Lorenz & Colossus
A far greater challenge to the codebreakers at Bletchley Park was the Lorenz cipher machine. Made by the Lorenz company, it was used exclusively for the most important messages passed between the German Army Field marshals and their Central High Command in Berlin. Its size meant that it was not a portable device like Enigma. Bletchley Park code breakers called the machine ‘Tunny' and the coded messages ‘Fish'.

Lorenz used the ‘International Teleprinter Code', in which each letter of the alphabet is represented by a series of five electrical impulses. Messages were enciphered by adding, character by character, a series of apparently randomly generated letters to the original text. Crucially, to decrypt the enciphered message, the receiving Lorenz simply added exactly the same obsuring letters back to the ciphertext. The obsuring letters were generated by Lorenz's 12 rotors, five of which followed a regular pattern, while another five followed a pattern dictated by two pin wheels. Cracking Fish again relied on determining the starting position of the Lorenz machine's rotors.

The great Cryptanalyst, John Tiltman broke the first Fish messages at Bletchley in 1941 using hand-methods that relied on statistical analysis, but by 1944 the Germans had introduced complications which made it virtually impossible to break Tunny by hand alone. Dr Max Newman and his team in the ‘Newmanry' were assigned the task of building machines to break Tunny.

Max Newman called in the help of Tommy Flowers, a brilliant Post Office Electronics Engineer. Flowers went on to design and build ‘Colossus'. The first Colossus machine arrived at Bletchley in December 1943. This was the world's first practical electronic digital information processing machine ˜ a forerunner of today's computers.

Lorenz had to be cracked by carrying out compex statistical analyses on the intercepted messages. Colossus could read paper tape at 5,000 characters per second and the paper tape in its wheels travelled at 30 miles per hour. This meant that the huge amount of mathematical work that needed to be done could be carried out in hours, rather than weeks.

Mark I Colossus was upgraded to a Mark II in June 1944, and was working in time for Eisenhower and Montgomery to be sure that Hitler had swallowed the deception campaigns prior to D-Day on June 6th 1944. There were eventually 10 working Colossus machines at Bletchley Park. These were all eventually dismantled, but starting in the 1994, one wasrebuilt by a team led by Tony Sale. The functioning rebuilt Colossus can nowbe seen at The National Museum of Computing at Bletchley park in Block Hwhere the original Colossi operated.