In the Second World War, he worked in the OKW/Chi, the cipher bureau of the High Command of the Wehrmacht. He worked in Chi IVc with duties that included scientific decoding of enemy crypts, the development of code breaking methods and working on re-cyphering systems not solved by practical decoding. He had a staff of 48. From March 1941 he lived in Berlin-Zehlendorf and was released from teaching duties in Frankfurt. Franz first successfully solved Mexican and Greek codes and then the M-138-A Strip Cipher of the US Department of Foreign Affairs (named by the Germans as Am-10). An electronic machine called a clock tower was used.
