On this day in 1941, the British navy captured the German submarine U-110 as it was attacking a convoy in the North Atlantic. After being seriously damaged by depth charges, the submarine surfaced and the sailors on board surrendered. The captain, Fritz-Julius Lemp, believed the sub was sinking. Upon realizing that it was not, he swam back to destroy secret materials and was never seen again.
A British boarding party got onto U-110 and stripped it of everything portable. This, along with the Battle of Stalingrad, turned out to be a major turning point in the European theater of World War II. That's because one of the things captured was an Enigma coding machine. Britain kept the incident secret, not even informing Roosevelt until 1942.
The Enigma machine was employed extensively by all branches of the Nazi military, who believed it to be unbreakable. It consisted of a keyboard into which the message could be entered by one person, and a set of illuminated letters from which the coded text could be read off by another. In between was a set of three, later five, rotors and a plugboard that together determined which letter was illuminated for which letter pressed.
The rotors changed with every keypress, effectively changing the encryption letter by letter. But in order for the receiving station to decrypt it, they needed the same plugboard settings and the same initial rotor positions used by the sender. So there was a codebook of daily settings to be used that was shared across the German military.
As it happens, the three rotor Enigma code was broken by Polish mathematicians prior to the breakout of hostilities, and without ever having seen the machine, and had even created a primitive computer they called a Bomb to automate the decryption. This required 6 Bombs, one for each possible rotor order. When the Germans added two more rotors, 60 Bombs would have been required to do the decoding and the Poles did not have the resources to build so many.
The Poles shared their decryption techniques with British Intelligence. Part of what the Polish technique depended on was German laziness. German intelligence, for instance, used the same rotor setting every day.
After the invasion of Poland, the Polish team, after many perilous journeys and some time in a Spanish prison. made it to Britain here they collaborated closely with the British decryption effort at Bletchley Park. When the Germans changed their practices, the Polish method no longer worked.
The British took on a major effort at decryption led by Cambridge mathematician Alan Turing, who designed his own Bomb that was able to analyze all 17,000+ possible settings of the Enigma. The materials captured from the U-110 were critical in this effort. There were several different Enigmas used by different branches of the German armed forces, and used in different ways as well. The Naval code was the most difficult to crack. Though the codebook wouldn't be valid forever, it was valid for long enough to develop a mathematical method of deriving the settings from the cipher text itself, which is then what the British Bomb was designed to do.
Most historians believe that the ability to read German messages shortend the war by several years.
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