World War I cryptography
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Trench codes were used by field armies of most of the combatants (Americans, British, French, German) in World War I.
- Zimmermann telegram
- Arthur Zimmermann
- MI1 British Military (Army) Intelligence
- Room 40 Royal Navy (Britain)
- Alastair Denniston Room 40
- James Alfred Ewing Room 40, first head
- Nigel de Grey Room 40
- William R. Hall ‘Blinker’ Hall, Room 40, second head
- Dilly Knox Room 40
- Oliver Strachey MI1
- William Montgomery (cryptographer) Room 40
- Playfair cipher
- Ernst Fetterlein was in the Tzarist Russian Ministry of Foreign Affairs from 1896, and solved (among others) German, Austrian and British codes. He was eventually made chief cryptographer with the rank of Admiral. With the Russian Revolution in 1917 he fled to Britain, and was recruited to Room 40 in June 1918 to work on Austrian, Bolshevik, and Georgian codes.
- The Russians used an overly complicated version of the Vigenère Cipher. It was broken within three days by Austro-Hungarian cryptanalyst Hermann Pokorny.
- The Tableau de Concordance was the main French diplomatic cipher.
Germany and Austria 
Germany and Austria intercepted Russian radio traffic, although German success at the Battle of Tannenberg (1914) was due to interception of messages between the Russian commanders in the clear!
The ADFGX and ADFGVX field ciphers were a modified polybius system with single order double columnar transposition and frequent key change, with letters optimized for Morse. It was later broken by the famous French cryptanalyst Georges Painvin.
Herbert Yardley began as a code clerk in the State Department. After the outbreak of war he became the head of the cryptographic section of Military Intelligence Section (MI-8) and was with the American Expeditionary Force in World War I as a Signals Corps cryptologic officer in France. He later headed the Cipher Bureau, a new cryptanalysis group started in 1919, immediately after World War I, and funded jointly by the State Department and the US Army.
Some American cryptography in World War I was done at the Riverbank Laboratory where Elizebeth Friedman, William F. Friedman and Agnes Meyer Driscoll worked. The Riverbank Laboratory, Chicago was privately owned by Colonel George Fabyan.