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Ultra was the designation adopted by British military intelligence in June 1941 for wartime signals intelligence obtained by "breaking" high-level encrypted enemy radio and teleprinter communications at the Government Code and Cypher School at Bletchley Park.[1] "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.[2] Several other cryptonyms had been used for such intelligence. British intelligence first designated it "Boniface"—presumably to imply that it was the result of human intelligence. The U.S. used the codename "Magic" for its decrypts from Japanese sources.
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[3] and other Italian and Japanese ciphers and codes such as PURPLE and JN-25.[1]
Many observers, at the time and later, regarded Ultra as immensely valuable to the Allies. Winston Churchill told King George VI: "It was thanks to Ultra that we won the war."[4] F. W. Winterbotham, quoted the western Supreme Allied Commander, Dwight D. Eisenhower, at war's end describing Ultra as having been "decisive" to Allied victory.[5] Sir Harry Hinsley, official historian of British Intelligence in World War II, made a similar assessment about Ultra, saying that it shortened the war "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.[6]
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Most Ultra intelligence was derived from reading radioed enemy messages that had been encrypted with various cipher machines. This was complemented by material derived from radio communications using other methods, such as radio 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 the British Government Code & Cypher School at Bletchley Park were allowed some time to build up experience of collecting and starting to decrypt messages on the various radio networks. Initially, German Enigma messages were the main source, with those of the German airforce predominating, as they used radio more, and their operators were particularly ill-disciplined.
"Enigma" refers to a family of electro-mechanical rotor cipher machines. These produced a polyalphabetic substitution cipher, and were widely thought to be 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. For instance, the Abwehr (the German military intelligence service) 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 before the war.
German military Enigma was first broken in December 1932 by the Polish Cipher Bureau, using a combination of brilliant mathematics, the services of a spy in the German office responsible for administering encrypted communications,[7] and a slice of good luck.[8] Thereafter the Poles read Enigma to the outbreak of World War II and beyond, in France.[9] However, at the turn of 1939 the Germans made their systems ten times more complicated, which required a tenfold increase in the Poles' decryption equipment, a need that they could not meet.[10] On 25 July 1939, just five weeks before the outbreak of World War II, the Polish Cipher Bureau handed reconstructed Enigma machines and their techniques for decrypting ciphers to their French and British allies.[11] 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."[12]
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.[13] It was the Poles' early start at breaking Enigma, and the continuity of their successful efforts, that enabled the Allies to hit the ground running when World War II broke out.[14]
In the spring of 1941, the Germans started to introduce on-line stream cipher teleprinter systems for strategic point-to-point radio links, to which the British gave the generic code-name Fish.[15] Several distinct systems were used, principally the Lorenz SZ 40/42 (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.
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 rotor-based cipher machine called the C-38.[6] This was broken from June 1941 onwards by the Italian subsection of the UK's GC&CS at Bletchley Park.[16]
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,[6] and reports of long interviews with Hitler.
The chief fleet communications code system 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. The Japanese did not use it for their top secret communications, instead developing their own, similar machine.
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 as "Boniface". The volume of the intelligence reports going out to commanders in the field built up gradually. Naval Enigma decoded in Hut 8 was forwarded from Hut 4 to the Admiralty Operational Intelligence Centre (OIC),[18] which were distributed initially under the codeword "HYDRO".[19] The term "Ultra" was adopted in June 1941.[20] The term "Ultra" was reportedly suggested by Commander Geoffrey Colpoys, RN, who served in the RN OIC.
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 Frederick William Winterbotham.[21] 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.
Fixed SLUs 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, 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.[22] 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.
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.[23]
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.[24]
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.[25] 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:[25] 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.[26]
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.[27]
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 Radó) eventually learned to take it seriously.
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.[28]
At Bletchley Park, extensive indexes were kept of the information in the messages decrypted.[29] 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.[30] 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.[31]
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.[6]
Some of the contributions that Ultra intelligence made to the Allied successes are given below.
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,[61][62] 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.
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.[6]
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.[63]
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),[64] and supplied enough information to support the idea that the Allies were unable to read Naval Enigma.[65]
By 1945 most German Enigma traffic could be decrypted within a day or two, yet the Germans remained confident of its security.[66] Had they known better, they could have changed systems, forcing Allied cryptanalysts to start over.
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 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.[67]
A second explanation relates to a misadventure of Churchill 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. 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 Axis to blame Ultra for their defeat.[68] 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 Poles 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.
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 one reason the British and Americans made the machines available. Switzerland even developed its own version of Enigma, known as NEMA, and used it into the late 1970s.
Some information about Enigma decryption did get out earlier, however. 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. The same year, David Kahn in The Codebreakers described the 1944 capture of a Naval Enigma machine from U-505 and mentioned, somewhat in passing, 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."[69] "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."[70] "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."[71]
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, a decision was taken to permit revelations about some Bletchley Park operations.
The United States National Security Agency (NSA) retired the last of its rotor-based encryption systems, the KL-7 series, in the 1980s.
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 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."
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.[72]
Knightley suggests that Ultra may have contributed to the development of the Cold War.[73] 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,[73] 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.
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