Type B Cipher Machine

Type B Cipher Machine. The Type B Cipher Machine, codenamed PURPLE by Allied cryptanalysts, was an advanced rotor machine used for high-level diplomatic communications by the Japanese Foreign Ministry during the late 1930s and World War II. Its complex electromechanical design, which employed telephone stepping switches instead of traditional rotors, generated a sophisticated polyalphabetic cipher that was considered unbreakable by its creators. The successful Allied decryption of PURPLE, primarily by American codebreakers, provided crucial intelligence that significantly influenced the course of the Pacific War.

History and Development

The development of the Type B machine was initiated by the Imperial Japanese Navy following the introduction of the earlier Type A Cipher Machine, known to the United States as RED. After American cryptanalysts, including the team at OP-20-G, began breaking RED ciphers in the mid-1930s, Japanese authorities sought a more secure system. The design and construction were led by Captain Kazuo Tanabe of the Imperial Japanese Navy and engineer Masamitsu Kihara, with the first machines delivered to the Foreign Ministry in 1939. Its deployment coincided with escalating tensions preceding major conflicts like the Second Sino-Japanese War and the attack on Pearl Harbor. The machine’s internal name was *Ō-bun In-ji-ki* (Type B Printing Machine for European Text), distinguishing it from systems used for kana characters.

Technical Description

The Type B machine was an electromechanical device that used a complex array of telephone stepping switches, similar to those found in contemporary Bell System exchanges, to perform its cryptographic functions. Unlike the pin-and-lug Enigma rotors used by Nazi Germany, the Type B employed a system of six banks of 25-point stepping switches, divided into groups of fast, medium, and slow switches, to create a polyalphabetic substitution cipher. It encrypted Roman letters and numbers, with the output printed directly onto paper tape, a feature it shared with the American SIGABA machine. The machine’s electrical pathways were governed by a plugboard and a complex arrangement of cables, creating a non-reciprocal cipher where the encryption and decryption processes differed. This design was more akin to the later American ECM Mark II than to contemporary European rotor machines.

Operational Use

The Type B machine was used exclusively for the most sensitive diplomatic communications between Tokyo and major Japanese embassies and consulates worldwide, including those in Berlin, Rome, Moscow, and Washington, D.C.. Messages encrypted on the system, termed "PURPLE" traffic by Allied intelligence, covered high-level strategic discussions, political negotiations, and intelligence reports. Key communications regarding the Tripartite Pact, diplomatic maneuvers prior to the attack on Pearl Harbor, and assessments of Soviet intentions during the Molotov–Ribbentrop Pact period were sent via these machines. The Japanese embassy in Berlin was a major hub for this traffic, facilitating coordination between the Axis powers throughout the North African campaign and the Eastern Front.

Cryptanalysis and Security

The security of the Type B machine was catastrophically compromised by a team of American cryptanalysts, most notably William F. Friedman and Frank Rowlett of the U.S. Army's Signal Intelligence Service. Working without ever capturing a physical machine, the team at Arlington Hall meticulously reconstructed the machine’s logical design through traffic analysis and brilliant cryptanalysis. They identified a critical cryptographic flaw: while the machine’s stepping switches provided complexity, the underlying cipher structure had mathematical weaknesses that allowed for the separation of variables. This breakthrough, achieved by September 1940, enabled the consistent decryption of Japanese diplomatic messages. The intelligence product, designated MAGIC, provided vital forewarning of Japanese intentions, though specific warnings about Pearl Harbor were tragically not acted upon decisively.

Variants and Legacy

The basic Type B design had no significant field variants, but its successor systems, developed after PURPLE’s compromise was suspected, included the JADE and CORAL cipher machines used by the Imperial Japanese Navy. The capture and study of Type B machines at the end of the Pacific War provided valuable insights for postwar cryptography in both the United States and the Soviet Union. The story of its breaking became a foundational legend of American signals intelligence, directly leading to the establishment of the National Security Agency (NSA). The machine itself remains a pivotal artifact in the history of cryptography, symbolizing both the peak of pre-computer cipher design and the ultimate vulnerability of even the most complex systems to determined analysis.