Enigma, the XYZ period (1939–1940)

Abstract

In July 1939, at the dawn of the Second World War, Polish cryptologists revealed their mastery of the Enigma encryption machine to their French and British counterparts. The Enigma machine was the main encryption method used by the German armed forces at the time. An alliance was born: XYZ (the letters are codes identifying each of the allies: X represents the French, Y represents the English, and Z represents the Polish). The Poles transmitted their methods to their allies, starting with the sheets invented by Zygalski. These sheets were made by the English, who then sent copies to the French and the Polish. These joined the French after the Polish defeat. With these sheets, from January 1940 onward, the allies were able to read the messages of the enemy. Unfortunately, in May 1940, the Germans changed their system, and the Zygalski sheets became obsolete. Only the English managed to continue breaking the Enigma keys via manual methods. In all, thousands of messages were read. In June, France capitulated, and consequently, the triple alliance ended. This story has been told before, but recently declassified French archives have allowed us to clarify several points, particularly the central place of the Cyclometer.