Mathematical cryptology. In 1932-1933 discovered a method (based on properties of groups of permutations) of breaking the Enigma cipher. (The method, developed later in England, contributed significantly to the Allies' victory in World War II.) In Poznan and Warsaw until 1939, during the war in France and England.
An Application of the Theory of Permutations in Breaking the Enigma Cipher - Marian Rejewski article (Typesetted version by Enrico Grigolon - November 2002).