Alan Turing papers on code breaking released by GCHQ
19 April 2012 Last updated at 18:50 BST
GCHQ has released two 70-year-old papers by Alan Turing on the theory of code breaking.
The papers, one entitled The Applications of Probability to Crypt, and the other entitled Paper on the Statistics of Repetitions, discuss mathematical approaches to the problem.
Turing played a key role in cracking the Enigma code which was used by the Nazi government and military to scramble messages during World War II.
The BBC's Chris Vallance reports.