Code-breaking feat re-enacted at Bletchley
6 February 2014 Last updated at 19:13 GMT
The National Museum of Computing in Bletchley Park, Milton Keynes, has staged a re-enactment of an attack by a Colossus computer on a German Lorenz cipher machine using a rebuilt Colossus computer.
The re-enactment comes 70 years after Colossus, Britain's first electronic computer, went into operation to help decrypt the messages of German High Command.
Andy Clark, a trustee of the museum, talked BBC News through the many complex processes involved in breaking the code, which included using the Tunny cipher machine, a British copy of the Lorenz machine which was built without ever seeing an actual Lorenz cipher.
Video journalist: Neil Bowdler