The Glasgow University knit forums is a wealth of knowledge and articles about the history of knitting and craft in general. This article from the Atlas Obscura includes accounts of remarkable women throughout modern wars who knitted messages into their knitting both on the home front and on the front lines.
Knitting became public property with War Office knitting parties to knit for the boys on the front lines. Unauthorised knitting patterns were banned due to rationing with the Make Do and Mend campaigns gaining momentum in the UK on the home front. Yarn companies such as Sirdar produced authorised patterns such as the one below to send out to loved ones overseas.
However, Knitting was taken to the front lines in many modern wars and used by female operatives to smuggle their messages past checkpoints and enemy guards to send their messages on to the authorities. from the American Revolutionary war to the Soviet Union, the old lady stereotype has been turned on its head by remarkable women using established codes. A far more secure method than pen and paper.
So next time you pick up a pattern, just think of the endless possibilities you have in your hand to create a secret message to your loved ones or other knitters in general, you will be carrying on a long line of espionage before you.
To all of those knitters who have gone before us. Ladies, We salute you!