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Red and Purple: A Story Retold (U)

''After studying the techniques used to solve World War II-era Japanese machine cipher systems, a small group of analysts attempted to use present-day techniques to solve these systems. The results were not what one might have expected.''

Introduction

Could modern computer techniques be used to solve the Japanese machine cipher systems of World War II? In June 1982, NSA analysts in the course "Japanese Cipher Devices of World War II" discovered that the task was not so easy. The purpose of the course, a part of the CA Summer Program in Innovative Cryptologic Education (SPICE), was to study the techniques used to solve Japanese machine cipher prior to and during World War II, and then to attempt to use modern techniques to solve these systems.

The seven individuals involved in the class included analysts from A, B, R, E, and Pl, and two CA interns. One of the CA interns, at the time on tour in E42, taught the course and wrote this article. For two months she researched material in the NSA archives and in the Cryptologic Collection of the NSA Library: Using original material, she prepared the machine data base on JEEP, along with problems from a course on Purple that is in the Cryptologic Collection.

The students were given five actual messages to analyze before they knew anything about the systems. The instructor had expected the properties she had studied in her research to surface in the initial analysis. This was not the case.

The problem, discovered during the analysis, was that the messages were not homogeneous. One was plain text, one was a variation on a later Japanese transposition system, two were Red, and the final one was of unknown origin (possibly Purple). There were not enough messages available to solve a system, and what was available did not demonstrate the properties that led to the breaking of the system. Though the class was unable to solve the system with the messages available, it was possible to study the historical solution of the system.

The Breaking of Red

The M-3 cipher machine, held by all the major embassies of Japan in the early 1930s, was modified on 1 December 1938 for extra secret messages. The modification was called M-3A. On 20 February 1939 the M-3A was replaced with M-3B. American analysts named the M-3 and M-3A cipher Red, while cipher from the M-3B was called Purple.