In the history of cryptography, 91-shiki ōbun injiki (九一式欧文印字機, "System 91 Typewriter for European Characters") or Angōki Taipu-A (暗号機 タイプ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 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. Parallel usage of the two systems assisted in the breaking of the Purple system.
The Red cipher 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.