference was a source of friction between them.[81]^[82] It has been noted with some irony that in Delhi, the British Ultra unit was based in a large wooden hut in the grounds of Government House. Security consisted of a wooden table flat across the door with a bell on it and a sergeant sitting there. This hut was ignored by all. The American unit was in a large brick building, surrounded by barbed wire and armed patrols. People may not have known what was in there, but they surely knew it was something important and secret.^{[citation needed]}

To disguise the source of the intelligence for the Allied attacks on Axis supply ships bound for North Africa, "spotter" submarines and aircraft were sent to search for Axis ships. These searchers or their radio transmissions were observed by the Axis forces, who concluded their ships were being found by conventional reconnaissance. They suspected that there were some 400 Allied submarines in the Mediterranean and a huge fleet of reconnaissance aircraft on Malta. In fact, there were only 25 submarines and at times as few as three aircraft.^[27]

This procedure also helped conceal the intelligence source from Allied personnel, who might give away the secret by careless talk, or under interrogation if captured. Along with the search mission that would find the Axis ships, two or three additional search missions would be sent out to other areas, so that crews would not begin to wonder why a single mission found the Axis ships every time.

Other deceptive means were used. On one occasion, a convoy of five ships sailed from Naples to North Africa with essential supplies at a critical moment in the North African fighting. There was no time to have the ships properly spotted beforehand. The decision to attack solely on Ultra intelligence went directly to Churchill. The ships were all sunk by an attack "out of the blue", arousing German suspicions of a security breach. To distract the Germans from the idea of a signals breach (such as Ultra), the Allies sent a radio message to a fictitious spy in Naples, congratulating him for this success. According to some sources the Germans decrypted this message and believed it.^[83]

In the Battle of the Atlantic, the precautions were taken to the extreme. In most cases where the Allies knew from intercepts the location of a U-boat in mid-Atlantic, the U-boat was not attacked immediately, until a "cover story" could be arranged. For example, a search plane might be "fortunate enough" to sight the U-boat, thus explaining the Allied attack.

Some Germans had suspicions that all was not right with Enigma. Admiral 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 Sea, and a British destroyer promptly showed up. The U-boats 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 B-Dienst, his own codebreaking group, had partially broken Royal Navy traffic (including its convoy codes early in the war),^[84] and supplied enough information to support the idea that the Allies were unable to read Naval Enigma.^[d]

By 1945, most German Enigma traffic could be decrypted within a day or two, yet the Germans remained confident of its security.^[85]

Role of women in Allied codebreaking

After encryption systems were "broken", there was a large volume of cryptologic work needed to recover daily key settings and keep up with changes in enemy security procedures, plus the more mundane work of processing, translating, indexing, analyzing and distributing tens of thousands of intercepted messages daily.^[86] The more successful the code breakers were, the more labor was required. Some 8,000 women worked at Bletchley Park, about three quarters of the work force.^[87] Before the attack on Pearl Harbor, the US Navy sent letters to top women's colleges seeking introductions to their best seniors; the Army soon followed suit. By the end of the war, some 7000 workers in the Army Signal Intelligence service, out of a total 10,500, were female. By contrast, the Germans and Japanese had strong ideological objections to women engaging in war work. The Nazis even created a Cross of Honour of the German Mother to encourage women to stay at home and have babies.^[72]

Effect on the war

The exact influence of Ultra on the course of the war is debated; an oft-repeated assessment is that decryption of German ciphers advanced the end of the European war by no less than two years.^[88][89] Hinsley, who first made this claim, is typically cited as an authority for the two-year estimate.^[90]

Would the Soviets meanwhile have defeated Germany, or Germany the Soviets, or would there have been stalemate on the eastern fronts? What would have been decided about the atom bomb? Not even counter-factual historians can answer such questions. They are questions which do not arise, because the war went as it did. But those historians who are concerned only with the war as it was must ask why it went as it did. And they need venture only a reasonable distance beyond the facts to recognise the extent to which the explanation lies in the influence of Ultra.

— Hinsley^[8]

Winterbotham's quoting of Eisenhower's "decisive" verdict is part of a letter sent by Eisenhower to Menzies after the conclusion of the European war and later found among his papers at the Eisenhower Presidential Library.^[91] It allows a contemporary, documentary view of a leader on Ultra's importance:

July 1945

Dear General Menzies:

I had hoped to be able to pay a visit to Bletchley Park in order to thank you, Sir Edward Travis, and the members of the staff personally for the magnificent service which has been rendered to the Allied cause.

I am very well aware of the immense amount of work and effort which has been involved in the production of the material with which you supplied us. I fully realize also the numerous setbacks and difficulties with which you have had to contend and how you have always, by your supreme efforts, overcome them.

The intelligence which has emanated from you before and during this campaign has been of priceless value to me. It has simplified my task as a commander enormously. It has saved thousands of British and American lives and, in no small way, contributed to the speed with which the enemy was routed and eventually forced to surrender.

I should be very grateful, therefore, if you would express to each and every one of those engaged in this work from me personally my heartfelt admiration and sincere thanks for their very decisive contribution to the Allied war effort.

Sincerely,

Dwight D. Eisenhower

There is wide disagreement about the importance of codebreaking in winning the crucial Battle of the Atlantic. To cite just one example, the historian Max Hastings states that "In 1941 alone, Ultra saved between 1.5 and two million tons of Allied ships from destruction." This would represent a 40 percent to 53 percent reduction, though it is not clear how this extrapolation was made.^[92]

Another view is from a history based on the German naval archives written after the war for the British Admiralty by a former U-boat commander and son-in-law of his commander, Grand Admiral

By the 1970s, 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. Switzerland developed its own version of Enigma, known as NEMA, and used it into the late 1970s, while the United States National Security Agency (NSA) retired the last of its rotor-based encryption systems, the KL-7 series, in the 1980s.

A second explanation relates to a misadventure of Churchill's between the World Wars, when he publicly disclosed information from decrypted Soviet communications. This had prompted the Soviets to change their ciphers, leading to a blackout.^{[citation needed]}

The third explanation is given by Winterbotham, who recounts that two weeks after

In 1967, Polish military historian Władysław Kozaczuk in his book Bitwa o tajemnice ("Battle for Secrets") first revealed Enigma had been broken by Polish cryptologists before World War II.

In 1967, David Kahn in The Codebreakers described the 1944 capture of a Naval Enigma machine from U-505 and gave the first published hint about the scale, mechanisation and operational importance of the Anglo-American Enigma-breaking operation:

The Allies now read U-boat operational traffic. For they had, more than a year before the theft, succeeded in solving the difficult U-boat systems, and – in one of the finest cryptanalytic achievements of the war – managed to read the intercepts on a current basis. For this, the cryptanalysts needed the help of a mass of machinery that filled two buildings.^[97]

Ladislas Farago's 1971 best-seller The Game of the Foxes gave an early garbled version of the myth of the purloined Enigma. According to Farago, it was thanks to a "Polish-Swedish ring [that] the British obtained a working model of the 'Enigma' machine, which the Germans used to encipher their top-secret messages."^[98] "It was to pick up one of these machines that Commander Denniston went clandestinely to a secluded Polish castle [!] on the eve of the war. Dilly Knox later solved its keying, exposing all Abwehr signals encoded by this system."^[99] "In 1941 [t]he brilliant cryptologist Dillwyn Knox, working at the Government Code & Cypher School at the Bletchley centre of British code-cracking, solved the keying of the Abwehr's Enigma machine."^[100]

Later, the 1973 public disclosure of Enigma decryption in the book Enigma by French intelligence officer Gustave Bertrand generated pressure to discuss the rest of the Enigma–Ultra story.^[101]

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.

A succession of books by former participants and others followed. The official history of British intelligence in World War II was published in five volumes from 1979 to 1988, and included further details from official sources concerning the availability and employment of Ultra intelligence. 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.

A 2012 London

F. W. Winterbotham, the first author to outline 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 America's NSA. "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."^[109]

Debate continues on whether, had postwar political and military leaders been aware of Ultra's role in Allied victory in World War II, these leaders might have been less optimistic about post-World War II military involvements.^[e]

Knightley suggests that Ultra may have contributed to the development of the Cold War.^[110] The Soviets received disguised Ultra information, but the existence of Ultra itself was not disclosed by the western Allies. The Soviets, who had clues to Ultra's existence, possibly through Kim Philby, John Cairncross and Anthony Blunt,^[110] may thus have felt still more distrustful of their wartime partners.

The mystery surrounding the discovery of the sunk German submarine U-869 off the coast of New Jersey by divers Richie Kohler and John Chatterton was unravelled in part through the analysis of Ultra intercepts, which demonstrated that, although U-869 had been ordered by U-boat Command to change course and proceed to North Africa, near Rabat, the submarine had missed the messages changing her assignment and had continued to the eastern coast of the U.S., her original destination.

In 1953, the CIA's

• Hut 6
• Hut 8
• Magic (cryptography)
• Military intelligence
• Signals intelligence in modern history
• The Imitation Game

Notes