Planning Overlord: The Impact of Intelligence on D-Day Erik Huizing Dr. Travers HTST 381 L01 Monday, April 10, 2000 "Funken ist Landesverrat" To use the radio is to betray the country. These were the words of General Erich Fellgiebel, head of the Heeresnachrichtenwesen (Army Intelligence), illustrating the importance of keeping important communications secret. Had his words been heeded, the Allies may not had the intelligence advantage they did. Military intelligence involves two distinct branches: collecting intelligence, and employing counter-intelligence against the enemy. Once information has been gathered against the enemy, it must be verified, then analysed for strategic value. Counter-intelligence involves both the denial of information to the enemy, as well employing deceptive practices to achieve strategic goals. While the Allies employed both disciplines, counter-intelligence and deception were much more useful, primarily because the choice of landing sites was based on geography rather than intelligence. The Allies were fortunate enough to have Ultra, which gave them the ability to decrypt secret German communications. These decrypts made vast contributions to the strategic picture as seen by the Allies. By the time the Allies began planning Overlord, Enigma messages could be decoded by Ultra in three hours. Three hours is insufficient to be of tactical use, but still holds strategic importance. Reports of German troop and supply movement, requests for resupply, and orders for battle are all vital pieces of information to know when planning an invasion. Secondly, the counter-measures taken by the Allies served two purposes. First, by taking serious precautions, the Allies had much more secure communications than the Germans. Second, the release of false information through double agents and the Double-Cross system, helped confuse the Germans about many things, including the details of any invasion. For centuries, there has been a need for secure communications. The success of battle depends orders being received without worry of them being intercepted by the enemy. To ensure security, messages are encrypted such that, ideally, only the sender and receiver can derive any useful meaning from the encoded message. In 1926, British Intelligence began to intercept messages that defied all types of known code breaking. The Germans had unleashed the Enigma, a mechanical message encoder. The creation of Arthur Scherbius, Enigma was an attempt at creating an unbreakable code. Prior to Enigma, most encryption systems used a simple substitution: one letter was simply substituted for another according to some 'key'. Their weaknesses lay in the fact that the 'key' used to encrypt the message did not change throughout the message. The 'key' used to encrypted a message with Enigma changed as each letter of the message is typed in. This is accomplished by three scramblers which rotate like an odometer as each letter is typed. Each scrambler can have one of 26 positions, one for each letter of the alphabet. Prior to sending a messa ge, the three scramblers are set to a certain sequence, which comes out of a book of published keys. The three scramblers can also be arranged in any order, thereby increasing the number of keys that can be used. At the end of the scramblers is a reflector that bounces the signal back through the scramblers, doubly encrypting the letter. An added effect of the reflector was that if encrypted text was entered into the Enigma, it came out as the uncoded message, meaning that an Enigma machine could be used for both encryption and decryption. This feature however, turned out to be a weakness. Such two-way systems are vulnerable to trial and error tests, which aided the British in breaking Enigma. Finally, a plug board is used for swapping the meaning of and six letters. In effect, there are over 10,000,000,000,000,000 possible initial settings for the Enigma. If it were possible to check one setting every minute, it would still take more than the age of the universe to check every single setting. Despite the overwhelming number of configurations the Enigma could take on, the Allies still managed to defeat it. As with previous encryption systems, Enigma's weakness lie in repetitions. Prior to the war, the Polish happened upon a commercial version of the Enigma, which revealed how the Enigma was designed, but was not useful for decrypting transmissions. However, this was more of a start than the English, French, or Americans had. Second, a French agent code named Rex, acquired documents regarding the operation of the military version of Enigma. These documents did not reveal the wiring of the three scramblers, but did contain enough information on how to each month. Each day has its own key. Using this day key, the sender picks a message key, ideally three random letters, and sends that sequence twice. At this point, both sender and receiver both reset their machines according to the message key. A key drawback in the Enigma system was its method of distributing the book of day keys. Should this book fall into enemy hands, it makes the job of determining the message key much easier. The fact that the message key was repeated once at the beginning of the message allowed Polish code breakers to begin deducing the wiring of the scramblers. The concept of using a different day and message key is one which adds security, however, it was the repetition of the message key which greatly aided the code breakers. They were eventually able to construct tables indicating how the first and fourth letters differed. Tables were also constructed for the second and fifth, and third and sixth letters as well. By using these tables, it became possible to determine when the scrambler had made a complete revolution, thereby reducing the number of unique settings of Enigma to check. By using the these tables, it finally became possible to determine what the day key was. The effect of this discovery was to reduce number of combinations from 10,000,000,000,000,000 to 105,456. While technically feasible, checking each of these settings required an army of code breakers, something Poland just didn't have. Fortunately, the British did. A team of code breakers and cryptanalysts had been struggling with Enigma since its introduction. Following the German invasion of Poland, all the work on Enigma was handed over to the British team at Bletchley Park. Building on the work of the Polish, the British, lead by Alan Turing, persisted in their work. Through a process of testing plain text against coded words, the British were eventually able to consistently decrypt Enigma transmissions. This system became known as Ultra. The ability to read German transmissions was a double edged sword. On one hand, Ultra enabled the Allies to track German troop movements, requests for resupply, and battle orders, in effect pointing out German weak points. Attacks based on Ultra decrypts could be very successful. On the other hand, if the Allied attacks appeared to be too lucky, the Germans may suspect their codes have been compromised. This indeed did happen, and extra security measures were added to Enigma: two additional scramblers, and six more plug board connections. These did not deter Bletchley Park, however. The basic functionality and layout of Enigma was known, and a great deal of work could be reused to determine the layout of the new scramblers. Another department of research at Bletchley Park was investigating the creation of a new method of encryption. Unfortunately, due to the need for national security, many discoveries were kept secret. One invention was a new method of encryption, touted as being unbreakable. As a result of Briti sh secrecy, the very same method was co-discovered by three Americans in 1977. RSA (named after its inventors, Riverst, Shamir and Adleman). RSA, and its British counterpart were revolutionary for two reasons. First, they relied on a one-way function to encrypt messages. A one-way function is a mathematical expression which cannot be easily reversed, such that many different inputs yield the same output. This means that a the message must be decrypted by a different process with a different key than were used to encrypt it. Second, this system made use of extremely large prime numbers (more than 100 digits) as keys. The usage of prime numbers was beneficial because of their properties when used in one-way functions. A second concept was also developed at Bletchley Park, but credit again was claimed first by the Americans. Public-key cryptography, the process by which two parties can safely agree on a key to use without worrying about eavesdroppers. Finally, in 1984, GHQ lifted its sec recy ban, allowing the people at Bletchley Park to claim credit for their inventions. Preparations for the invasion required the decisions of where to land, and how many troops to land. Geography, not intelligence, decided where the Allies would land. Pas-de-Calais was ruled out because it was too obvious a landing sector and it was too well defended. Further information about the placement and movement of German divisions in France gave SHAEF (Supreme Headquarters for Allied Expeditionary Force) a clear picture of the location and strength of German divisions. The beaches of Normandy crossed the boundary between the 7th Army and the 15th Army. In addition, German armour was being kept away from the coast. Rommel had wanted to keep the Panzer divisions close to the beaches in order to effectively support the infantry. However, due to chain of command conflicts, he was unable to get his wish, even as the paratroopers would capture or destroy bridges granting access to the beaches, denying the Germans an opportunity to launch a counterattack. Secondly, control of the sky would be required, so the beaches had to be within range of Allied air bases. Other requirements were that it should be difficult for the Germans to transfer reserves to the invasion site quickly or easily. COSSAC (Chief of Staff to Supreme Allied Commander for operation Overlord) determined that there should not be more than 12 twelve full strength, first rate divisions in addition to the troops defending the coast. In addition, during the first two months, they should not be able to move more than fifteen divisions from other fronts. Finally, the flow of German reinforcements should be limited to three divisions on D-Day, five on D+2, and nine on D+9. Lastly, an element of surprise was necessary. Surprise requires keeping the invasion plans secret, and possibly using deception to confuse enemy intelligence. In February 1940, the Inter-Services Security Board (ISSB) had been set up to provide cover for British operations and prevent information leaks. During the planing of Overlord, the ISSB suggested the implementation of many security precautions to keep invasion plans secret. Access to coastal areas became restricted, denial of travel to and from Ireland, and in addition, a visitors ban was also imposed on the coastal area from the Humber to Penzance, the area between Mildord Haven and Portishead, and surrounding areas. These moves were supported by both MI 5, the War Office, and COSSAC, but strongly opposed by the Home Office, the Ministry of Home Security, and the Ministry of Health. Winston Churchill in particular said "We must be aware of handing out irksome for irksome's sake". On one side, tight restrictions on the population, including travel and communication off the island made it increasingly difficult for Germans to gather intelligence on British operations and plans. On the other hand, this security comes at the price of reduced civil liberties. Additionally, how would German intelligence view these acts of isolation? A committee was formed to review the ISSB's recommendations, and decided that the recommendations were inadequate and added further recommendations: "diplomatic representatives of all neutral governments, including the government of Eire, should be prohibited from sending or receiving uncensored communications, whether by bag or cipher, and from leaving Great Britain... Artificial delays to bring the period between dispatch and receipt up to one month should be imposed on mail to foreign countries. For mail to POWs in Germany, this period should be three months (this was though to be a very easy way for an enemy agent to communicate in plain language code)... Overseas telegrams should be paraphrased and artificially delayed." On March 1, all the recommendations were approved, with the exception of those regarding diplomacy. Restrictions were lifted against Russia and the Americans. The visitors ban would become effective April 10, while the communications restrictions would become effective midnight April 17. How effective were these measures? It cannot be said for certain. Certainly, they impeded the information flow into and out of England. There were information leaks and security breeches, however. In March, MI 5 reported that there had been several communications with unauthorised personal regarding the construction of towable Mulberry harbours, which would aid the landing of supplies following the invasion. A look at Ultra decrypts from the time leading up to D-Day suggests that the German ideas about an invasion was ill informed, if not, confused. With German intelligence cut off from the real events in England, the Allies had the opportunity to create false information. The deceitful actions took many forms, from false radio transmissions, to using double agents, from creating full size model ships and aircraft, to using Patton's reputation. Operation Fortitude and the Double-Cross System encompassed the transfer of false information to German Intelligence. The goal of Fortitude was to create false radio traffic, construct fake bases, ships, aircraft, and army divisions. Fortitude was a two-part plan. The first half, Fortitude North, involved convincing the Germans that an invasion would take place in Norway. Following a link up with Soviet forces, a second invasion aimed at Pas-de-Calais would be launched. The success of Fortitude is evident in the facts that the Germans were expecting landings in Norway and southern France when the Allies landed on the beaches of Normandy: " (FAIR INDICATIONS OSLO & OSLO HARBOUR DEFENCE FLOTILLA) IN CONNECTION WITH)) THE LARGE SCALE LANDING ON THE CHANNEL COAST STARTED ON SIXTH THAT SURPRISE APPEARANCE OF THE ALLIES IN ITS AREA ALSO WAS TO BE RECKONED WITH AND ORDERED HEIGHTENED VIGILANCE " To add credibility to Fortitude North, Montgomery's messages were sent by land line from Portsmouth to Dover where they were broadcast from. This created the illusion that Montgomery was in Dover, planning for the invasion at Calais. This level near-complete deception was accomplished in many ways. First, a certain level of subtly in the messages was necessary. An uncoded radio message stating the British will land in Norway will not be convincing in the least. However, a message stating that the 80th division requests skis and winter gear will lead the enemy to believe that an invasion in the north is being planned. Another matter of concern with Fortitude was diverting attention from the Normandy beaches to other possible landing sites, with the goal of forcing the Germans to spread their forces thinly amongst many possible invasion sites. The second phase, Fortitude South, was aimed at creating further confusion after D-Day. Again using Pas-De-Calais as the intended target, Fortitude South radio messages suggested that the main assault would be launched against Calais by the 1st US Army Group. A beachhead would be established, and 50 divisions would be brought ashore. Radio transmissions also stated that the Normandy invasion was merely a diversionary attack. The amount of deception employed by the Allies, plus the cooperation with the Resistance, which blew bridges, sabotaged tanks and railways, and cut communication lines, ensured that the Germans would have great difficulty containing and suppressing the invasionary forces. By the time the invasion occurred, the Germans had suspected these acts of sabotage, but were unable to draw any concrete conclusions: "FIELD LUFTGAU WEST FRANCE & FRANCE AWARE ON SIXTH)) THAT ACCORDING ALLIED BROADCAST ON FIFTH TERRORIST GROUPS IN FRANCE & FRANCE UNDER ORDERS TO ATTACK RAILWAYS, IMPORTANT ROADS, CANALS, TELEPHONE AND TELEGRAPH LINES WITHIN TWO FOUR OR FOUR EIGHT HOURS. APPRECIATED THAT SABOTAGE TO BE EXPECTED ESPECIALLY AREAS TOURAINE & TOURAINE AND LE MANS & LE MANS, RAILWAY SABOTAGE EASY BRITTANY & BRITTANY AND AREA SARTHE & SARTHE, TELEPHONE SABOTAGE THROUGHOUT FRANCE & FRANCE" Lastly, throughout the Africa and Sicily campaigns, the German High Command kept a close eye on General Patton. Believing him to be the Allies' secret weapon, very close attention was paid to his actions and whereabouts at all times. As part of their deception plans, Patton and his staff were sent on high profile visits Corsica, and throughout Sicily. Patton's visits created the appearance that the Allies would be planning landings at Sicily and Corsica, possibly led by Patton. The fact that such a successful, high profile general was on these trips only helped seal the idea that there would be invasions from Sicily or Corsica. The Double-Cross system took a different approach. Captured German spies were turned against their country, then sent back with misleading information from the Allies. Fortunately for the Allies, some of these spies were considered very dependable by German Intelligence, and their information was taken very seriously . Part of the Double-Cross system emerged after Admiral "Blinker" Hall suggested using double agents to recently appointed Director of Naval Intelligence, John Godfrey . Agents were used as go-betweens for Naval Intelligence to the Germans. After gaining their trust, an obsolete cipher was sold to the Germans. Finally, "top secret" messages were encrypted with this cipher, and broadcast. The Germans would use the cipher purchased earlier to decode these apparently top secret communications between Naval Intelligence and other high ranking organisations. Intelligence alone cannot win battles, however when used properly, it can prevent the enemy from accurately determining plans, and additionally confusing them further as to the true nature of a nation's intentions. While intelligence reports did not have a direct influence on the actual selection of the invasion beaches, it still played a key role in Overlord. Namely, keeping the Germans distracted with false invasion plans, and confused by contradictory information. Additionally, intelligence (primarily Ultra Decrypts) informed the Allies of troop positions and strengths. In closing, its worth considering how important Ultra was to the Allies. Enigma's main weakness was that the messages were broadcast over radio. Anybody, including the British, were free to listen in. The radio waves were a rich source of information for Ultra. However, as the Germans retreated back into German territory, they no longer had to make use of radio. Instead, there were land lines, which were difficult to eavesdrop on. As soon as the main method of German communication was land line, and not radio, the volume of Ultra decrypts dropped severely. This sudden loss of information rendered the Allies effectively blind. This is evident by the fact that the Germans were able to launch a surprise counter-offensive in late December 1944. There were no Ultra decrypts to indicate that such an offensive was being planned. However good information is, what's more important is how its used. Intelligence alone does not win wars. "Wars may be fought with weapons, but they are won by men. It is the spirit of men who follow and the man who leads that gains the victory." - Patton Bibliography Primary Sources Ultra Decrypts May-June 1944, D180 S7U46 Ultra Decrypts June 1944, D180 S7U46 Hinsley, F. H. British Intelligence in the Second World War, Volume 4. Cambridge, Cambridge University Press, 1993. Secondary Sources Ambrose, Stephen. Citizen Soldiers: The U.S. Army from The Normandy Beaches to the Bulge to The Surrender of Germany. New York, Touchstone, 1997 Ambrose, Stephen. D-Day: The Climactic Battle of World War II. New York, Touchstone, 1994. Bath, Allan Harris. Tracking the Axis Enemy: The Triumph of Anglo-American Naval Intelligence. University of Kansas Press, 1998. Bennett, Ralph. Intelligence Investigations: How Ultra Changed History. Portland, Frank Cass, 1996. Calvocoressi, Peter. Top Secret Ultra. New York, Pantheon Books, 1980. D'Este, Carlo. Patton: A Genius for War. New York, Harper Perennial, 1995. Hinsley, F. H. British Intelligence in the Second World War, Abridged Edition. Cambridge, Cambridge University Press, 1993. Kahn, David. The Codebreakers: The Comprehensive History of Secret Communication from Ancient Times to the Internet. New York, Schribner, 1967, 1996. Kanh, David. Seizing the Enigma: The Race to Break the German U-Boat Codes 1939-1943. Boston, Houghton Mifflin Company, 1991. Knuth, Donald. The Art of Computer Programming, Volume 2: Seminumerical Algorithms. Berkeley, Adison-Wesley, 1998. Schneier, Bruce. Applied Cryptography. New York, John Wiley & Sons, Inc. 1996 Singh, Simon. The Code Book: The Evolution of Secrecy from Mary, Queen of Scots to Quantum Cryptography. New York, Doubleday, 1999.