RED (Type A) Cipher Machine

1. Algorithm / Product Overview

1.1. The RED designation and operational context

RED was the cover name used by the U.S. Army Signal Intelligence Service (SIS) for a Japanese diplomatic machine cipher. The Type A family used by the Japanese Ministry of Foreign Affairs is also associated with the Japanese designation 91-shiki ōbun-injiki; the number 91 corresponds to imperial year 2591, or 1931. The name Angōki Taipu-A also appears in historical descriptions.

Japan's move toward machine encipherment did not occur on a single date. Devices in the Type 91 lineage developed during the first half of the 1930s, while the NSA historical account states that the Foreign Ministry adopted the Type A machine for communications security in 1935 and that SIS encountered and named the traffic RED in that year. Keeping the date of the machine lineage separate from institutional adoption and from the date on which American analysts identified the system avoids a common chronological ambiguity.

RED must not be confused with the RED BOOK / RED Naval Code used by the Imperial Japanese Navy between the wars. The naval system was a codebook family; the RED discussed here was a machine-generated diplomatic cipher.

2. History and Design Lineage

2.1. Security concerns after Yardley

Herbert O. Yardley's 1931 book The American Black Chamber disclosed substantial information about American cryptanalytic activity and contributed to Japanese concern about the security of existing diplomatic methods. NSA's historical narrative treats that disclosure as an important reason the Japanese Foreign Ministry moved increasingly toward machine systems.

The technical lineage of the Type 91 family included equipment obtained from Boris Hagelin's company and based on older designs associated with Arvid Damm. Historical accounts state that Hagelin, concerned that his patents would be copied, supplied older Damm equipment rather than his most advanced rotor machines. Japanese engineers reverse-engineered the device and developed their own family of machines. One distinctive Japanese design choice was to divide the alphabet into groups of six and twenty characters.

2.2. The “sixes and twenties” split

RED enciphered diplomatic text written in the Latin alphabet and produced groups suitable for cable or telegraph transmission. Under contemporary international telegraph charging rules, pronounceable strings could be cheaper to transmit than arbitrary code groups. The machine therefore processed a six-letter group separately from the remaining twenty letters, producing output with a more syllable-like structure. In early descriptions the six-letter group corresponded to vowels, with Y treated as a vowel.

American cryptanalysts referred to this structure as the “sixes and twenties.” Later RED variants allowed the six-letter set to vary, but the 6/20 separation itself survived and became an important structural clue when the successor PURPLE system was studied.

3. Mechanism

3.1. Half-rotor concept

A conventional rotor machine maps input contacts to output contacts according to the current rotor position. Historical technical descriptions of RED instead discuss arrangements comparable to half rotors that processed the two letter groups separately. American replica machines likewise used two half rotors so that the six-letter and twenty-letter sets could be solved independently.

The main mechanical characteristics of the machine can be summarized as follows:

• A rotor-like section used circularly arranged contacts together with contact strips representing the alphabet.
• Mechanical stepping changed the mapping from an input character to an output contact.
• Related rotor concepts, in modified form, appeared in later machines such as the Hagelin B-211.
• The division of the alphabet into two groups continued in the PURPLE lineage.
• Historical descriptions refer to arrangements involving all 26 strips and two 60-contact sets on the wheel face.
• A blocked-tooth stepping mechanism advanced the rotor, and some technical descriptions identify a special stepping behaviour around position 47.

Terminology for these mechanical details differs across historical sources, so I do not treat the summary above as a complete electrical reconstruction of the original device. For cryptanalysis, the decisive property is the separate processing of two letter classes and the statistical structure that this leaves in the ciphertext.

4. Cryptanalysis

4.1. Independent solutions

British and American teams attacked RED independently. Secondary cryptologic histories report that Hugh Foss and Oliver Strachey solved the system in Britain in 1934 and that Harold Kenworthy's shop produced a replica known as the J machine the following year.

In the United States, SIS began working on Type A traffic in 1935. Frank Rowlett and Solomon Kullback recovered the keying system and machine pattern; NSA biographies and historical material place the transition to regular intelligence production in the 1936-1937 period and record the first RED translation in February 1937. On the naval side, Agnes Meyer Driscoll's work against the related ORANGE / M-1 machine also contributed knowledge because the systems had closely related mechanical ideas.

SIS initially used a descriptive label such as “Japanese Code Machine,” then adopted the cover name RED as part of a colour-based nomenclature. The Americans also constructed a functional replica to accelerate decryption, using two half rotors to process the six-letter and twenty-letter groups separately.

4.2. Transition from RED to PURPLE

From late 1938 onward, the Type B / PURPLE machine began appearing at the most important diplomatic posts; NSA records state that the Japanese Foreign Ministry introduced the new equipment in February 1939. Less important posts continued to use RED, so the two systems operated in parallel for a period.

That overlap gave cryptanalysts examples of the same or closely related traffic under two systems. More importantly, PURPLE preserved the six/twenty division already known from RED. Knowledge gained from RED therefore provided a useful structural foothold for the much harder PURPLE problem. After roughly eighteen months of concentrated work, the American team built a machine that reproduced PURPLE's scrambling behaviour without having seen the original Japanese device.

5. Intelligence Value

RED decrypts did not by themselves produce all of the strategic intelligence associated with Japanese diplomatic systems, but they gave direct insight into Japanese foreign policy. NSA historical material notes that RED decrypts revealed information about secret provisions surrounding the 1936 Anti-Comintern arrangements. Later RED and PURPLE diplomatic traffic also helped American analysts follow the evolving relationship among Germany, Italy, and Japan and the negotiations surrounding the Tripartite Pact.

Secondary histories also report that decrypted information about the sea trials of the Japanese battleship Nagato contributed to performance-related changes to USS North Carolina (BB-55) while the latter was being designed. It is more accurate to treat this as one input to a broader intelligence and engineering assessment than to claim that a single RED message directly redesigned the ship.

6. Evaluation

Describing RED as merely “a simple machine” misses the more useful cryptanalytic lesson. Its weakness came from the interaction of structure and procedure: the alphabet was separated into two classes, a related design choice persisted in successor equipment, some traffic was repeated across systems, and operational habits exposed regularities that analysts could exploit. The security of a cipher machine therefore depends not only on mechanical complexity, but also on key space, statistical leakage, operating procedure, and whether the same information reappears through another channel.

RED is best understood through its mechanical design, the cryptanalytic work directed against it, and the historical record that distinguishes it from later Japanese cipher systems such as PURPLE. The chronology below follows those technical and intelligence sources without treating RED and PURPLE as interchangeable systems.

References

• **[1]** National Security Agency. (2021). Red and Purple. NSA/CSS Cryptologic History.
• **[2]** National Security Agency. (2021). Early Japanese Systems. NSA/CSS Cryptologic History.
• **[3]** Alan G. Konheim. (2007). Computer Security and Cryptography. Wiley-Interscience.
• **[4]** David Kahn. (1996). The Codebreakers: The Comprehensive History of Secret Communication from Ancient Times to the Internet, Revised Edition. Scribner.
• **[5]** Christopher Andrew. (1996). For the President's Eyes Only. HarperCollins, p. 105. ISBN 978-0-06-092178-1.
• **[6]** Friedrich Ludwig Bauer. (2007). Decrypted Secrets: Methods and Maxims of Cryptology. Springer, pp. 154-158.
• **[7]** Stephen Budiansky. (2000). Battle of Wits: The Complete Story of Codebreaking in World War II. Free Press, pp. 84-88.
• **[8]** Hervie Haufler. (2003). Codebreakers' Victory: How the Allied Cryptographers Won World War II.
• **[9]** Michael Smith. (2000). The Emperor's Codes: The Breaking of Japan's Secret Ciphers. Arcade Publishing, pp. 45-47.
• **[10]** David Kahn. (1967). The Codebreakers: The Story of Secret Writing. Macmillan. ISBN 978-0-684-83130-5. OCLC 59019141.
• **[11]** John J. G. Savard. The RED Machine. Historical technical note; accessed in the source set used for the 2021 version.