The Enigma was a highly sophisticated mechanical encryption device that had a keyboard and looked superficially like a typewriter. The German engineer Arthur Scherbius developed it in 1923 from a design by Dutchman HA Koch; the German Army and Navy saw its potential and bought it in 1929, and later the Luftwaffe became enthusiastic users. The Germans firmly believed that it was completely secure. It seemed ideal for units operating in North Africa and Soviet Russia where communications relied on radio. In its simplest form, for every letter it sent there were hundreds of millions of possible solutions. However, the Germans forgot how few letters there are in the alphabet; that no letter could stand for itself; and that the machine had no number keys so that figures had to be spelled out. The Poles began deciphering signals in 1932, the French in 1938 and the British in February 1940. For the British the secrecy of the project was at such a high level that they classified it as 'Ultra Secret', and so it became ULTRA. When Germany invaded the Soviet Union it was decided that ULTRA intelligence would be passed to the USSR via a spy ring run by the GRU (Soviet Military Intelligence) based in Switzerland with the code-name 'Lucy'. In this way it was hoped that Moscow would not realize what an invaluable intelligence resource was available to the West. The Enigma Machine For the British Chiefs of Staff in London, following the fighting at Stalingrad and in the Caucasus, there was particular concern because of the threat that a German victory there would pose for Iran and Iraq and Western oil supplies. The intelligence came partly from Luftwaffe Enigma decrypts and partly from what the British military mission in Moscow could glean. Stalin assured Churchill that the Soviet Caucasus front would hold until the onset of winter. The Chiefs of Staff knew from a decrypt of a Japanese message that the Germans were greatly exaggerating the casualties that they reported they were inflicting on the Soviets. On 7 September 1942 British intelligence read an Enigma message that confirmed that the main German effort would now be focused on Stalingrad. It became clearer with every day that passed that the outcome of the battle on the whole Soviet southern front hinged on the outcome of the fighting at Stalingrad. By 14 September the Chiefs of Staff expected the city to fall at any moment, but a week later they were confident that the Soviets would hold. By the end of the month there was real optimism throughout Whitehall with the realization that the Germans now faced a second winter in Russia on a greatly extended line against a Soviet Army that would still be capable of tying down large enemy forces.