With the rise of easily-intercepted wireless telegraphy, codes and ciphers were used extensively in World War I. The decoding by British Naval intelligence of the Zimmermann telegram helped bring the United States into the war.

Trench codes were codes used for secrecy by field armies in World War I. 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. 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.