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trench code
In cryptography, trench codes were codes used for secrecy by field armies in World War I. A reasonably-designed code is generally more difficult to crack than a classical cipher, but of course suffers from the difficulty of preparing, distributing, and protecting codebooks.
However, 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 earthworks 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. To be sure, trench-raiding parties could sneak into enemy lines and try to snatch codebooks, but then an alarm could be raised and a code quickly changed. They were changed on a regular basis anyway.
==French Army==
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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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