Engineering Hitler's Downfall. Gwilym Roberts

signals were encoded using the highly sophisticated Enigma and Lorenz encrypting machines, amongst others, which produced coded messages that the Germans regarded as unbreakable.

      Thanks to incredibly brilliant decrypting work by the Bletchley Park cryptologists, coupled with the development of electro-mechanical computing machines, the Enigma (and later, even more sophisticated systems) was broken. As a result, the Allied High Command was informed of German plans and dispositions, often within a few hours of their radio transmission.

      Many women worked at Bletchley Park, some as cryptologists but several more providing various support services. In particular there were many hundreds of Wrens (WRNS – Women’s Royal Naval Service) who assisted in the operation of the decrypting machines. A few Wren officers also served as decrypting officers in the troopships converted from the fast peacetime passenger liners whose speed enabled them to cross the oceans unescorted. If Bletchley Park decrypts suggested U-boats might be approaching, signals were sent instructing them to change course.

      Down Memory Lane

      At the end of the war the decoding machines and many relevant papers were destroyed. (The machines now on display at the museum in Bletchley Park are recently constructed replicas.) Due to the loyalty of those who worked there during the war, all of whom were bound by the Official Secrets Act, the whole Bletchley Park operation remained unknown to the public until the 1970s and 1980s when there started to be leaks about its activities; following this some of those involved wrote their memoirs. Lately its operations have been described in a number of books and portrayed in films and television programmes. Bletchley Park’s present-day successor is the Government Communications Headquarters (GCHQ) at Cheltenham.

      The decoded information, called Ultra, had a profound effect on the outcome of the Battles of the Atlantic and Alamein, and the North Africa and north-west Europe campaigns. It has been claimed that the knowledge gained through Ultra shortened the war by two years. After the entry of the USA into the war American cryptanalysts worked alongside their British counterparts at Bletchley Park. ‘It was thanks to Ultra,’ Churchill is credited with saying, ‘that we won the war.’

      Enigma

      Originally invented by a German engineer around 1920, early Enigma encoding machines were used commercially but were later adopted by the German military who also upgraded them. The Poles were also working on them; by December 1932 they had broken the German cyphers. They also built replica machines and, five weeks before the outbreak of the Second World War, they gave replica equipment to the British GC&CS and their French equivalent together with details of their decryption techniques.

      Courtesy of Greg Goebel

      The Enigma machines worked on electro-mechanical principles and had five rotors, each with a variety of settings which were generally changed on a daily basis, meaning time was always of the essence – it was calculated that there could have been 159x1018 possible daily keys.

      Separate coding systems were used by each of the German services, adding to the difficulties of their decryption. As the numbers working at Bletchley Park increased, wooden huts were built in the grounds, with each hut’s occupants dealing with separate aspects of the decrypting operation. Hut 6 under Gordon Welchman was responsible for dealing with Army and Air Force codes, while Navy codes were the responsibility of Hut 8, which was initially controlled by Alan Turing and subsequently by Hugh Alexander.

      In the early days the successful cracking of a code relied to a certain extent on insights and inspired guesses by the cryptologists as to the likely behaviour of the German operatives of the machines. Some frequently used insights were named after the person who first proposed them e.g. the Herivel tip, or Herivelismus, after John Herivel and Parkerismus after Reg Parker.

      Outstanding work by the GC&CS cryptanalysts enabled some messages to be broken as early as 1940. Further clues as to the workings of the Enigma machines and of the naval codes used were obtained by the capture of German weather boats off Iceland in 1940–41; however, as code settings were changed each month they could be broken only for a limited period. Then, in May 1941, U-boat U-110 attacked a convoy though depth charges fired by the escorting vessels forced her to surface, whereupon her crew abandoned ship. Instead of ramming and sinking her, the captain of HMS Bulldog ordered one of his officers, Sub-Lieutenant David Balme RN, to be rowed over, board the submarine, and retrieve anything of interest. Among the prize items of equipment he took back to Bulldog were an Enigma machine and current code books, which were then rushed to Bletchley Park in great secrecy. (Balme was awarded the DSC.) Next, in October 1942, U-559 was sunk in the eastern Mediterranean. Before she went under, three crew members of the attacking ship, HMS Petard, were able to board her and recover the code books. Sadly, the submarine sank suddenly, taking two of the boarding party with her.

      Lorenz and Tunny

      In mid-1941 the Germans started using the more sophisticated Lorenz cypher machines; some 12 months later, the German High Command began to use the system for its high-level communications with its Army Commands. The Lorenz signal traffic was known as Tunny by the Bletchley Park cryptanalysts. As the result of an error by a German transmitter in late August 1941, by January 1942 John Tiltman and Bill Tutte had worked out the complete logical structure of the cypher machine, thereby enabling Tunny to be decoded. This remarkable piece of reverse engineering was later described as ‘one of the greatest intellectual feats of World War 2’.

      Tiltman, John Hessell Brigadier CMG CBE MC (1894–1982)

      A British Army officer, he moved to intelligence work after being wounded during the First World War, initially working with the Indian Army and then at the Government Code and Cypher School (GC&CS). His intelligence work was largely connected with cryptography, and he showed exceptional skill at cryptanalysis. He was considered one of Bletchley Park’s finest cryptanalysts on non-machine systems and worked with Bill Tutte on the cryptanalysis of the Lorenz cypher. In 1944, he was appointed deputy director of GC&CS and continued there until 1949 when he moved to the US Army Security Agency as a liaison officer. He eventually became a consultant and researcher at the US National Security Agency. In September 2004, he was inducted into the ‘NSA Hall of Honor’, the first non-US citizen to be recognised in that way. The NSA commented:‘His efforts at training and his attention to all the many facets that make up cryptology inspired the best in all who encountered him.’

      Tutte, William (Bill) Thomas OC FRS FRSC (1917–2002)

      The son of a gardener, he became a British, and later a Canadian, mathematicianand code-breaker. Having graduated with a degree in chemistry from Trinity College, Cambridge, he worked at Bletchley Park, initially on Italian Navy codes but from mid-1941 on Tunny, the signal traffic generated by the Lorenz coding machine. His brilliant and fundamental deciphering work led to the code being broken and, eventually, to the creation of the Colossus computing machine. Post-war he emigrated to Canada where he held senior positions at the universities of Toronto and Waterloo. He had a number of significant mathematical accomplishments, including foundational work in the fields of graph and matroid theories.

      Turing, Alan (1912–54)

      A mathematician and computer scientist, Turing was appointed a fellow of King’s College, Cambridge, in 1935 at the early age of 23. At the outbreak of war he was recruited to the Government’s code-breaking staff at Bletchley Park where he was instrumental in breaking the German naval codes. With Gordon Welchman he designed the Bombe decoding machine