Enigma, although probably the most famous, was by no means the only cypher to be broken at Bletchley Park. Lorenz was a highly sophisticated cypher used personally by Hitler and his High Command. As D-Day approached, breaking 'Tunny', as Lorenz was known, was to become increasingly important. Colonel John Tiltman and Bill Tutte managed to work out how the Lorenz machine operated, despite the fact that no one at Bletchley Park had ever seen one.

But many of the Tunny messages still took several weeks to decypher; far too long for the intelligence to be of use. Mathematician Max Newman became convinced that the answer lay in building a computing machine such as that described in a pre-war thesis by Alan Turing.

Three men got together at Bletchley Hall and built a machine, using valves, that could automate the code cracking. It could try lots of different keys (the same way a person would) but it could do it a lot faster. They called this machine Colossus, and it was finished in December 1943.

Tommy Flowers - many years later

The key to Colossus' success was the ability to store possible keys in its vacuum tube memory. Intercepted, encrypted messages were punched onto tape. The tape was sped through the machine at 30 miles per hour (5000 characters a second) while a possible key was tried with each cycle. Colossus did not actually decode ciphered messages; it found the correct wheel settings for another machine, called a Tunny. The Tunny was basically a reverse-engineered Lorenz, which deciphered the text upon receiving correct wheel settings.