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| Japanese Red |
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From Wikipedia: "In the history of cryptography, 91-shiki injiki (System 91 Printing Machine) or Angoki Taipu-A ("Type A Cipher Machine"), codenamed Red by the United States, was a diplomatic cryptographic machine used by the Japanese Foreign Office before and during World War II. A relatively simple device, it was quickly broken by western cryptographers. The Red cipher was succeeded by the "Purple" machine ("97-shiki obun inji-ki") which used some of the same principles. Parallel usage of the two systems assisted in the breaking of the Purple system.
The Red cypher should not be confused with the Red naval code, which was used by the Imperial Japanese Navy between the wars. The latter was a codebook system, not a cipher.
OPERATION
The Red machine encrypted and decrypted texts written in Latin
characters (alphabetic only) for transmission through the cable
services. These services charged a lower rate for texts that could be
pronounced than for random strings of characters; therefore the machine
produced telegraph code by encyphering the vowels separately from the
consonants, so that the text remained a series of syllables.] (The
letter "Y" was treated as a vowel.) The "sixes and twenties" effect (as
American analysts referred to it) was a major weakness which the
Japanese continued in the Purple system.
Encryption itself was provided through a single half-rotor; input
contacts were through slip rings, each of which connected to a single
output contact on the rotor.] Since both the vowels and consonants were
passed through the same rotor, it had sixty contacts (the least common
multiple of six and twenty); wiring ensured that the two groups were
kept separate. The slip rings were connected to the input keyboard
through a plugboard; again this was organized to keep the vowels and
consonants separate.]
The rotor turned at least one step after each letter. The amount of
rotation was controlled by the break wheel, which was connected to the
rotor, and which had up to forty-seven pins in it. Up to eleven of these
pins (in a predetermined set of positions) were removable; in practice,
from four to six pins were removed. Rotation of the wheel stopped when
the next pin was reached; therefore, if the next pin were removed, the
rotor would advance two places instead of one.] The irregular pattern of
rotation produced an Alberti cipher.
HISTORY
The vulnerability of Japanese code systems was made public in 1931 when Herbert Yardley published The American Black Chamber, a popular account of his code breaking activities for the US government in which he discussed the breaking of Japanese codes and their use during the Washington Naval Conference. These revelations prompted Japanese to look into machine cyphers.