Coded letters sent from a prisoner of war back home to his parents in Saltash have been deciphered by experts at Plymouth University.
John Pryor, a sub-lieutenant who was captured while trying to evacuate soldiers from Dunkirk during the Second World War, used MI9 codes to send vital military intelligence to his commanders in London.
His son Stephen Pryor said: "I had known for 30 years that my father had these letters, but he could not remember the full code and so their contents lay hidden.
"His letters from the camps were always addressed to my grandfather but would have already passed through German and British censors, the latter picking up any coded requests."
Mathematicians, historians and geographers worked together to crack the codes used by MI9 to conceal information going in and out of prisoner of war camps across Europe.
The letters reveal how the Allies were trying to engineer escapes from the camps and show how the prisoners of war were passing on intelligence.
Dr Harry Bennett, associate professor of History at the university, said: "Coded messages played a huge part in the war effort on both sides, as they were undoubtedly the best way to get messages or instructions through.
"The MI9 code was especially important, as their chief mission was to source equipment and supplies for prisoners of war who would then attempt to orchestrate an escape.
"But from these letters we now know they were also passing on information about key German sites, such as munitions dumps. The letters go to emphasise just how invaluable the code writers were to the Allied war effort."
The experts discovered certain signals which indicated whether or not a letter contained messages, and then within them the coded words alternated every fourth and fifth word.
However, if those words happened to be "but" or "the" it triggered an intricate alphabetical sequence and, in John Pryor's letters, he used this code to hide requests for items such as maps and passports.
Stephen Pryor said the letters help to give an insight into his father's experience.
"My father was among the tens of thousands of young men who as PoWs lost the best years of their youth and could never hope to regain them.
"But I can now see that despite their plight, he and his peers took incredible risks and it has only made me admire their resilience and ingenuity even more."
