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Two 70-year-old papers by Alan Turing on the theory of code breaking have been released by the government's communications headquarters, GCHQ.

It is believed Turing wrote the papers while at Bletchley Park working on breaking German Enigma codes.

A GCHQ mathematician said the fact that the contents had been restricted "shows what a tremendous importance it has in the foundations of our subject".

It comes amid celebrations to mark the centenary of Turing's birth.

The two papers are now available to view at the National Archives at Kew, west London.

GCHQ was able to approximately date the papers because in one example Turing had made reference to Hitler's age.

The papers, one entitled The Applications of Probability to Crypt, and the other entitled Paper on the Statistics of Repetitions, discuss mathematical approaches to code breaking.

The principal challenge facing Turing, and those who worked at Bletchley Park, was cracking a secret code used by the Nazi government and military to scramble messages.

Establishing the settings the Germans' Enigma machines had used proved vital to the codebreaking effort, ultimately providing the Allies with a significant advantage, particularly against German submarine forces.

According to the GCHQ mathematician, who identified himself only as Richard, the papers detailed using "mathematical analysis to try and determine which are the more likely settings so that they can be tried as quickly as possible."

Bletchley Park went on to use bombes - large electro-mechanical machines worked on by Turing - to help identify the correct settings.

Richard said that GCHQ had now "squeezed the juice" out of the two papers and was "happy for them to be released into the public domain".

He added that the work of Bletchley Park was held in high regard by GCHQ. "I think we are very proud of the history of our organisation and like to think that we are their successors," he said.

Bletchley Park, which now celebrates the work of the war-time code-breakers, is planning a number of activities to mark the centenary.

One is to build a secure speech system, developed by Turing, called Delilah.

The system which encoded and decoded voice communications, was intended to be used in a similar way to a telephone scrambler.

A recreation of the system is being built by a team led by volunteer John Harper.

"Alan Turing just had brilliant ideas way ahead of their time which were terribly important to the future of the world if you like," Mr Harper said.