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(Kennedy McConnell was an R.A.F. electrical engineer working on the Turing designed "Bombe" decoding machines. In 2003 he produced a professionally filmed lecture series, which has been copied on to videotape and DVD. The full series, comprising approximately seven hours of detailed historical analysis, can be viewed at Dundee Central Library. There are additional copies of the film at the Bletchley Park Trust, the Scottish National Museum, the Imperial War Museum and the National War Museums of America, Australia, and Canada. An abstract of Part 1 follows).

The portable encoding machine used by all German Forces during the 1939/1945 war was the ENIGMA. This ingenious electro-mechanical device was designed to transpose plain text messages into scrambled output before radio transmission. Because of its complex mechanism and variable circuitry, the Enigma could produce an astronomical number of permutations. The encoded text was made even more baffling by daily changes in settings. Consequently, the Germans remained convinced throughout the war that the Enigma codes could not be broken. Nevertheless, British cryptographers and engineers succeeded in doing so. The resultant intelligence, which was codenamed ULTRA, made a major contribution to the Allied victory.

The main components of the Enigma were the keyboard, lamp board, scrambler unit and plug board. The keyboard had the alphabet arranged in three rows, but there were no numbers or punctuation keys. The same three rows layout was repeated in the lamp board, but behind each letter was a 4.5 volts electric bulb. When the key was pressed then a letter was illuminated. The most important part of the machine was the scrambler unit, in which three interchangeable rotors were mounted on a common shaft. Each rotor was cross-wired internally. The assembly was linked to the keyboard by a ratchet mechanism, so that each time a key was pressed then the right hand rotor moved forward one position. After a predetermined number of letters, the second rotor was engaged and moved in the same way. This sequence was continued when the third rotor was activated. Five differently wired rotors were provided for each Army and Air Force Enigma, and eight for the Naval version. The order and setting of the three rotors were changed daily at 00.01 hours. Consequently, the Army and Air Force machines offered 60 daily options (any 3 from 5) while the Naval Enigma provided 336 options (any 3 from 8)

The plug board (or stecker board) consisted of 26 double sockets arranged in the same alphabetical sequence as the keyboard and lamp board. These sockets could be interconnected by means of twin flexible cables. For example, A to X, C to N, R to Y, etc. The Enigma code book, which was changed monthly, usually specified 10 pairs of letters to be interconnected in this manner. The remaining six letters were short circuited by special plugs. These were known as ‘self steckers’. When any letter key was pressed, current from the 4.5 volts battery flowed from the keyboard via the stekker board to enter the scrambler unit. After passing through the internal wiring of the three rotors, the return path was via the reflector (or umkerwalze) and back through the three rotors to exit from the scrambler. The current then returned via the stekker board and finally reached the lamp board to light one of the bulbs. For example: if key L was pressed, then bulb M would light up, but only after the current had passed through stekkers A/N, rotors 3/4/2 and stekkers V/R. However, if the same letter key was pressed repeatedly, a different bulb would light up each time. The combination of five interchangeable rotors and ten pairs of stekkers enabled the Enigma machine to produce approximately 150,000,000,000,000 encoding positions. However, one inherent weakness in the design was that no letter could be transposed as itself. For example, pressing letter key N could never light up bulb N.

During the first few months of the war, an elite team of cryptographers, mainly mathematicians and linguists, was recruited and stationed at Bletchley Park Manor in Buckinghamshire. The objective was to utilise their exceptional talents to attack the Enigma codes. They soon realised that their task was theoretically impossible. Consequently, they concentrated on exploiting inherent flaws in the Enigma design and operating procedures, such as careless errors made by operator.

The most important of these clues was called CRIBS, which was the term used to describe any words or phrases suspected to be part of an intercepted signal.
Routine daily reports and weather forecasts were fruitful sources. These were frequently transmitted at the same time every day. The cryptographers then tried to deduce the exact position of the crib within the complete message.

The next step was to check for letters which were repeated in both the crib and the encoded text. From these pairings and their positions, an interlinked diagram was constructed. These diagrams, called MENUS, became input programmes for unique machines called BOMBES. This name was originated by three brilliant Polish mathematicians who had achieved limited success in breaking the Enigma codes before Poland was invaded in September 1939. Their names were Marian Rejewski, Jerzy Rozycki and Henryk Zygalski. The reason for choosing this name for their code breaking apparatus is debatable, but the most feasible explanation is that it ticked like a bomb when in operation.

The first British bombe was commissioned at Bletchley Park in March 1940. The mathematical logic on which the machine operated was developed by one of the code breakers called Alan Turing, who was a famous British mathematician. Another cryptographer, Gordon Welchman, improved greatly on Turing’s original concept. A detailed technical description of the design and construction of the Turing bombe is outwith the scope of this article. However, in basic terms, this amazing machine was able to test the menus against every possible permutation of Enigma rotor selection and settings at high speed. The 17,576 permutations produced by one set of Enigma rotors could be searched in about 17 minutes.

The purpose of the bombe was to find circuits which would identify positions where the menu matched the rotor sequence and settings for that particular key. High speed sensing relays then stopped the bombe at the relevant settings. However, bombes were constantly being improved and later versions were fitted with an attachment which typed out the letters concerned without stopping the machine.

The bombes functioned for 24 hours per day and 7 days per week. They were operated on a rotating shift system by WRENS (Women’s Royal Naval Service) Each bombe was attended by two Wrens. Before each test run, the menu was ‘plugged up’ to programme the bombe. These complex machines were prone to develop minor electrical or mechanical faults, which could result in ‘false stops’ or ‘missed stops’. Consequently, teams of RAF and GPO technicians were available at all times to correct such faults and also provide preventive maintenance. When the earlier bombes produced a ‘stop’, the operators checked the relevant combination of rotors and settings. As explained above, later bombes typed out this information but continued to run. These ‘stops’ were relayed to the cryptographers who could then evaluate them as possible solutions. The stekker pairings had to be deduced. However, this deduction was made easier with the knowledge that there must be eight self stekkers. When the bombe had found a match which verified the menu, then all the messages transmitted on that Enigma key for the remainder of the same day could be decoded. The next stage was to translate the German text into English. This was done by expert linguists who were familiar with German military terminology. Specially trained Intelligence Officers then analysed the translated messages and graded them according to their importance. The distribution of Ultra to Chiefs of Staff in Whitehall was by teleprinter.

Ultra for overseas war zones had to be transmitted by radio. Strict security was essential because it would have been disastrous if the Germans had been able to decode any of these signals. The enciphering machine used for this radio traffic was called TYPEX. The Germans were never able to break the Typex codes. To minimise the security risk at the receiving end, Special Liaison Units were attached to the headquarters of all overseas commands. The SLU
personnel decoded the incoming signals and passed them to the relevant Staff Officer. After these decodes had been studied, they were returned to the SLU to be destroyed.
Ultra played a crucial role in the Battle of the Atlantic from 1940 to 1943. The British war effort relied heavily on imported food, oil, raw materials and weapons. Consequently, the North Atlantic supply routes had to be kept open. After the Royal Navy had eliminated the German battleships, such as the Bismarck, the main threat to the convoys of merchant ships came from German submarines, known as U-Boats. (Unterseeboots). The U-Boats hunted in groups called ‘Wolf Packs’, which usually operated in mid Atlantic where the convoys could not be protected by shore based aircraft. As soon as any U-boat spotted a convoy, then the others in the Wolf Pack were alerted by radio and converged on their prey. These tactics wreaked havoc with the convoys. During 1940/1941, several million tons of shipping were sunk and Britain’s survival was threatened. Merchant ships were being sunk faster than replacements could be built. The situation was critical.

As explained above, the German naval Enigma incorporated eight rotors, which made the codes much more difficult to decipher. However, the U-boat codes were eventually broken in May 1941. This success had a dramatic impact on the battle because the Admiralty was now able to route convoys away from the wolf pack. Also, Allied air and naval forces were able to find more U-boats and their supply ships. This transformation was so remarkable during the remainder of 1941 that Admiral Doenitz became convinced that the U-boat codes were being broken. Despite assurances from German Naval Intelligence that the codes were unbreakable, he arranged for all naval Enigma machines to have a fourth rotor fitted to increase the number of encoding positions by 26. This ingenious modification involved thinning down the reflector, or Umkehrwalze, in the scrambler unit in order to make room for the fourth rotor. This extra rotor was not linked to the keyboard and had to be moved manually. There were two versions with different internal wiring. These were known as beta and gamma. This version of the naval Enigma, known as M4, was introduced in February 1942.

The cryptographers at Bletchley Park were baffled by the four rotor output, which was codenamed TRITON by the Germans and SHARK by the British. The three rotor Bombes were now unable to operate effectively. The loss of Ultra intelligence meant that Allied convoys were again sailing blind and shipping losses escalated to critical levels. Britain was now struggling to survive and sustain the war effort.

Several factors helped the cryptographers in their desperate attempts to penetrate SHARK. One flaw in the use of the four rotor Enigma was that land based weather stations were equipped with only three rotor machines.. Consequently, daily weather reports from U-boats to had to be transmitted in three letter codes. This was done by locking the fourth rotor in a neutral position. Another important event which provided valuable clues was the capture of Enigma codebooks from U-559 in October 1942. Two seamen sacrificed their lives to rescue these vital documents. Their names were Lieutenant Anthony Fasson and Able Seaman Colin Grazier. Both were awarded the George Cross posthumously for their bravery. The Germans believed that the code books had sunk with the submarine and failed to change them. The SHARK code was finally broken in December 1942. This long awaited breakthrough was a turning point in the battle. Once again, the Admiralty could divert convoys away from the Wolf Packs. By this stage of the war, long range Liberator aircraft had closed the mid Atlantic gap and airborne radar equipment was much improved. Many more escort destroyers were available and small aircraft carriers, known as Convoy Escort Carriers, had also been introduced.

The result was that U-boat losses reached an unsustainable level and Admiral Doenitz was forced to concede defeat in May 1943. The Battle of the Atlantic was won by the courage and sacrifice of all who sailed in the convoys or hunted the deadly Wolf Packs. However, there is no doubt that Ultra intelligence was the secret weapon that helped to achieve this victory.

Kennedy McConnell via Dundee Central Library

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