Type A Cipher Machine

The Red cipher was succeeded by the Type B "Purple" machine (九七式印字機, 97-shiki ōbun injiki, "System 97 Typewriter for European Characters") which used some of the same principles.

The Red machine encrypted and decrypted texts written in Latin characters (alphabetic only) for transmission through the cable services.Per International Telegraph Union regulations at the time, pronounceable words in telegrams were charged a lower rate than unpronounceable code groups;[1]: 842–849  therefore the machine produced telegraph code by enciphering the vowels separately from the consonants, so that the text remained a series of syllables.

[5] The system was introduced in 1930–1931 (the 91 in the designation refers to the Japanese imperial year 2591),[6] using a reverse-engineered version of a machine supplied by the firm of Boris Hagelin.

[7] Hagelin's most sophisticated systems were rotor machines similar to those used in World War II, but as he did not trust the Japanese to honor his patents, he sent a more primitive device designed by Arvid Damm instead.

The British solution came first, with Hugh Foss and Oliver Strachey working out the code in 1934, and Harold Kenworthy's shop producing a replica, the "J machine", a year later.

In the Army SIS group, the system was broken by Frank Rowlett and Solomon Kullback; for the navy, Agnes Driscoll is generally credited.

[5][10] Reports of the sea trials of the battleship Nagato were also decoded, leading to important changes to the USS North Carolina (BB-55), then being designed, in order to match the performance of the Japanese ship.