Top secret documents devised by Alan Turing, which should have been destroyed under wartime rules, have been found during renovations of Bletchley Park where they were used in roof cavities to stop draughts.
The documents have been identified as 'Banbury sheets', papers punched with holes to allow manual comparison of enciphered texts. Turing devised the sheets as part of his efforts to crack Nazi naval communications. They were found jammed into holes in the ceiling of Hut 6.
Conditions in the room were known to be poor, leading code breakers there to stuff wastepaper into holes to keep out the cold.
The documents should have been destroyed under wartime rules to maintain secrecy.
Bletchley Park Trust chief executive Ian Standen told The Times the papers were found in September 2013 and are being thawed for public display next month.
"Discovering these pieces of code-breaking ephemera is incredibly exciting and provides yet more insight into how the code breakers actually worked," Standen said.
"The fact that these papers were used to block draughty holes in the primitive hut walls reminds us of the rudimentary conditions under which these extraordinary people were working."
Former Auxiliary Territorial Service member Betty Webb, 90, who worked at Bletchley Park on German police messages that would unveil the Holocaust, described the conditions as medieval.
The papers demonstrated the Banburismus process which Turing developed to infer information details about probable settings used by the Nazi Enigma machine. Holes were punched in two sheets of paper and overlaid. Those holes that aligned indicated that deduction of the settings was possible.
Those papers, along with scrawling of numbers and letters, were uncovered during the £8m (A$15.6m, $12.1m) renovations.
Bletchley Park curator Gillian Mason said the documents were pivotal.
"These are the actual documents used by code breakers, and in terms of the code breaking process they are pivotal,” Mason told The Times. "I can just see these people beavering away. There is lots of pencil and crayon activity." ®