Secret code in PoW letters cracked 70 years on But now, 30 years after discovering their existence, Mr Pryor's family have read the secret messages hidden in his letters home. Dr Harry Bennett, associate professor of history at Plymouth 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 writes were to the Allied war effort. The research project was started by Barbara Bond, a pro-chancellor at Plymouth University. In a chance conversation with Stephen Pryor, governor of Plymouth University, she discovered his father John had written a number of coded letters from German PoW camps. The pair tried but failed to decipher the codes and turned to David McMullan, a mathematics professor who used the little information he had about MI9 codes to painstakingly go through John Pryor's messages. He finally managed to decipher the hidden contents of the letters. Professor McMullan found certain signals which indicated whether a letter contained messages. Within them the coded words alternated every fourth and fifth word. If the words were "but" or "the," an intricate alphabetical sequence was triggered and in John Pryor's letters, he used this code to hide requests for items such as maps and passports. 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. My father was among 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. An escape committee inside the PoW camps decided what needed to be included in the messages. Information, or requests for equipment needed to engineer an escape, would then be passed on to the camp's designated code writer, who knew how to hide the messages in his letters. British officials warned Mr Pryor's parents that some of his letters home would contain the secret messages but they were not given the code needed to unlock them. They were also not informed which letters would contain messages. Mr Pryor died in 2011 and his son, Stephen, continued the family's efforts to uncover the secret messages.