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.

摘要

1939 年 2 月 20 日，美国陆军密码破译者将其命名为“紫色”的新型日本外交密码机投入使用。到 1939 年 4 月 10 日，陆军信号情报局 (SIS) 取得了惊人的进展。SIS 密码破译者发现，出于密码原因，PURPLE 保留了将罗马字母拆分为一组 6 个字母和一组 20 个字母（即 6 个字母和 20 个字母）的方式，这种拆分在 PURPLE 的前身 RED 中也曾观察到。他们遇到了一个熟悉的问题，很快就能够恢复 6 进制的加密表，但 18 个月后，他们仍然对 20 进制的加密感到困惑。然后，1940 年 9 月 20 日，SIS 密码破译员 Genevieve Grotjan 有了一个发现这为紫色机器的恢复开辟了道路。