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Concept
Type B Cipher Machine
Summary
In the history of cryptography, the "System 97 Typewriter for European Characters" (九七式欧文印字機 kyūnana-shiki ōbun injiki) or "Type B Cipher Machine", codenamed Purple by the United States, was an encryption machine used by the Japanese Foreign Office from February 1939 to the end of World War II. The machine was an electromechanical device that used stepping-switches to encrypt the most sensitive diplomatic traffic. All messages were written in the 26-letter English alphabet, which was commonly used for telegraphy. Any Japanese text had to be transliterated or coded. The 26-letters were separated using a plug board into two groups, of six and twenty letters respectively. The letters in the sixes group were scrambled using a 6 × 25 substitution table, while letters in the twenties group were more thoroughly scrambled using three successive 20 × 25 substitution tables.
The cipher codenamed "Purple" replaced the Type A Red machine previously used by the Japanese Foreign Office. The sixes and twenties division was familiar to U.S. Army Signals Intelligence Service (SIS) cryptographers from their work on the Type A cipher and it allowed them to make early progress on the sixes portion of messages. The twenties cipher proved much more difficult, but a breakthrough in September 1940 allowed the Army cryptographers to construct a machine that duplicated the behavior (was an analog) of the Japanese machines, even though no one in the U.S. had any description of one.
The Japanese also used stepping-switches in systems, codenamed Coral and Jade, that did not divide their alphabets. American forces referred to information gained from decryptions as Magic.
The Imperial Japanese Navy did not cooperate with the Army in pre-war cipher machine development, and that lack of cooperation continued into World War II. The Navy believed the Purple machine was sufficiently difficult to break that it did not attempt to revise it to improve security. This seems to have been on the advice of a mathematician, Teiji Takagi, who lacked a background in cryptanalysis.
Official source
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