In 1926, the German Navy adopted, as its top cryptographic device, a modified civilian Enigma machine; in 1928 the German Army followed suit. The complexity of the system was much increased in 1930 by the introduction of a plugboard ("Steckerbrett"), albeit with only six connecting leads in use. In December 1932, Marian Rejewski made what historian David Kahn describes as one of the greatest advances in cryptologic history, by applying pure mathematicsthe theory of permutations and groupsto breaking the German armed forces' Enigma machine ciphers. Rejewski had worked out the precise interconnections of the Enigma rotors and reflector, after the Bureau had received, from French Military Intelligence Captain Gustave Bertrand, two German documents and two pages of Enigma daily keys (for September and October of that year). These had been obtained by a French military intelligence agent, a German codenamed "Rex", from an agent who worked at Germany's Cipher Office in Berlin, Hans-Thilo Schmidt, whom the French codenamed "Asché".