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An Enigma-machine rotor
Ultra (sometimes capitalised ULTRA) was the name used by the British for intelligence resulting from decryption of encrypted German radio communications in World War II. The term eventually became the standard designation amongst the Allies for all intelligence from high-level cryptanalytic sources. The name arose because the code-breaking success was considered more important than the highest security classification available at the time (Most Secret) and so was regarded as being Ultra secret.
Much of the German cipher traffic was encrypted on the Enigma machine, hence the term "Ultra" has often been used almost synonymously with "Enigma decrypts". However, once decrypts of the Lorenz SZ 40/42 became available, they had greater intelligence value.
Polish 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."
Until the name "Ultra" was adopted, there were several cryptonyms for intelligence from this source, including Boniface. For some time thereafter, "Ultra" was used only for intelligence from this channel.
F.W. Winterbotham, in The Ultra Secret (1974), quotes the western Supreme Allied Commander, Dwight D. Eisenhower, as at war's end describing Ultra as having been "decisive" to Allied victory in World War II.
The Sources Of Ultra Intelligence
The German Sources
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A typical Bletchley intercept sheet, before decryption and translation.
A typical Bletchley intercept sheet, after decryption.
ULTRA intelligence was largely derived from German cipher traffic. These messages were mostly generated on several variants of an electro-mechanical rotor machine called "Enigma." The Enigma machine was widely thought to be in practice unbreakable in the 1920s, when a variant of the commercial Model D was first used by the German Navy. The German Army, Navy, Air Force, Nazi party, Gestapo, and German diplomats all used Enigma machines, but there were several variants (e.g., the Abwehr used a four-rotor machine without a plugboard, and Naval Enigma used different key management from that of the Army or Air Force, making its traffic far more difficult to cryptanalyse). Each variant required different cryptanalytic treatment. The commercial versions were not 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 required immense effort and investment.
In the Pacific theater, the Japanese cipher machine dubbed "Purple" by the Americans, and unrelated to the Enigmas, was used for highest-level Japanese diplomatic traffic. It was also cracked by the US Army's Signal Intelligence Service 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 German strategy and intentions, reports on direct inspections (in one case, of Normandy beach defenses) by the ambassador, and reports of long interviews with Hitler.
The Japanese are said to have obtained an Enigma machine as early as 1937, although it is debated whether they were given it by their German ally or bought a commercial version which, except for plugboard and actual rotor wirings, was essentially the German Army/Air Force machine.
The Ultra Summaries
Initially, Army- and Air Force-related intelligence derived from SIGINT sources (mainly Enigma decrypts) was compiled in summaries at GC&CS (Bletchley Park) Hut 3. The summaries were subsequently distributed under the codeword "BONIFACE", presumably to imply that they were the result of human intelligence operations. The Admiralty (Royal Navy) produced its own intelligence summaries at the RN Operational Intelligence Centre (OIC), 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.
The Distribution Of Ultra
To The Army & The 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.
The Radio & Cryptography
The communications system was founded by Brigadier Sir Richard Gambier-Parry, who was Head of MI6 Section VIII from 1938-1946 and was based at Whaddon Hall in Buckinghamshire, UK. Ultra summaries from Bletchley Park were sent over landline to the radio transmitter site of Section VIII at Windy Ridge. From there they were transmitted over radio to the destination SLU.
The communications element of each SLU was called "Special Communications Unit" or SCU. Radio transmitters were constructed at 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.
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.
The 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, an "innocent" 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.
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 1979-1988. 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.
The Ultra's 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 summarizes the wartime consequences in the last chapter of his book.
During the Battle of France (May–June 1940), the Allies, especially the French, failed to take proper advantage of Ultra, though Ultra intelligence helped organize the evacuation of the British Expeditionary Force (BEF) from Dunkirk.
In the summer of 1940, British cryptanalysts, who were successfully breaking German Air Force Enigma-cypher variants, were able to give Churchill information about the issuing of maps of England and Ireland to the Sealion invasion forces. Ultra also revealed to the British that the threat of invasion was over, when on September 17 Hitler authorized the dismantling of aircraft loading ramps at Dutch airfields.
During the Battle of Britain, Air Chief Marshal Sir Hugh Dowding employed Ultra intelligence for optimal deployment of the limited RAF Fighter Command assets.
Breaking of some messages (not in German Enigma) led to the defeat of the Italian Navy at Cape Matapan (27-29 March 1941), preceded by another "fortuitous" search-plane sighting. British Admiral Cunningham also did some fancy footwork at a hotel in Egypt to prevent Axis agents from taking note of his movements and deducing that a major operation was planned.
Ultra intelligence was of considerable assistance to the British (Montgomery being "in the know" about Ultra) at El Alamein in Western Egypt in the long-running battle with the Afrika Korps under Rommel.
Intelligence from signals between Adolf Hitler and General Günther von Kluge was of considerable help during the campaign in France just after the Allied D-Day landings, particularly in regard to estimates of when German reserves might be committed to battle.
On the other hand, the Red Army was aware of the German buildup, dispositions and precise time of attack prior to the Battle of Kursk, even without the Ultra information provided to them. However, some evidence suggests that the Lucy spy ring in Switzerland, which provided crucial information about Stalingrad and Kursk, was actually a conduit for passing Ultra information to Moscow without detection by the Germans.
The Battle Of The Atlantic
It is commonly claimed that breaking of German Naval Enigma shortened the war by a year, but given its effects on the Second Battle of the Atlantic alone, that might be an underestimation.
An exhibit in 2003 on "Secret War" at the Imperial War Museum, in London, quoted British Prime Minister Winston Churchill telling King George VI, "It was thanks to Ultra that we won the war." Churchill's greatest fear, even after Hitler had suspended Operation Sealion and invaded the Soviet Union, was that the German submarine wolf packs would succeed in strangling sea-locked Britain. He would later write, in Their Finest Hour (1949), "The only thing that ever really frightened me during the war was the U-boat peril." A major factor that averted Britain's defeat in the Battle of the Atlantic was her regained mastery of Naval Enigma decryption.
There were, however, also other technologies, equipment and tactics that moved the Battle of the Atlantic in the Allies' favor. As the air gap over the North Atlantic closed and convoys received escort carrier protection, anti-submarine aircraft became efficient hunter-killers with the use of centimetric radar and airborne depth charges. Improvements to Huff-Duff (radio-triangulation used as part of ELINT) meant that a U-boat could be located even if its messages could not be read. (Simply avoiding a known submarine often sufficed.) Improvements to ASDIC (SONAR), coupled with Hedgehog, enhanced the likelihood of sinking a U-boat.
The Bombing Of German Cities
From February 1942, when Air Marshal Arthur Harris became Air Officer Commanding of Bomber Command, the RAF implemented large-scale night area bombardment of German cities. The destruction of city centres not only killed civilians and destroyed factories, houses and railways, but damaged and degraded the telephone and telex network. The long-distance cable network would usually be quickly repaired, and the telex network was closed down by the Allies only on 8 May 1945. However, as the war progressed, a surge in communications demand by German command-and-control, and the effect of unrepaired destruction, forced the Germans to rely ever more heavily on encrypted radio traffic, which the Allies were able to read.
After D-Day, with the resumption of the strategic bomber campaign over Germany, Harris remained wedded to area bombardment. Historian Frederick Taylor argues 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" in every large German city.
Harry Hinsley closes a counter-factual-historical assessment of the role of Enigma decryption:[I]f [absent Enigma-decryption] the U-boats had delayed the invasion [of France] only by months, till the spring of 1945, other considerations would have come into play. As it was, the invasion of Normandy was carried out on such tight margins in 1944 that it would have been impracticable—or would have failed—without the precise and reliable intelligence provided by Ultra about German strengths and order of battle. Carried out in 1945, [Operation Overlord] would have failed more decisively—or, more likely, these other considerations would have necessitated further delay. Germany's V-weapon offensive against the United Kingdom, and especially against some of the invasion ports along the south coast, would have been in full swing, creating immense destruction throughout southern England. [Germany] would have finished the Atlantic Wall. From early in 1945, as Ultra revealed, [Germany] would have brought into service revolutionary new U-boats and jet and rocket aircraft....
Who can say what different strategies [the Western Allies] would have pursued? 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 recognize the extent to which the explanation lies in the influence of Ultra.
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."
It remains a subject of debate concerning 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 Cold War. The Soviets received information of disguised Ultra material, but the existence of Ultra itself was not disclosed by the Western Allies. The Soviets, who had clues to its existence, possibly through Kim Philby and Anthony Blunt, thus may have felt to have reason to be distrustful of their partners.
- Gannon (2006) p. xx
- Welchman (1982) p. 289
- F.W. Winterbotham, The Ultra Secret, New York, Dell, 1974, pp. 16–17.
- Bamford, J. (2001), Body of Secrets, Doubleday, pp. 17, ISBN 0-385-49907-8
- Patrick Beesly (1977), Very Special Intelligence - The Story of the Admiralty's Operational Intelligence Center 1939–1945, Sphere Books Limited, pp. 36, ISBN 0-7221-1539-3
- Nigel West (1986), GCHQ The Secret Wireless War 1900–86, London: Weidenfeld and Nicolson, pp. 136, ISBN 0-297-78717-9
- Nigel West (1986), GCHQ The Secret Wireless War 1900–86, London: Weidenfeld and Nicolson, pp. 162, ISBN 0-297-78717-9
- F.W. Winterbotham (1975), The Ultra Secret, London: Futura, ISBN 0860072681, OCLC 2817319
- Until 1943, Group Captain Winterbotham was also head of GCHQ Hut 3.
- Nigel West (1986), GCHQ The Secret Wireless War 1900–86, London: Weidenfeld and Nicolson, pp. 138, ISBN 0-297-78717-9
- Nigel West (1986), GCHQ The Secret Wireless War 1900–86, London: Weidenfeld and Nicolson, pp. 152, ISBN 0-297-78717-9
- Geoffrey Pidgeon (2003), The Secret Wireless War: The Story of MI6 Communications 1939–1945, UPSO Ltd, ISBN 1-84375-252-2, OCLC 56715513
- In addition, there existed SCU3 and SCU4, which supported Y Service radio intercepting and direction finding facilities. These units were formed from assets of the former Radio Security Service, after it was reassigned to MI6 and they were not involved in Ultra dissemination.
- Patrick Beesly (1977), Very Special Intelligence - The Story of the Admiralty's Operational Intelligence Center 1939–1945, Sphere Books Limited, pp. 142, ISBN 0-7221-1539-3
- Bill Momsen (2007). "Codebreaking and Secret Weapons in World War II, Chapter IV: 1941-42". Nautical Brass. http://home.earthlink.net/~nbrass1/4enigma.htm. Retrieved 2008-02-18.
- Mallmann-Showell, J.P. (2003), German Naval Code Breakers, Hersham, Surrey: Ian Allan Publishing, ISBN 0-7110-2888-5, OCLC 181448256
- Coincidentally, German success in this respect almost exactly matched in time an Allied blackout from Naval Enigma.
- David Kahn, "Enigma Unwrapped," New York Times Book Review, 29 December 1974, p. 5.
- F.W. Winterbotham, The Ultra Secret, p. 15.
- Ladislas Farago, The Game of the Foxes, p. 664.
- Ladislas Farago, The Game of the Foxes, p. 674.
- Ladislas Farago, The Game of the Foxes, p. 359.
- Calvocoressi (2001) p. 90
- Winston Churchill, Their Finest Hour, pp. 598–600.
- Taylor, Fredrick, Dresden:Tuesday 13 February 1945, (NY): HarperCollins, ISBN 0-06-000676-5, (Lon): Bloomsbury. ISBN 0-7475-7078-7, pp. 202
- F.H. Hinsley, "Introduction: The Influence of Ultra in the Second World War," Codebreakers: The Inside Story of Bletchley Park, edited by F.H. Hinsley and Alan Stripp, Oxford University Press, 1993, pp. 12–13.
- Christopher Kasparek writes: "Had the... postwar governments of major powers realized... how Allied victory in World War II had hung by a slender thread first spun by three mathematicians [Rejewski, Różycki, Zygalski] working on Enigma decryption for the general staff of a seemingly negligible power [Poland], they might have been more cautious in picking their own wars." (Review of Michael Alfred Peszke, The Polish Underground Army, the Western Allies, and the Failure of Strategic Unity in World War II, 2005, in The Polish Review, vol. L, no. 2, 2005, p. 241.
- Phillip Knightley (1986), The Second Oldest Profession, W.W. Norton & Co, p. 173-5, ISBN 0-393-02386-9
- 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) , 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, The Game of the Foxes: The Untold Story of German Espionage in the United States and Great Britain during World War II, New York, Bantam Books, 1971.
- 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. (1992), "Introduction: The influence of Ultra in the Second World War", in Hinsley, F.H.; Stripp, Alan, Codebreakers: The inside story of Bletchley Park, Oxford: Oxford University Press, 1993, ISBN 978-0-19-280132-6
- Hinsley,Sir Harry (ed.) the official history of British intelligence in World War II
- 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 0684831309.
- 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 (1978), Ultra goes to War, London: Grafton, ISBN 0586202803 Focuses on the battle-field exploitation of Ultra material.
- 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.
- Welchman, Gordon (1997), The Hut Six Story, M&M Baldwin, pp. 158, ISBN 0-947712-34-8 Describes briefly production of intelligence in BP Hut 3.
- West, Nigel (1986), The SIGINT Secrets: The Signals Intelligence War, 1900 to Today
- Winterbotham, F.W. (2000) , The Ultra secret: the inside story of Operation Ultra, Bletchley Park and Enigma, London: Orion Books Ltd, ISBN 9780752837512, OCLC 222735270
- Winton, John (1988), Ultra at Sea
United Kingdom intelligence agencies and organisations
Current agencies Government Communications Headquarters (GCHQ) · Security Service (MI5) · Secret Intelligence Service (MI6) · Defence Intelligence Staff (DIS) · Serious Organised Crime Agency (SOCA) · Joint Intelligence Committee (JIC) Defunct agencies MI1 (code breaking) · MI2 (geographic info) · MI3 (geographic info) · MI4 (maps and aerial photos) · MI7 (propaganda) · MI8 (signals intelligence) · MI9 (resistance aid) · MI10 (technical analysis) · MI11 (military security) · MI12 (censorship) · MI14 (Germany desk) · MI15 (aerial defence) · MI16 (scientific intelligence) · MI17 (secretariat) · MI19 (interrogation) ·
Naval Intelligence Division · Committee of Imperial Defence · Directorate of Military Intelligence