They were the coded letters sent home from the war which Sub-lieutenant John Pryor hoped would give Britain vital military intelligence.
The British PoW was being held in a German camp but was allowed to write to his family due to 'good behavior' in the camp.
Sub Lt Pryor used the privilege to hide secret messages in his letters home, including disguising news of sunken British submarines by referring to 'the vegetable patch'.
Now Pryor's coded letters - which he hoped would reach commanders in London - have been cracked 70 years after he wrote them.
Sub Lt wrote in one letter: He wrote: 'Many seeds are left, being saved from several plants which did very well some time ago.
'Our last year's harvest was extremely good. Well worth repeating again for this year.'
But it meant: 'HMS Undine attack failure. Trawler depth-charged, scuttled in 70 feet, three burnt.'
The secret codes in John's letters also described information on the camp and requests for maps and a passport to help him escape.
John, who was captured as he tried to evacuate soldiers from Dunkirk, never told his family about the codes until he was an old man - and couldn't remember what they meant.
The secret codes were deciphered after his son Stephen Pryor, a chancellor at Plymouth University, showed the letters to a PhD student studying POW escape plans.
They were then deciphered by a team of mathematicians, historians and geography experts - 70 years after they were written.
Stephen, of Saltash, Cornwall, said: 'I had known for 30 years that my father had these letters but their contents lay hidden.
'His letters from the camps were always addressed to my grandfather but would have already passed through German censors.
'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.'
The codes were devised by MI9 - the British Directorate of Military Intelligence Section 9, a department of the War Office between 1939 and 1945.
During World War II it was tasked with aiding resistance fighters in enemy occupied territory and recovering Allied troops who found themselves behind enemy lines.
It also tried to communicate with British prisoners of war and sent them advice and equipment.
Experts say an escape committee inside the PoW camps decided what needed to be included in the messages.
John's codes were deciphered by a team including David McMullan, a mathematics professor who used information about MI9 codes.
Professor McMullan found certain signals which indicated whether a letter contained messages - with the coded words alternated every fourth and fifth word.
In John's letters the words 'but' or 'the' would lead to an intricate alphabetical sequence which contained hidden information.
In one letter Mr Pyror ingeniously disguised a message about a scuttled British sub in a passage about the prisoners' vegetable patch.
Researchers found the signal for the secret message was the phrase 'many seeds are left' - which they deciphered as 'scuttled.'
From there they were able to deduce a coded message about the sinking of a British submarine, the HMS Undine, which was destroyed in 1940.
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.'
Mr Pryor wrote his memoirs in 1980 and attempted to include extracts of the letters - but by then could not remember how the codes worked. He died in 2011.