RED (Type A) Cipher Machine

1. Algorithm or Product Overview

1.1. Summary

The RED Type A cipher machine was a cryptographic device used by the Japanese Ministry of Foreign Affairs before and during the Second World War.

The machine was also known as the 91-shiki ōbun-injiki, or System 91 Typewriter for European Characters, and as Angōki Taipu-A. The number 91 referred to the Japanese imperial year 2591.

Because the machine used a comparatively simple operating principle, Western cryptanalysts broke it within a relatively short time. The PURPLE Type B machine, also known as the 97-shiki ōbun inji-ki, used a related principle and was likewise broken.

The name RED should not be confused with the RED Naval Code used by the Imperial Japanese Navy. That system was a codebook rather than a cryptographic machine algorithm.

2. About the Algorithm or Product

2.1. History

In 1931, Herbert Yardley disclosed that the Japanese cryptographic system used during the Washington Naval Conference had been broken. His detailed account in The American Black Chamber prompted Japanese cryptologists to review their methods.

The system was introduced in 1930 and 1931 through a reverse-engineered design supplied by Boris Hagelin’s company. Hagelin’s most advanced systems were rotor machines similar to those later used during the Second World War. Distrusting the Japanese over patents, however, he sent a more primitive device designed by Arvid Damm. This became the basis of the Japanese design. Japanese cryptologists subsequently modified the machine so that vowels were encrypted separately.

2.2. Operation

The RED Type A cipher machine encrypted and decrypted text written with Latin-alphabet characters for transmission through cable services.

At the time, telegram text was transmitted through cable services. Under International Telegraph Union regulations, pronounceable words were charged at a lower rate than unpronounceable code groups. The machine was therefore designed to generate and transmit separate telegraph-code groups for vowels and consonants. The letter Y was treated as a vowel.

The basic system, which American analysts called the “sixes and twenties,” was later retained in the PURPLE machine and created a significant weakness.

2.3. Mechanism and Details

A rotor machine generally uses a disk with 26 contacts arranged in a circle on each side.

As the rotor moves, the contacts on the two sides conduct through different mappings.

The mechanism used a rotor with 26 circular contacts on one side and a shaft with 26 contact strips. Movement routed each character to a different position. This type of component was described as a half rotor.

A modified form of a similar rotor arrangement was later used in the Hagelin B-211.

The division of the alphabet into vowels and consonants continued in the PURPLE machine.

The mechanism used all 26 strips and two sets of 60 contacts on the wheel face, one set connected sequentially.

When the mechanism encountered a blocked tooth, the rotor advanced by one position. At position 47, however, the gear continued to advance for the current character.

3. Cryptanalysis

3.1. Work Performed

Three independent groups successfully broke the RED Type A algorithm. In Britain, Hugh Foss and Oliver Strachey studied the system in 1934. One year later, a replica known as the J machine was constructed in Harold Kenworthy’s workshop.

The American effort had roots reaching back to 1925. Frank Rowlett and Solomon Kullback of the Army’s Signals Intelligence Service broke the algorithm. Work on the ORANGE M-1 machine, which also used two half rotors, and the earlier cryptanalytic work of Agnes Driscoll contributed to the effort. The Americans constructed a replica to accelerate decryption. It used two half rotors to process vowels and consonants separately.

The SIS initially called the system the Japanese Code Machine. It later considered that name a security risk and assigned the designation RED.

3.2. Cryptanalytic Details

The PURPLE machine began replacing RED in 1938, although early installations were limited to major organizations. Less important embassies and consulates continued to use the older system.

Similar design weaknesses, particularly continued use of the “sixes and twenties” split, made PURPLE vulnerable. After approximately 18 months of work, its algorithm was broken and provided important intelligence through the end of the war.

3.3. Evaluation

Intelligence obtained directly from solving RED was not initially decisive, but the effort later supported more important intelligence. American cryptanalysts first supplied relevant details concerning the Tripartite Pact. The resulting decrypts also contributed to critical modifications and operational success involving USS North Carolina, BB-55.

4. References

“Pearl Harbor Review — RED and PURPLE.” National Security Agency. Accessed 2020-12-13.

Andrew, Christopher. For the President’s Eyes Only. HarperCollins, 1996, p. 105. ISBN 978-0-06-092178-1. Accessed 2020-12-13.

Bauer, Friedrich Ludwig. Decrypted Secrets: Methods and Maxims of Cryptology. Springer, 2007, pp. 154-158.

Budiansky, Stephen. Battle of Wits: The Complete Story of Codebreaking in World War II. New York: Free Press, 2000, pp. 84-88.

Haufler, Hervie. Codebreakers’ Victory: How the Allied Cryptographers Won World War II. 2003, p. 114.

Smith, Michael. The Emperor’s Codes: The Breaking of Japan’s Secret Ciphers. New York: Arcade Publishing, 2000, pp. 45-47.

Kahn, David. The Codebreakers: The Story of Secret Writing. New York: Macmillan, 1967. ISBN 978-0-684-83130-5. OCLC 59019141.

Savard, John J. G. “The RED Machine.” Accessed 2020-12-13.