Colossus Rebuild
Colossus Rebuild
The Colossus Gallery display allows easy access for viewing.

Photo: Colossus © LSA August 2015.
| EXIT | Colossus Rebuild | Early Days | The Mark 1 | The Equipment | Milestones | Terms |
| Photo Gallery |


More than sixty-five years ago, the covert activities at Bletchley Park were said to have shortened the war by several years as well as forming the foundations of modern communications.

Early Days
Early Days
Early days in H Block.

The new tape drive/ pulley system was in the process of being re-engineered, but had not yet been fitted.

Photo taken through the viewing window: The 'bedstead' for Colossus (Rebuild) is assembled © LSA Oct 1994. In the background, Tony Sale discusses the progress with a colleague.
Colossus Mark 1  
Colossus Mk 1
Colossus (as seen in October 1999) is nearing completion.

"The tape-pulley system is complete and the paper tape runs up to speed and is easily scanned by the optical reader system. Two out of the five code elements are working, only another 3 circuit elements to duplicate!"

Photo: Colossus (Rebuild) © LSA Oct 1999.
Colossus 2012  
Colossus 2012
Colossus Mark 2 in the newly refurbished gallery.


Photo: Colossus (Rebuild) © LSA June 2012.
The Equipment  
The Equipment
Standard GPO kit was used in the rebuild, but some components are updated versions of those which would have been available during the war.

Photo: Motor uniselectors on Colossus © LSA August 2015.
Photo Gallery  
Photo Gallery
Click on the Photo Gallery for a series of images.


Photo: The Colossus Story © LSA August 2015.

  • On 18th July 1994 HRH The Duke of Kent formally opened the Bletchley Park Museums and inaugurated the Colossus (Mark 1) Rebuild Project.
  • On 6th June 1996 (the anniversary of D Day) HRH The Duke of Kent came back and formally switched on Colossus in the presence of its designer Tommy Flowers.
  • On 20th May 2004, Tony Sale videoed the further developed Colossus Mk 2 setting all five K wheels on the BREAM cipher text.
  • On 1st June 2004, the 60th anniversary (1944-2004) of a Colossus Mk 2 in Bletchley Park, a video of the rebuilt Mk 2 was shown to 120 people at a commemorative event in the Science Museum in London.
  • On 17th December 2004, after careful consideration of all the evidence, including advice from English Heritage, the Secretary of State decided to List H Block as a Grade II building. It was added to the list having been judged to be a building of special architectural or historic interest.
  • 2005 the Mk2 rebuild continued, while the ultimate fate of Bletchley Park as a whole site was still uncertain.
  • In March 2012 the new gallery allowed visitors to walk all the way around Colossus to more easily view its workings.
Cryptography Terms

The Lexicon of Cryptography ('Most Secret', Bletchley Park 1943) from ENIGMA, a novel set against the historical background of events at Bletchley Park, by Robert Harris.

To assist in cracking the German intercepts, a whole vocabulary of terms evolved and were recorded in a 'Lexicon of Cryptography'.

A piece of evidence (usually a captured code book or length of plaintext) which provides clues for the breaking of a cryptogram. Codebreaker, Tilly Knox is said to have stated "..without question, the crib... is the single most essential tool of any cryptanalyst."

A message written in cipher or in some other secret form which requires a key for its meaning to be discovered.

The coincidence of two different cryptograms, each transmitted in a different cipher, yet containing the same original plaintext, the solution of the one thereby leading to the solution of the other.

To steal enemy cryptographic material. Any object stolen from the enemy that enhances the chances of breaking his codes or ciphers.

The original, intelligible text, as it was before encipherment, revealed after successful decoding or cryptanalysis.

To remove one layer of encipherment from a cryptogram which has been subjected to the process of super-encipherment. In essence, a message which has been coded once and then re-encoded to give twice the security.

The sounds made by an enemy wireless transmitter immediately before it begins to broadcast a coded message.

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