Ultra (cryptography)

For other uses, see Ultra (cryptography) (disambiguation).

Rotors from an Enigma-machine

Ultra was the designation used by the British in World War II for signals intelligence obtained by "breaking" high-level encrypted enemy radio and teleprinter communications. "Ultra" eventually became the standard designation among the western Allies for all such intelligence. The name arose because the intelligence thus obtained was considered more important than that designated by the highest British security classification then used (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". However, Ultra also encompassed decrypts of the German Lorenz SZ 40 and 42 machines that were used by the German High Command, and the Hagelin machine and other Italian and Japanese ciphers and codes such as PURPLE and JN-25.

Polish Cipher Bureau reconstructions of the Enigma machine and techniques for decrypting ciphers produced on it, were presented as a gift by Polish Military Intelligence to their French and British allies in Warsaw on July 26, 1939, just five weeks before the outbreak of World War II. It was not a moment too soon. Former Bletchley Park mathematician-cryptologist Gordon Welchman has written: "Ultra would never have gotten 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."

In addition to the "Ultra" security classification, several cryptonyms were used for such intelligence, including "Boniface" in Britain—presumably to imply that it was the result of human intelligence operations—and "Magic" in the U.S.

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.

F.W. Winterbotham, in The Ultra Secret (1974), quotes the western Supreme Allied Commander, Dwight D. Eisenhower, at war's end describing Ultra as having been "decisive" to Allied victory in World War II. Sir Harry Hinsley, official historian of British Intelligence in World War II, made a similar assessment about the war in the Atlantic, the Mediterranean and Europe, saying that it was shortened "by not less than two years and probably by four years"; moreover, in the absence of Ultra, it is uncertain how the war would have ended.

Sources of ULTRA intelligence

A typical Bletchley intercept sheet, before decryption and translation.

A typical Bletchley intercept sheet, after decryption.

German sources

Much of the Ultra intelligence was derived from radioed German messages, many of which were enciphered on several variants of an electro-mechanical rotor machine called "Enigma." This produced a polyalphabetic substitution cipher, and 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 military. 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 as secure; Dilly Knox, of GC&CS, is said to have broken one during the 1920s.

Later in the war, in 1941, the Germans introduced on-line stream cipher teleprinter systems for strategic point-to-point radio links, 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 cipher systems were also successfully cryptanalysed, particularly Tunny, which the British thoroughly penetrated. It was eventually attacked using the Colossus computers, which were the first digital program-controlled electronic computers. Although the volume of intelligence derived from this system was much smaller than that from Enigma, its importance was high because it produced primarily strategic level intelligence.

In addition to Enigma and Fish decryptions, Ultra intelligence was supplemented with material derived from radio communications using other methods, such as radio traffic analysis and direction finding.

After the War, interrogation of German cryptographic personnel led to the conclusion that German cryptanalysts understood that cryptanalytic attacks against Enigma were possible but they had been thought to require impracticable amounts of effort and investment.

Italian sources

On entering the war in June 1940, the Italians were using book codes for most of their military messages. The exception was the Italian Navy which, early in 1941, started using a version of the Hagelin C-series rotor-based cipher machine called the C-38m. This was broken from June 1941 onwards by the Italian subsection of the UK's GC&CS at Bletchley Park.

Japanese sources

In the Pacific theater, the Japanese cipher machine dubbed "Purple" by the Americans, was used for highest-level Japanese diplomatic traffic. It produced a polyalphabetic substitution cipher, but unlike the Enigma machines, it was not a rotor machine, being built around electrical stepping switches. It was cracked by the US Army's Signal Intelligence Service and disseminated under the codeword MAGIC.

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 Germany's assessments of the military situation, of 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 chief, and most secure, command-and-control communications scheme used by the Imperial Japanese Navy was called JN-25 by the Americans. By early 1942, the latter had made considerable progress into decrypting Japanese naval messages.

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 internal wirings, was essentially the German Army / Air Force machine.

Distribution of Ultra

Initially, Army- and Air Force-related intelligence derived from signals intelligence (SIGINT) sources—mainly Enigma decrypts—was compiled in summaries at GC&CS (Bletchley Park) Hut 3. The Admiralty (Royal Navy) produced its own intelligence summaries at the RN Operational Intelligence Centre (OIC), which were distributed under the codeword "HYDRO".

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

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 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. 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 at 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 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. 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.

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.

Use of Ultra intelligence

Most deciphered messages, often about relative trivia, were not alone sufficient as intelligence reports for military strategists or field commanders. The organisation, interpretation and distribution of intelligence derived from messages from Enigma transmissions and other sources into intelligence was a subtle business. This was not recognised by the Americans before the attack on Pearl Harbor, but was learnt very quickly afterwards.

At Bletchley Park, extensive indexes were kept of the information in the messages decrypted. 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. The indexes 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.

The first decryption of a wartime Enigma message was achieved by the Poles at PC Bruno on 17 January 1940, albeit one that had been transmitted three months earlier. Little had been achieved by the start of the Allied campaign in Norway in April. At the start of the Battle of France on 10 May 1940, the Germans made a very significant change in the indicator procedures for Enigma messages. However, the Bletchley Park cryptanalysts had anticipated this, and were able—jointly with PC Bruno—to resume breaking messages from 22 May, although often with some delay. The intelligence that these messages yielded was of little operational use in the fast-moving situation of the German advance.

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.

Some of the contributions that Ultra intelligence made to the Allied successes are as follows.

• During the Battle of Britain, Air Chief Marshall Sir Hugh Dowding Commander-in-Chief of Fighter Command received Ultra intelligence reports, and had a teleprinter link from Bletchley Park to his headquarters at Bentley Priory. Ultra intelligence kept him informed of the German strategy, of the strength and location of various Luftwaffe units and often provided advanced warning of bombing raids (but not of their specific targets). These contributed to the British success. Dowding was unfairly criticized by those who did not receive Ultra intelligence, but did not disclose his source.

• Ultra intelligence provided a great deal of information about the Germans' planned Operation Sea Lion to invade England in 1940. Had the invasion taken place, Britain would have been much better prepared.

• The most significant piece of Ultra intelligence of 1940 came from an Enigma message on 12 August which, when deciphered, read KNICKEBEIN KLEVE IST AUF PUNKT 53 GRAD 24 MINUTEN NORD UND EIN GRAD WEST EINGERICHETET, which translates as "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. Ultra intelligence then continued to play a vital role in the so-called Battle of the Beams.

• In the Western Desert Campaign, Ultra intelligence helped Wavell and Auchinleck to prevent Rommel's forces from reaching Cairo in the Autumn of 1941.

• Ultra intelligence from Hagelin decrypts, and from Luftwaffe and German naval Enigma decrypts, helped sink some 40–60% of the ships supplying Rommel in North Africa. To disguise the source of the intelligence, "spotter" submarines and aircraft would be sent to observe the ships. The aircraft, submarines or their radio transmissions would be observed by the Axis vessels, leading them to think that it was reconnaissance rather than code breaking that lay behind the Allied successes. 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 three aircraft.

• Ultra contributed to Montgomery's success at the Second Battle of El Alamein by providing warning of Rommel's planned left-flank attack.

• It also provided evidence that the Allied landings in French North Africa were not anticipated.

• The British and Australian navies' success under Admiral Cunningham over an Italian battle fleet in March 1941 in the Battle of Cape Matapan, was greatly aided by Ultra intelligence of the sailing of one battleship, six heavy and two light cruisers plus destroyers to attack Allied convoys carrying troops from Egypt to Greece.

• Although the Allies lost the Battle of Crete in May 1941, the Ultra intelligence that a parachute landing was planned, meant that heavy losses were inflicted on the Germans and that fewer forces were captured.

• Ultra intelligence provided evidence that Operation Barbarossa, the German invasion of Russia was being planned. Although this information was "leaked" to Stalin, he refused to believe it. 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. 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. Transatlantic convoys were then diverted away from the U-boat "wolf packs", and U-boat supply vessels sunk. On 1 February 1941, 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 to and from the U-boat base at Kiel.

• The Abwehr was the intelligence and counter-espionage service of the German High Command. Ultra intelligence from their transmissions led to the remarkable state of affairs that allowed Britain's Security Service (MI5) to give a categorical assurance that all the German spies in Britain were controlled as double agents working for the Allies under the Double Cross System. This allowed some remarkable deceptions to be perpetrated.

• After the Japanese Attack on Pearl Harbor, the Americans quickly devised a method of utilizing the previously disorganised intelligence which bore fruit in the Battle of the Coral Sea in May 1942 which, although a tactical victory for the Japanese in terms of ships sunk, proved to be a strategic victory for the Allies.

• The biggest prize of the American deciphering of JN-25 messages was at the Battle of Midway in June 1942. The forewarned U.S. Navy set up an ambush, and four Japanese aircraft carriers and a heavy cruiser were sunk in exchange for one American aircraft carrier and a destroyer. The heavy losses in carriers and veteran aircrews constituted irreparable damage to the Japanese Navy.

• Ultra contributed very significantly to the monitoring of German developments at Peenemünde and the collection of intelligence about the V-1 flying bomb and the V-2 rocket from 1942 onwards.

• A JN-25 decrypt of 14 April 1943 provided details of Admiral Yamamoto's forthcoming visit to Balalae Island, and on 18 April his aircraft was shot down, killing this man who was regarded as irreplaceable.

• 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—including Operation Mincemeat ('The Man who Never Was')—were convincing Hitler and the German high command.

• 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.

• The massive operation that led up to Operation Overlord, the Allied invasion of France included planned deception of the enemy. Operation Bodyguard was designed to lead the Germans to believe that the invasion of north-western Europe would come later than was actually planned, and to threaten attacks at other locations than the true objective, including the Pas de Calais, the Balkans, southern France, Norway, and Soviet attacks in Bulgaria and northern Norway. Operation Fortitude was designed to induce the Germans into believing that the invasion would either be in Norway, or in northern France in the Pas de Calais rather than Normandy. Ultra intelligence, from both Enigma and the Lorenz cipher (Tunny) decrypts, showed that the Germans did not anticipate the Normandy landings and even after they had started, they believed the line that had been fed to them that Normandy was only a feint, and that the real invasion would be in the Pas de Calais.

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. 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 SLU.

The communications element of each SLU was called "Special Communications Unit" or SCU. Radio transmitters were constructed at Whaddon 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: SCU1 (Whaddon Hall), SCU2 (France before 1940, India), SCU3 (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.

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.

Lucy

An intriguing question concerns the alleged use of Ultra information by the "Lucy" spy ring, headquartered in Switzerland and apparently operated by one man, Rudolf Roessler. This was an extremely well-informed, rapidly 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, via 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 that it provided was accurate and timely, however, and Soviet agents in Switzerland (including their chief, Alexander Rado) eventually learned to take it seriously.

Safeguarding of sources

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, it is said, more disciplined about such measures than the Americans, and this difference was a source of friction between them. It was a little bit of a joke 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 flap across the door with a bell on it and a sergeant sat 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.

Ultra information was used to attack and sink many Axis supply ships bound for North Africa; but, as in the North Atlantic, every time such information was used, a believable alternative explanation had to be provided. Before any Axis ship was attacked, it was first "spotted" by a scout plane. One scout would be directed to the ship's known location, but at least two other planes would be sent out to other areas. This ensured that the enemy would see multiple scouting missions, and also prevent Allied pilots from realizing that the Allies already knew where Axis ships would be. (Pilots might be shot down and captured, and in any case such rumors would certainly leak to the Axis.)

In one case, 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 a mysteriously precise attack, arousing German suspicions of a security breach. To distract them 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.

In the Battle of the Atlantic (1939–1945) the precautions were taken to the extreme. In most cases where the Allies knew from intercepts the locations of U-boats in mid-Atlantic, the U-boats were not hunted 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. 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), supplied enough information to support the idea that the Allies were unable to read Naval Enigma.

Of course, in other cases Ultra intelligence could be taken advantage of with little or no risk of a compromise. One example was 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, giving all 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.

Post-war disclosures

While it is obvious why Britain and the U.S. 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, and 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 cryptologic blackout. The third explanation is given by Winterbotham, who recounts that two weeks after V-E Day, on 25 May 1945, Churchill requested that former recipients of Ultra intelligence not divulge the source or the information that they had received from it, in order that there be neither damage to the future operations of the Secret Service nor any cause for the Allies' enemies to blame Ultra 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, 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.

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 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."

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." "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." "In 1941 [t]he brilliant cryptologist Dillwyn Knox, working at the Government Code & Cypher School at the Bletchley center of British code-cracking, solved the keying of the Abwehr's Enigma machine."

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, in 1974, that a decision was taken to permit 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 postwar consequences

There has been controversy about the influence of Allied Enigma decryption on the course of World War II. It has also been suggested that the question should be broadened to include Ultra's influence not only on the war itself, but also on the post-war period.

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 U.S.' 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."

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.

Knightley suggests that Ultra may have contributed to the development of the Cold War. 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 and Anthony Blunt, may thus have felt still more distrustful of their wartime partners.

The mystery surrounding the discovery of the sunk U-869 off the coast of New Jersey by divers Richie Kohler and John Chatterton was unraveled 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.

See also

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

References

• Bamford, James (2002) [2001], Body of Secrets (New ed.), Arrow Books Ltd, ISBN 978-0099427742
• Bertrand, Gustave (1973), Enigma ou la plus grand énigme de la guerre 1939–1945 (Enigma: The Greatest Enigma of the War of 1939–1945), Paris: Librairie Plon
• Budiansky, Stephen (2000), Battle of wits: The Complete Story of Codebreaking in World War II, Free Press, ISBN 978-0684859323 A short account of World War II cryptology which covers more than just the Enigma story.
• Calvocoressi, Peter (2001) [1980], Top Secret Ultra, Kidderminster, England: M & MBaldwin, ISBN 0 947712 41 0
• Churchill, Winston, Their Finest Hour, Boston, Houghton Mifflin, 1949.
• Copeland, B. Jack (2003), Colossus: The Secrets of Bletchley Park's Codebreaking Computers, Oxford: Oxford University Press (published 2006), ISBN 0-19-284055-X, OCLC 238755360
• Farago, Ladislas (1974) [1971], The game of the foxes: British and German intelligence operations and personalities which changed the course of the Second World War, Pan Books, ISBN 9780330234467 Has been criticised for inaccuracy and exaggeration
• Gannon, James (2002), Stealing Secrets, Telling Lies: How Spies and Codebreakers Helped Shape the Twentieth Century, Washington, D.C.: Brassey's, ISBN 978-1574883671 pp. 27–58 and passim
• Gannon, Paul (2006), Colossus: Bletchley Park's Greatest Secret, London: Atlantic Books, ISBN 978 1 84354 331 2
• Gores, Landis Ultra: I Was There, LuLu Publishing, Inc., 2008 http://www.landisgores.com/ultra.html
• Hinsley, F.H.; Stripp, Alan, eds. (1993), Codebreakers: The inside story of Bletchley Park, Oxford: Oxford University Press, ISBN 978-0-19-280132-6
• Hinsley,Sir Harry (ed.) the official history of British intelligence in World War II
• Hinsley, Sir Harry (1996) [1993], The Influence of ULTRA in the Second World War, retrieved 31 August 2010 Transcript of a lecture given on Tuesday 19th October 1993 at Cambridge University
• Jones, R. V. (1978), Most Secret War, London: Book Club Associates, ISBN 978-0241897461
• Kahn, David, The Codebreakers: The Comprehensive History of Secret Communication from Ancient Times to the Internet, 2nd edition, New York, Scribner, 1996, ISBN 0-684-83130-9.
• Kahn, David, "Enigma Unwrapped," New York Times Book Review, 29 December 1974, p. 5. Review of F.W. Winterbotham's The Ultra Secret.
• Kahn, David (1991), Seizing the Enigma: The Race to Break the German U-boat Codes, 1939-1943, Houghton Mifflin Co., ISBN 978-0395427392 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.
• Lewin, Ronald (2001) [1978], Ultra goes to War (Penguin Classic Military History ed.), London: Penguin Group, ISBN 978-0141390420 Focuses on the battle-field exploitation of Ultra material.
• Northridge, A. R. (1993), Pearl Harbor: Estimating Then and Now, Central Intelligence Agency, retrieved 5 September 2010
• Parrish, Thomas, The American Codebreakers This book, earlier published as The Ultra Americans, concentrates on the U.S. contribution to the codebreaking effort.
• Rejewski, Marian 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.
• Sebag-Montefiore, Hugh (2000), Enigma: the Battle for the Code, Cassel Military Paperbacks, ISBN 0-304-36662-5, OCLC 53122520 This book focuses largely on Naval Enigma, includes some previously unknown information—and many photographs of individuals involved. Bletchley Park had been the author's grandfather's house before it was purchased for GC&CS.
• Singh, Simon (2000), The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography, London: Fourth Estate (published 1999), ISBN 1-85702-879-1 This provides a description of the Enigma, as well as other codes and ciphers.
• Smith, Michael (2007) [1998], Station X: The Codebreakers of Bletchley Park, Pan Grand Strategy Series (Pan Books ed.), London: Pan McMillan Ltd, ISBN 978-0-330-41929-1
• Welchman, Gordon (1997), The Hut Six Story, M&M Baldwin, p. 158, ISBN 0-947712-34-8 Describes briefly production of intelligence in BP Hut 3.
• West, Nigel (1986), GCHQ: The Secret Wireless War, 1900-86, London: Weidenfeld and Nicolson, ISBN 0-297-78717-9
• Winterbotham, F.W. (2000) [1974], The Ultra secret: the inside story of Operation Ultra, Bletchley Park and Enigma, London: Orion Books Ltd, ISBN 9780752837512, OCLC 222735270
• Wilkinson, Patrick (1993), "Italian naval ciphers", in Hinsley, F.H.; Stripp, Alan (eds.), Codebreakers: The inside story of Bletchley Park, Oxford: Oxford University Press, ISBN 978-0-19-280132-6
• Winton, John (1988), Ultra at Sea