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According to David Khan’s The Codebreakers, thousands of British soldiers were injured in a battle of Ovillers-la-Boiselle in 1916, due to a commander’s battlefield telephone call being intercepted by the enemy.
Following that massacre, General August Dubail of the French Army asked the cryptographers of the War to come up with some kind of encryption mechanism for telephone conversations so that the constant interceptions wouldn’t spell disaster for troops.
What the office of cryptography produced was a "carnet de chiffre," or notebook of ciphers. Any word that might give away troop positions or plans was in the notebook, along with a corresponding code. Whenever a telephone conversation would take place, the caller would be required to spell out these special words in code.
The Germans were about a year behind in developing these "trench codes," but they eventually caught up. They began in March of 1917 with a small code where certain bigrams, or combinations of two letters, replaced key words or letters. These codes evolved into more notebooks, some of which were changed as often as every fifteen days towards the end of the war. The French referred to the codes as "KRUSA," because those were the 5 letters they invariably started with.
However, a crucial difference between the French and German codes was something called Superencipherment, which is encoding something already encoded. The lack of such gave the allies an edge, leading to the breaking of the German trench code.