The German inventor Konrad Zuse wrote a proposal for a cipher machine during the winter of 1939–1940. The document was prepared at the Eastern front during WWII and reached the German military authorities, possibly with the help of Kurt Pannke, a manufacturer of calculating equipment. Zuse's scheme was recently found in his Nachlass. It is documented here for the first time. Zuse's offer was rejected by the military, so the software-based cipher machine described in the letter never materialized.