Trench code

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Trench codes (a form of cryptography) were codes used for secrecy by field armies in World War I.^[1][2] Messages by field telephone, radio and carrier pigeons could be intercepted, hence the need for tactical World War I cryptography. Originally, the most commonly used codes were simple substitution codes, but due to the relative vulnerability of the classical cipher, trench codes came into existence. (Important messages generally used alternative encryption techniques for greater security.) The use of these codes required the distribution of codebooks to military personnel, which proved to be a security liability since these books could be stolen by enemy forces.^[3]

By the middle of World War I the conflict had settled down into a static battle of attrition, with the two sides sitting in huge lines of fixed earthwork fortifications. With armies generally immobile, distributing codebooks and protecting them was easier than it would have been for armies on the move. However, armies were still in danger of trench-raiding parties who would sneak into enemy lines and try to snatch codebooks. When this happened, an alarm could be raised and a code quickly changed. Trench codes were changed on a regular basis in an attempt to prevent code breakers from deciphering messages.^[1]