The Enigma cipher machine

Charles Cooper

Probs and Stats

16 April 2002

The Enigma Machine

After World War One it became apparent to the German military leaders that during the war the British were intercepting and deciphering their secret messages. This coupled with British reports of how these deciphered messages enabled them to “better fight the war.”[1] These facts made it abundantly clear that a need for a better ciphering system was crucial for the German military. With this they began to search for a system that would ensure the confidentiality of their communications. They found the answer in a machine that was presented to them originally in 1918.

The machine was called Enigma; it was invented by a German electrical engineer named Arthur Scherbius. The Enigma machine was an entirely different ciphering machine, it used an electrical system to change a plain text messages into code. The greatest selling point was even if the machine fell into enemy hands it would still be useless. The secret of the machine lay in its rotors. These rotors were actually like gears; every time a keystroke was made a rotor would turn 1/26 of a rotation. The machines that were used at the beginning of World War Two employed three rotors; once the first rotor rotated twenty-six times, the second rotor moved one position.

On the Enigma there was also a plug board. This enabled letters to be ciphered by attaching cables from one outlet to another letter. This increased the ways at which a message could be ciphered made the number of possible starting positions between two and three billion. All of the rotors on the Enigma were interchangeable which added more protection, because each rotor was wired differently. Therefore, unless one knew which rotor went where, and what position each rotor started in, the Enigma machine was useless. It would take 100 machines working around the clock 5.8 years to exhaust all of the possibilities.[2] The navy furthered the security of messages by supplying seven wheels instead of just three like the army; the first three were used when communicating with the army, the second two were held in reserve, and the last two were used when communicating with itself. At the time of its use it was the epitome of ciphering machines. Without an actual machine and the cipher key and the correct placement of each rotor would it be impossible to understand a ciphered message.

The Battle of the Atlantic began on the third of September 1939, when U-30 commanded by Julius Lemp, one of the most successful of Germany’s U-boat commanders, sank the unarmed British liner Athenia, which he mistook for a cruiser. This attack on an unarmed vessel caused the British to think that Germany had begun unrestricted submarine warfare as it had done in the First World War. This, in turn, caused them to mobilize all of their anti-submarine counter-measures. This however, was not the case at the beginning of the war; German U-boat captains were under direct orders to follow international rules when engaging merchant vessels.[3] These included warning and allowing for passengers to escape before firing upon any merchant vessel. Once Lemp realized his mistake it was already to late.

The German U-boats were extremely successful at the beginning of the war. “The submarines were sinking more ships than Britain and the United States were building.”[4] Britain was being choked to death. It was estimated that, to keep Great Britain’s war machine healthy and running smoothly, 31 million tons of goods were needed. By 1941 they were receiving less than 28 million tons.[5] The success of the U-boats was due largely to the fact that German military intelligence had broken the British code; they knew where the convoys would be. They were then able to secretly dispatch their submarines, using the Enigma machine to send coded information; submarines could be repositioned to ambush the vulnerable merchant vessels. At the outbreak of war the German U-boats operated alone. However, in early 1940, they adopted a new tactic. The U-boats traveled in groups called wolf packs, increasing their effectiveness because the U-boats were then able to overwhelm the destroyers that escorted the convoys.

The allied attempts to decipher German codes began long before World War Two; at about the same time that Hitler began his rise to power. Poland had become extremely proficient at reading the encrypted messages of both the Russians and the Germans. Since the recreation of their country in 1918, the Poles felt threatened by both the Germans and the Russians, one of the major ways in which they protected their country was by knowing exactly what either enemy was doing. The skill that the Polish intelligence had acquired; enabled then to save their capital and their country from conquest by the Russian Army in the early 1920’s. In 1926 intelligence experts in Poland began to notice a change in German naval cryptograms.[6] The entire format was completely different, from the number of times a letter appeared, to the lack of repeated numbers. Then, in 1928, Polish intelligence noticed a similar change in the cryptograms of the Germany army. The Poles began to suspect a machine ciphered the cryptograms, this notion was confirmed by either spies, or German radioman chatter. Once the Polish became aware of the fact that it was an Enigma machine that was doing the encryption they bought and tested a commercial model of the device. It was soon realized that the commercial version was different from the military version and one could not translate the other.

As the Poles realized this new technology they also began to understand the fact the cryptology was changing.[7] Cryptology was no longer language based - it had become almost entirely mathematical. Classical substitution ciphers are attacked using letter frequency and a general understanding of the language. Using statistics it is only a matter of time before a classical substitution cipher is solved. The enigma however is much more complicated because letter frequency has no bearing on the code. With this realization the Polish intelligence service brought in mathematicians from universities and educated them on the principles of encryption and decryption.[8]

While the Polish were attempting to use math to crack the Enigma, the French felt that the Enigma could only be beat with the aid of keys obtained from Germany. The French managed to buy from a German officer various information about the machine. However, what the French purchased did not give them enough information to decrypt German cryptograms, so they took their intelligence to the British and the Polish. The British were not able to make anything new of the information. The Polish, on the other hand, with their already extensive understanding of the Enigma machine, were able to learn to a great deal more. The intelligence that was bought from the German’s explained a great deal about the importance of the positioning of the wheels and the workings of the rotors. The documents also described the plug board, which was not present on the commercial version of the machine that the Polish had bought. Even with all of this knowledge about the machine, the Polish informed the French that without a key to the correct position of the rotors any decryption would be impossible. The French then went back to their contact and purchased the keys to the Enigma machine for several months in 1932.[9] These documents were sent to the Polish who used to them to work on recreating the wiring of the rotors. After three months of work Marian Rejewski, Poland’s leading mathematical cryptanalyst was able to recreate the wiring to rotor one, and soon after that the wiring to the rest of the rotors. In doing this he accomplished what the Germans thought was impossible. While Poland recreated a military version of the Enigma machine they were still unable to read the Germans messages on a regular basis.[10] That was the beauty of the Enigma machine.

With the threat of war upon the horizon, Poland called British and French officials to Warsaw. There they were given a tour of the Polish code breaking facilities and then, most importantly, they were shown the replica Enigma machine, and the machine that the Poles had created to help them determine the key for a certain message, called a bombe. They were then given Enigma replicas and documents on all of the Polish findings. The British and French graciously accepted these gifts and immediately set out to create a system for finding the key for each message. When Hitler invaded Poland the Polish code-breakers destroyed all of their documents and machines and escaped to France, there they began working in the French intelligence service.

While the progress that mathematicians were able to make on deciphering the Enigmas complex inner workings was substantial, it became clear that the best way to decipher the Germans radio traffic, especially Naval radio traffic was to obtain the necessary items from the Germans. It was especially important to obtain German naval ciphers because the German navy, by the start of the war, equipped their enigmas with five rotors instead of three making their messages much harder to decipher than those of the Luftwaffe or the Army. It was also crucial because Britain was being choked to death by the unrestricted German submarine warfare.

The British received another gift in February of 1940. The German U-boat U-33 was headed towards one of Britain’s most important shipbuilding ports.[11] There it was to lay mines in the harbor and then sneak back out into the Atlantic Ocean. The U-boat planned to use the cover of darkness to sneak into the British harbor. Unfortunately for the U-boat, the H.M.S. Gleaner, a minesweeper was patrolling the area for mines. U-33 was discovered and depth charges were deployed as U-33 sat on the bottom. When it became clear that underwater escape was impossible, the boat’s skipper, Lieutenant Dresky, ordered for the men to destroy classified materials and to prepare to abandon ship. Then he ordered an emergency surface. On the surface the boat might have been able to out run the minesweeper.[12] When the submarine broke the surface she was immediately seen by the Gleaner, which turned to ram the U-boat. Realizing that escape was impossible Dresky ordered all of the men to abandon ship, and for the boat to be scuttled.[13] When this order was given, three men were supposed to drop the rotors from the Enigma machine into the water. Two of the men accomplished this task with out any problems. The third, however, forgot to drop the three rotors he was given. The British found the rotors in his pants and immediately sent them to Bletchley Park, Britain’s massive intelligence compound located in England.[14] The rotors that were confiscated were the previously undetermined rotors six and seven. This, along with the recovery of rotor eight and the regulations for sending an enciphered message from U-13 still did not make deciphering possible on a regular basis. By mid-1940 British hopes of solving the Enigma were dwindling, and it began to search for other methods to read the German’s naval messages.

In a commando raid executed by the British on the occupied Norwegian shoreline, a small patrol vessel was hit by several salvos from the British destroyer Somali, which was escorting the commandos and their landing craft. After the commandos disembarked from the various ships, the Somali was patrolling the waters. The executive officer of the destroyer saw the opportunity to board the burning ship to rescue injured sailors and perhaps capture classified documents. Most of the objects that were recovered from the burning wreckage where things that the British Intelligence service already had, including two rotors to an Enigma machine, however, the cipher keys for the entire month of February in 1940, were also captured.[15] The British used these keys to read all of the messages from February and then with information from these solutions, the British were able to read all of the traffic for that April, and then May. While the British were reading and deciphering old messages, it still brought to light one important fact, the naval Enigma was solvable.

One of the most important captures of the war occurred in May of 1941. U-110 under the command of Julius Lemp, the same man who commanded U-30 at the out set of war, was on its second cruise of the war off the coast of Greenland. It spotted the British convoy OB 318. Firing several torpedoes at the convoy U-110 was caught by surprise by the escort ship, H.M.S. Aubrieta who immediately fired depth charges in the direction of the submerged submarine. The depth charges caused major damage to the U-boat, forcing her to surface and surrender. Some confusion on board the escorts led to the firing of small arms at those clamoring on to the deck of the doomed U-boat.[16] The submarine was not scuttled and a boarding party from H.M.S. Bulldog was launched. The party boarded the ship to find it deserted, with all of its classified material still intact. It became evident that the submarine was not going to sink. A towline was set up in order to try and capture the submarine. Mean while the entire U-boat was being searched and gutted. The boarding crew could not speak German so anything that was not clearly pleasure reading was taken off the submarine. An Enigma machine was retrieved from the U-boat, this was not the most important item found on board. The British already had several Enigma machines, but the codebooks were also found and proved to be invaluable. These codebooks allowed for “major insight” into German naval operations. This whole operation was completed in total secrecy; which was crucial so that the Germans did not change their encryption patterns. These documents enabled the British intelligence service to decipher a German naval message in around six hours.[17] This was a phenomenal feat for the British.

In February of 1942, Enigma under went another change. A fourth wheel was added to the machine. Officials in the German high command had noticed a decrease in the rate at which their U-boats were being able to find allied convoys. Several security measures were added to the Enigma machine in order to increase security including another wheel. The British received another break in the fall of 1942; U-559 was attacked and forced to surface in the Mediterranean Sea. The crew attempted to scuttle the U-boat, a maneuver they had not practiced. Because of this it took some time before the U-boat sank, a boarding crew from H.M.S. Petard managed to get aboard the sinking boat and retrieve many classified documents. These documents aided the British in decrypting messages from the new four-rotor cipher machines.[18]

The Americans also had a hand in the race to crack the German messages. They had intelligence services that were setup to try and decipher intercepted radio traffic. The United States Navy also captured a German U-boat. U-505 was patrolling Caribbean waters when it was spotted and attacked by an American anti-submarine airplane. The American attack rendered the U-boat un-submersible. The U-505 did manage to escape this attack and limp back to France where it underwent repairs. Plagued by different mechanical problems and necessary repairs, the boat was held near port for much of the rest of the war. However on June 4^th, 1944, she came under the attack of three American destroyers. The destroyers managed to severely damage the German submarine and forced her to surface. The submarine was then boarded and was captured with fairly minor damage.[19] There were several other U-boats captured throughout the war each one providing its own little piece to the puzzle. The heroic efforts of many allied sailors along with the tireless search by allied intelligence officials enabled the allies to better protect the invaluable cargo that had to traverse the treacherous Atlantic Ocean. The combined effort of many men and women enabled the allies to ultimately win the Battle of the Atlantic.

[1]David Kahn, Seizing the Enigma: The Race to Break the German U-boat Codes 1939-1943 (Boston, 1991), 39.

[17]Jak P.Mallmann Showell, Enigma: U-boats Breaking the Code (Annapolis, 1988), 74.

[19] ¹⁹Jak P.Mallmann Showell, Enigma: U-boats, 92.