Genevieve Grotjan’s “great discovery”

Abstract On 20 February 1939, a new Japanese diplomatic cipher machine that U.S. Army codebreakers would name PURPLE came into use. By 10 April 1939, the Army’s Signal Intelligence Service (SIS) had made startling progress. SIS codebreakers had discovered that, for no cryptographic reason, PURPLE maintained the split of the Roman alphabet into a set of 6 letters and a set of 20 letters—the sixes and the twenties—a split that had been observed in PURPLE’s predecessor RED. They had a familiar problem, and they were quickly able to recover the enciphering table for the sixes But 18 months later they were still puzzled by the enciphering of the twenties Then, on 20 September 1940, Genevieve Grotjan, an SIS codebreaker, made a discovery that opened the way for the recovery of the PURPLE machine. This paper explores the patterns for which Grotjan was searching and concludes with what she found.