Tomorrow is the 65th anniversary of D-Day, the debarkation of Allied troops in Normandy. While the Soviets had already done the brunt of the work against the Nazis on Eastern Front, the 1944 landing marked a decisive turn in the war. Another decisive historical turn took place earlier that same week: Colossus Mark II, the world’s first programmable, digital, electronic computer, became operational. Not only was it a milestone in computing, it was one of the main contributors to the Normandy Landing’s success. The prototype, Colossus Mark I, was used by Britain’s Ultra code-cracking team at Bletchley Park to decrypt Nazi messages sent by the Lorenz SZ40/42 (“Tunny”) cipher machine, beginning in February 1944.

While Enigma is the best-known Nazi encryption machine, it was generally used for field operations; the Tunny was used for higher-level communications, so Colossus enabled the Allies access to Hitler’s inner channels.

The Colossus computer greatly reduced the time it took to decode messages: what once took weeks now took days. The improved Colossus Mark II, five times faster than the Mark I, was introduced on June 1, 1944, and was vital to D-Day’s success, as it decoded messages showing that Hitler had bought the Allies’ various strategic bluffs which deflected German defenses from the world’s largest amphibious landing.

The computer, created by engineer Tommy Flowers, read teleprinter characters from a paper tape transcript of intercepted messages. Vacuum tubes were used to perform the Boolean calculations to decrypt the messages, which were outputted to an electric typewriter.

Colossus was originally designed by Flowers before the war, while he was employed by the British Telephone Establishment, to automate telephone-call switching. One of the chief consultants for the project was Alan Turing, whose contributions to British wartime code breaking is overshadowed by Turing Machines.

By Mark Alvarez