The History of the Lorenz Cipher and the Colossus Machine

The first message encrypted using the Lorenz cipher was intercepted in early 1940 by a group of policemen in the UK. John Tiltman, a codebreaker at Bletchley Park, recognized the early messages as being enciphered in the Vernam manner, meaning that they depended on the addition of a sequence of obscuring numbers to the sequence of the text, and were given the code name "Tunny." Due to the characteristics of Vernam ciphers, Tiltman reasoned that if they intercepted two messages that started at the same point in the obscuring sequence, they could add the characters together to come out with the original texts. They called these "depths."

On August 30, 1941, code breakers at Bletchley Park received the longest "depth" they had ever intercepted, giving them an incredibly long stretch of the obscuring characters used in the Lorenz cipher. From here, Bill Tutte, along with others, spent months working out how the numbers were generated, and in 1942, a machine was built, called the Tunny, that could successfully decode messages if they knew the initial wheel positions and the wheel patterns.

While the Tunny proved that they could break the Lorenz cipher, it was taking four to six weeks to decipher a single message, rendering it practically useless. Max Newman, a mathematician, resolved that he could build an electronic machine that could perform the same basic function of the Tunny, but in a much faster manner. His new machine, called the Heath Robinson, worked well enough to show that his concept could be put to use, but still had a few problems. To fix these, he went to Tommy Flowers, a Post Office electronics engineer, who designed and built the Colossus. He began work on the new project in March 1943, finishing it in December. It was first used at Bletchley Park in January 1944, successfully deciphering German messages encoded with the Lorenz cipher in a fraction of the time it had previously taken. In June 1944, the Colossus Mark II machine was built that was five times faster than the original Colossus and was easier to program, but operated in the same basic manner as the original. This new and improved machine was able to read 5,000 characters per second and carry out 100 calculations at a time.

With the implementation of the Colossus in 1944, a huge number of German messages could be decoded in a timely fashion. By the end of the war, there were ten Colossus Mark II computers in operation at Bletchley Park. The information received from intercepted messages and decoded by the Colossus proved to be crucial in determining the outcome of the war. Arguably the most important intelligence obtained from these messages was knowledge of where Hitler's troops were located during the D-Day attacks, allowing the Allies to carefully decide which beaches to attack.

Today, the Colossus is credited with being the first digital computer, however, for many years, it was forgotten about. Directly following the end of World War II, the British destroyed eight out of the ten Colossus machines at Bletchley Park, due to paranoia of the Russians gaining secret information about it during the Cold War. The remaining two, which were kept by British Intelligence, were subsequently destroyed in the 1960s, and information about the Colossus didn't reach the public until the 1970s.

Bletchley Park