By Richard Vaughan

It was built to crack Nazi codes during the Second World War and for 30 years its existence was hidden from the public, but today veterans of Colossus, the world’s first electronic digital computer that was at all programmable, gathered to celebrate its 70th anniversary.

Designed by British telephone engineer Tommy Flowers, Colossus was built at Bletchley Park, Buckinghamshire, to speed up the code-breaking of the complex Lorenz cipher used by Hitler and his generals of the Wehrmacht.

Key aspects of the machine's design were based on the pioneering work of the celebrated British mathematician Alan Turing, particularly on his formalisation of the concepts of “algorithms” and “computation”, which now form the basis of all computer science.

By the end of the war there were 10 Colossi being used. They are believed to have dramatically shortened the length of the war and saved countless lives.

Yesterday, veterans of the Colossus project, as well as their families, gathered at Bletchley Park to see a re-enactment of the code-breaking process, from intercepting to decrypting the code using a working Colossus replica.

Colossus Mark I was first put to use on 5 February 1944, and it would typically take four hours to decrypt the Lorenz code being used that day, enabling the Allies to read Nazi messages and make important military decisions.

According to the National Museum of Computing, it is even thought that the Allies were reading some of the decrypted messages before they reached the intended recipients in Germany.

The Colossus occupied the size of an average living room, weighed five tonnes and used 8kW of power, but could read 5,000 characters per second – faster than anything that had been achieved before – allowing it to decrypt the 63 million characters of the Lorenz machine.

Tim Reynolds, chair of the National Museum of Computing, said: "The achievements of those who worked at Bletchley Park are humbling.

“Bill Tutte's ingenuity in deducing out how the Lorenz machine worked without ever having seen it, the skill of those in the Testery who broke the cipher by hand, and Tommy Flowers' design of the world's first electronic computer, Colossus, to speed up the code-breaking process are feats almost beyond comprehension.

“The fifth of February will be a proud day for the museum to host the Colossus and Tunny veterans who are able to make the journey. This day is in honour of all the men and women who worked on breaking the Lorenz cipher.”

Questions for debate and discussion

1. Explain how the Colossus was used during the Second World War and how it helped.
2. In your opinion, is spying ever ethical? Explain your reasoning.
3. Do you know of any codes? What are they and who uses them?
4. Why, in the age of wifi and high definition, do we celebrate the anniversary of comparatively ‘primitive’ technology?

Relevant resources

Breaking the Enigma code

  • Teach your students about how code-breaking has influenced the outcomes of historical events.

Prime factors cryptography

  • Students must use their knowledge of prime factorisation to crack the code and find the bad maths joke!

Breaking the genetic code

  • Code isn’t just for computers! Get your students thinking about genetic code with this fun packed lesson.

Alan Turing: Father of modern computer science

  • Students will learn all about Alan Turing as well as developing their research and media skills with this cross-curricular lesson.