Trench code In cryptography , trench codes were code s 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 codebook s . 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 . German Army . 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 . U . S . troops . The Americans were relative newcomers to cryptography when they entered the war , but they did have their star players . One was Parker Hitt , who before the war had been an Army Signal Corps instructor . He was one of the first to try to bring US Army cryptology into the 20th century , publishing an influential short work on the subject in 1915 . He was assigned to France in an administrative role , but his advice was eagerly sought by colleagues working in operational cryptology . Another Signal Corps officer who would make his mark on cryptology was Joseph Mauborgne , who in 1914 , as a first lieutenant , had been the first to publish a solution to the Playfair cipher . When the Americans began moving up to the front in numbers in early 1918 , they adopted trench codes and became very competent at their construction , with a Captain Howard R . Barnes eventually learning to produce them at a rate that surprised British colleagues . The Americans adopted a series of codes named after rivers , beginning with " Potomac " . They learned to print the codebooks on paper that burned easily and degraded quickly after a few weeks , when the codes would presumably be obsolete , while using a font that was easy to read under trench conditions . Communications discipline . However , American codemakers were often frustrated by the inability or refusal of combat units to use the codes — or worse , to use them properly . A soldier engaged in combat doesn ' t always feel the need to do things " by the book " even when there are very good reasons to do so , and generals on the front line felt that they had other things to worry about . One codemaker suggested that the best way to address the problem was to publicly hang a few offenders , but he lacked the authority to do so . The British and French were already familiar with such problems in " communications discipline " . They hadn ' t completely solved the problems either , but they had at least managed to get it through the heads of most of their signalmen that if they didn ' t have time to properly encrypt a message , they shouldn ' t bother trying ; send the message unencrypted , or " in the clear " . A partially or badly encrypted message could undermine a cipher or code system , sometimes completely , which made an unencrypted message far preferable .