SIGINT played a key role in World War II. In the 1920s, the knowledge that cryptanalysis had achieved important results in several countries during World War I led to the development of new and improved cipher methods and especially new cipher machines. These were produced not only for diplomatic and military communications, but for use in business. American Edward Hugh Hebern had been working since 1917 to develop a rotor-driven machine, but he secured a U.S. patent only in 1924.
Also in 1917, German Arthur Scherbius experimented with a similar machine; he secured a German patent on 23 February 1918. He was followed in October 1919 by Dutch inventor Hugo Alexander Koch and Swede Arvid Gerhard Damm. In 1923, Scherbius purchased the Koch patents and produced a new rotor-driven cipher machine known as "Enigma." Scherbius demonstrated the Enigma machine in 1923 at Bern and in 1924 at the World Postal Congress at Stockholm. He developed several versions of the Enigma; the commercial version, Enigma-D, was purchased by several countries, including the Netherlands, Sweden, Switzerland, Czechoslovakia, the United States, Great Britain, and Japan.
The Enigma influenced production of cipher machines in other countries for military uses. In Sweden, engineer Boris Hagelin began his improved machine series; in Great Britain, "Type X" was an improved Enigma. In Japan, a special version was constructed, and experiments led to the "97-shiki-O-bun In-ji-ki" for high-level diplomatic communications (later identified by the Americans with the code name "Purple"). During the Spanish Civil War, the Nationalist side and their allied Italians employed Enigma-D machines, and the Italians also used their Enigma version known as "Alfa." The Poles developed their own cipher machine, "Lacida," and the Czechs experimented with a version driven by compressed air rather than electricity. Boris Hagelin successfully marketed his machines in Sweden and France, later with the Italian navy, and finally in the United States, where 140,000 copies of his M-209 machine were built.
The Germans relied heavily on the Enigma machine for military communications during the war. Only in 1975, with the publication of Frederick Winterbotham's book The Ultra Secret, did it become generally known that Allied cryptanalysts had unlocked the secrets of the Enigma machine and that Allied intelligence had been able to read the most secret German military and diplomatic communications, although not in real time. Winterbotham's book was only the first in a flood of publications about Allied code-breaking. These included work published by the Poles, who in 1932 and 1933 had been the first to crack the secrets of the Enigma; the French, who had delivered to the Poles cipher materials secured from a German agent; the British, who established at Bletchley Park a decryption center; and the Americans, who also came into this operation and worked to break the Japanese codes.
The concentration on Enigma obscured the fact that there was not one Enigma machine, but rather several versions. Moreover, Bletchley Park could break only some of the cipher key nets of Enigma, depending on the number of signals sent in the daily settings of the various key nets. Concentration on Enigma also led to neglect of the many other machine- or hand-cipher systems of other nations, which were tackled by the cryptanalysts of many countries with mixed success.
The German defeat of Poland in September 1939 came so rapidly that Polish cryptanalysts had virtually no impact on that campaign. Evacuated by way of Romania to France, however, they joined the French Deuxi?me Bureau (intelligence service), which was operating in close conjunction with the British. But Allied efforts against the German Enigma key nets only had limited success at first and did not greatly influence Allied operations until mid-1940. With the defeat of France, that nation's cryptanalysts went to the unoccupied southern zone. From there, some Poles went on to Algeria and then joined the British at Bletchley Park.
On the Axis side, the German naval decryption service, the xB-Dienst, enjoyed considerable success in breaking the hand-cipher systems of the Royal Navy, which at this time did not use machines but rather super-enciphered codebooks. This fact enabled German warships in most cases to evade superior British forces. It also provided the Germans with information about Allied convoys.
In May 1940, Bletchley Park at last was able, with only short delays, to decrypt the main operational Enigma key net of the German Luftwaffe. This was aided by the introduction of the first "bombes"—devices developed by mathematicians Alan Turing and Gordon Welchman to determine the daily settings of the German machines. These intercepts, now known by the intelligence code name of ultra, influenced the Battle of Britain, as daily reports by the German air groups revealed their strengths and intended operations. Using this information, Britain's "home chain" radar, and a system of coast watchers, the British could use fewer fighter aircraft with maximum efficiency, sending fighters aloft at the right time to intercept incoming German bombers.
But Bletchley Park could not then break the Enigma key nets of the German army. Of special importance at the time was breaking the German navy key nets, especially those used by the U-boats. The German Enigma machine was known to the British from a replica the Poles had constructed and delivered to France and Britain in August 1939, but the problem was the wiring of the cipher-rotors. Only five of these were known; but the German navy used eight, and these had to be identified, together with information about the cipher settings, which changed daily. This was only accomplished in May and June 1941.
Despite decoding delays, the Allies were able to read German radio signals and thus had the means to destroy their surface supply organization in the Atlantic. This in turn forced the Germans to cancel commerce raiding operations by their large surface combatants. Also, from early August 1941, it became possible to decrypt U-boat radio traffic, which made it possible for the Allies to reroute their Atlantic convoys around the German U-boat dispositions. This prevented the sinking of perhaps 300 merchant ships in the second half of 1941.
Not all German officials had confidence in the security of the ciphers. Admiral Karl Dönitz tried several times after mid-1941 to improve the system. He introduced a code for the grid map positions and a separate cipher key net for the Atlantic U-boats. Then, on 1 February 1942, Dönitz ordered the introduction of an improved cipher machine known as "M-4" that had a divided "Umkehrwalze B" reflective rotor and an additional, fourth, rotor, the so-called "Greek Beta." This change produced an 11-month blackout at Bletchley Park in reading German U-boat traffic. At the same time, xB-Dienst was able—partially in 1942 and more so in 1943—to decrypt the Anglo-American super-enciphered codebook "Naval Cipher No. 3" used for communication with the convoys. Although there were decoding delays, the decryption led to excellent results for the U-boats in the North Atlantic until early spring 1943.
On 30 October 1942 in the Mediterranean, a German U-boat was forced to surface, and a specially trained British boarding party was able to salvage important cipher materials, especially the "weather short signal book." This was used to find "cribs," the cipher/clear text compromises that aided in determining daily settings of the MC4 machine. Thus, from mid-December 1942, Bletchley Park could once again send to the submarine tracking room of the Admiralty the dispositions of the German U-boat Wolf Packs in order to reroute convoys. But there were now so many Wolf Packs that new tactics had to be found to avoid repetition of the heavy losses incurred in mid-March, when the Germans introduced a new weather short signal book, producing a new blackout. By concentrating the available bombes to U-boat traffic, Bletchley Park was again able after only 10 days to break the U-boat cipher. Coupled with this, the Allies committed additional antisubmarine forces, very-long-range aircraft, hunter-killer groups of destroyers and the first escort carriers, radar in escort ships and light-weight radar in aircraft, high-frequency direction finders, and Leigh lights. The turning point came in late May 1943, when Dönitz redeployed his submarines from the North Atlantic convoy routes to the Central Atlantic and to distant operational areas. At the same time, Bletchley Park decrypted the orders, enabling the British to relocate their air groups from the convoy routes to Britain over to a strong offensive against U-boat routes in and out of the Bay of Biscay.
In June 1943, the British realized that the Germans had decrypted their signals, and the Admiralty changed to a new super-enciphered code, Naval Cipher No. 5, which led to a blackout for xB-Dienst. On 1 July, the Germans introduced a new "Greek rotor C/Gamma," but this led to only a short interruption in decryption at Bletchley Park because of the introduction of a new high-speed bombe. Development in the United States of high-speed bombes led to the transfer in November 1943 of decryption work on the U-boat cipher to the U.S. decryption organization Op-20G, while Bletchley Park concentrated on the German Enigma ciphers of the German air force and the army, both of which had substantially increased the number of their key nets.
Bletchley Park's first break into the German army ciphers had come as the Germans prepared to invade the Soviet Union in June 1941. This provided information about German deployment of forces to Poland and preparation of trains specifically to transport prisoners of war, clear proof that the deployments were not simply an effort to blackmail the Soviet Union. During the invasion itself, Bletchley Park was able to intercept and read a great many German signals. Because the British did not trust the security of the Soviet code and cipher systems, decryptions from the German forces on the Eastern Front were transmitted to Moscow in a special secure cipher and only under cover stories. It is as yet unknown if the Soviets were aware from their spies in Britain, including Kim Philby and especially John Cairncross at Bletchley Park, of the source of the information and, if so, whether Soviet leader Josef Stalin trusted it. Also, while the German army, navy, and air force decryption services were able to read Soviet codes and some ciphers and use the information thus obtained in operations, we do also not know how extensively and when the Soviets could decrypt German signals.
Signals intelligence was of great importance in the Mediterranean Theater. When Italy entered the war on 10 June 1940, the Allies could read many of the Italian codes and ciphers. In addition, some ciphers were captured from Italian submarines forced to surface. But the Italians soon changed many of their systems, and decryption fell off sharply as a result. Then, in February 1941, Bletchley Park cracked the German air force cipher "Light Blue" and the Italian version of the Enigma known as "Alfa," used for radio communication between Rome and the Dodecanese Islands. These successes had important consequences, most notably in defeating the Italian navy at the Battle of Cape Matapan in March 1941.
In May 1941, decryption of the German signals provided excellent intelligence concerning German plans to invade Crete, but the British defense of the island failed because the German air force controlled the skies and because the intercepts included misleading information about a German seaborne invasion. As it transpired, this was not a significant German effort, but it led the British to shift defensive assets to the north away from the airfields, where the main German assault occurred. The Battle of Crete revealed both the advantages and disadvantages of SIGINT regarding enemy intentions.
From June 1941, Bletchley Park's decryption of the Italian Hagelin naval cipher machine "C-38m" used in communications between Supermarina in Rome and Tripoli had important consequences for Axis resupply of Axis forces in the western desert. Axis shipping losses increased dramatically in the second half of 1941 and 1942, when it became possible to vector Allied submarines and surface warship strike groups from Malta to intercept Italian convoys between Sicily and the Tunisian coast and Tripoli. The British always tried first to send reconnaissance aircraft to report the target so the signals they sent prevented the Italians from recognizing the true source of the information.
Decrypted signals had to be used judiciously. Thus Afrika Korps commander General Erwin Rommel exaggerated the evils of his supply situation in order to gain additional support, causing British Prime Minister Winston L. S. Churchill twice to order his commanders in the field to begin offensives that then failed in Rommel's counterattacks. Rommel also learned much about British force strengths in Egypt because the U.S. military attaché in Cairo used a code broken by the Italians for his reports. Allied commanders also made different use of SIGINT results in operations extending from El Alamein to Tunisia and during the landing operations at Sicily, Salerno, and Anzio. Revelations of the ultra secret in 1975 did force a reconsideration of the military reputations of several Allied commanders, including Field Marshal Bernard Montgomery.
In connection with the preparation of the Normandy Invasion, the Allies in Operation fortitude successfully used radio deception to convince the Germans of the presence in southeast England of a U.S. Army group commanded by Lieutenant General George S. Patton, a force that in fact did not exist. This false information led Adolf Hitler to believe that the Normandy Invasion was a feint and that the Allies planned to make their major cross-Channel attack in the Pas de Calais area. Hitler thus held back his panzer divisions of the Fifteenth Army from Normandy until too late and then sent them piecemeal into action.
The western Allies also succeeded in the decryption of the German army teleprinter cipher machine used for communications between the highest levels from the army High Command and field armies. This was made possible by development of the first electronic precursor of the later computers, the "Colossus" machine, which became operational in spring 1944. It proved important in halting the German counterattack against the breakthrough at Argentan and led to the encirclement of strong German forces at Falaise. But Allied reliance on ultra intelligence, as the German decryptions were known, meant the German Ardennes Offensive (Battle of the Bulge) of late December 1944 took the Allies unawares, as the Germans had observed strict radio silence and used only secure land lines for communication.
Other belligerents and also neutral powers employed SIGINT during the war. The Finns cracked many Soviet codes, and the Swedish mathematician Arne Beurling broke into the German naval teleprinter Siemens T-52 cipher machine. German teleprinter lines running through Swedish territory were tapped there. Swedish intelligence officers, who favored the Allied side, delivered information to the British naval attaché at Stockholm. Thus the Admiralty learned of the German plan to attack convoy PQ 17 in July 1942 with large surface ships, destroyers, submarines, and aircraft, whereon British First Sea Lord Admiral of the Fleet Sir Dudley Pound ordered the convoy to scatter without waiting for the report of the departure of the German Task Forces. This led to catastrophe for the convoy, as German aircraft and U-boats sank two-thirds of its merchant ships. The rising number of cipher key nets employed by the Germans also overstretched Bletchley Park capacities. Thus even by mid-1942, of about 50 key nets then in use, Bletchley Park could only decrypt about 30, and these with varying time delays and gaps.
SIGINT also had great influence in the Far East Theater. In September 1940, American cryptanalysts under the leadership of William Friedman broke the Japanese diplomatic Purple cipher. (Recently it has been revealed that both the Germans and Soviets were also able to break Purple.) Thus, during negotiations with Japanese in 1941, the U.S. State Department not only knew the documents the Japanese diplomats would present but also their specific negotiating instructions. Purple intercepts provided clear evidence in late 1941 that the Japanese had decided to break off negotiations. But the diplomatic communications gave no hints of Japanese military plans, and the Japanese army codes were still difficult to decrypt. The Japanese navy's super-enciphered codes, especially JN.25—an earlier version of which had been broken—could not be read after changes in the codebooks and super-enciphering tables. Thus U.S. military and naval leaders had to depend on traffic analysis and direction finding, which did provide clear evidence, supported by optical observations, of the deployment of Japanese forces for attacks against the Philippines, Malaya, and the Netherlands East Indies. There was, however, no direct indication of an attack against Pearl Harbor, which thus came as a great surprise.
Breaking the new version of JN.25 and other Japanese codes took time, because reconstructing the tables and codebook was always difficult after changes. In 1942, by clever evaluation of vague indications, Commander Joseph J. Rochefort concluded that the Japanese planned first to invade New Guinea and then to strike at Midway. This conclusion enabled Pacific Fleet Commander Admiral Chester Nimitz to counter the Japanese moves, leading to the Battles of the Coral Sea and Midway.
During the struggle for the Solomon Islands in late 1942 and 1943, U.S. cryptanalysts learned to decrypt the Japanese codes more quickly, and by 1944 they could in most cases provide timely decryptions of Japanese signals in support of strategic operations. Of special importance was the success in breaking the so-called "Maru" cipher for Japanese logistical support to the islands they held in the Pacific. This allowed U.S. submarines to be directed with considerable accuracy to intercept positions, which in turn produced rising losses in the already thinly stretched Japanese merchant ship capacity and in Japanese warships. SIGINT closely supported General Douglas MacArthur's Southwest Pacific Forces from New Guinea to the Philippines and the Central Pacific Forces under Admiral Nimitz against the Marshall, Caroline, and Mariana Islands and finally against Iwo Jima and Okinawa.
In the Indian Ocean area, British cryptanalysts were able to break Japanese army and air force codes. On the other side, the Japanese had only limited success in decryption operations. In consequence, they employed mainly traffic analysis and direction finding for their military and naval operations.
There is no doubt that SIGINT was of great importance to all belligerents in the war. It is also true that the Allies were much more effective in its use. SIGINT alone did not win the war for the Allies, but undoubtedly it significantly shortened the length of the conflict.
Alvarez, David. Allied and Axis Signals Intelligence in World War II. London: Frank Cass, 1999.; Bauer, Friedrich L. Decrypted Secrets. Methods and Maxims of Cryptology. New York: Springer, 1997.; Beesly, Patrick. Very Special Intelligence: The Story of the Admiralty's Operational Intelligence Centre, 1939–1945. London: Greenhill Books, 2000.; Bertrand, Gustave. Enigma: ou la plus grande enigme de la guerre 1939–1945. Paris: Plon, 1973.; Boyd, Carl. Hitler's Japanese Confidant: General Oshima Hiroshi and Magic Intelligence, 1941–1945. Lawrence: University Press of Kansas 1993.; Drea, Edward J. MacArthur's Ultra: Codebreaking and the War against Japan, 1942–1945. Lawrence: University Press of Kansas, 1992.; Hinsley, F. H., et al. British Intelligence in the Second World War. 5 vols. London: Her Majesty's Stationery Office, 1979–1990.; Lewin, Ronald. The Other Ultra: Codes, Ciphers and the Defeat of Japan. London: Hutchinson, 1982.; Rohwer, Jürgen, and Eberhard Jäckel, eds. Die Funkaufklärung und ihre Rolle im Zweiten Weltkrieg. Stuttgart, Germany: Motorbuch Verlag, 1979.; Welchman, Gordon. The Hut Six Story: Breaking the Enigma Codes. New York: McGraw-Hill, 1982.; Winterbotham, Frederick W. The Ultra Secret. London: Weidenfeld and Nicolson, 1975.; Winton, John. Ultra in the Pacific: How Breaking Japanese Codes and Ciphers Affected Naval Operations against Japan, 1941–1945. London: Leo Cooper 1993.