The BBC reports that two papers by Alan Turing, believed to have been written while he was working at Bletchley Park, have been released by GCHQ. The papers, ‘The Applications of Probability to Crypt’, and ‘Paper on the Statistics of Repetitions’, apparently use mathematical analysis to try and determine which are the more likely Enigma settings so that they can be tried as quickly as possible.
The article quotes “a GCHQ mathematician” saying that GCHQ had now “squeezed the juice” out of the two papers and was “happy for them to be released into the public domain”, but that the fact that the contents had been restricted “shows what a tremendous importance it has in the foundations of our subject”. The two papers are now available to view at the National Archives at Kew, west London.
Source: Alan Turing papers on code breaking released by GCHQ.