The submarine in a 'vegetable patch': Code in letters sent home by British PoW in WWII to help Allies is revealed after 70 years
The letters sent back home by Sub-Lieutenant John Pryor from a German prison camp seemed innocent enough.
They often started with 'My Dear Mummy & Daddy' and talked about mundane things such as gardening and a 'vegetable patch'.
But the British wartime letters actually contained hidden messages, which have only now been deciphered by academics.
Those letters contained secret messages for the British military - and involved a fiendishly complex alphanumeric code.
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 - and never told his family about them.
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.