Trench code

Trench Codes (a form of cryptography) were codes used for secrecy by field armies in World War I. 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 mathematical encryption for extra security, though. 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.

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. Vast numbers of men were sacrificed in futile offensives to break these lines, with the usual result being little more than a dent of a few kilometers at best. 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.

The French began to develop trench codes in early 1916. They started out as telephone codes, implemented at the request of a general whose forces had suffered devastating artillery bombardments due to indiscretions in telephone conversations between his men. The original telephone code featured a small set of two-letter codewords that were spelled out in voice communications. This grew into a three-letter code scheme, which was then adopted for wireless, with early one-part code implementations evolving into more secure two-part code implementations. The British began to adopt trench codes as well.

The Germans started using trench codes in the spring of 1917, evolving into a book of 4,000 codewords that were changed twice a month, with different codebooks used on different sectors of the front. The French codebreakers were extremely competent at cracking ciphers but were somewhat inexperienced at cracking codes, which require a slightly different mindset. It took them time to get to the point where they were able to crack the German codes in a timely fashion.

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