The M-94 was a W.W.II United States Army Signal Corps cipher device used from the early 1920's up till 1942 as a low level, tactical, cryptographic encoding/decoding device. A version used by the U.S. Navy was called CSP-488. It was also called a Jefferson device and was similar in type (if not identical) to Major Bazeries Cipher Device or Bazeries Cylinder. Over time, these units were replaced by the M-209 cipher machine.
Per Louis Kruh, about 9,432 of the M-94's were made between their adoption in 1921 and their replacement by the M-209 on August 9, 1943 . He notes about 150,000 of the M-209's were made.
A view of the M-94 showing wheel markings. (Courtesy firstname.lastname@example.org)
M-94 is composed of a central shaft on which is mounted a number of revolving disks containing alphabets on their peripheries. The disks can be removed from the central shaft and replaced in any given prearranged order. After the disks are mounted on the shaft in the order agreed upon, the plain text of the message to be enciphered is aligned across the device in a horizontal line by revolving the disks one by one. The cipher text is chose at random from the horizontal lines of letters resulting from this plain-text alignment. Succeeding lines of plain text are enciphered in the same manner until the entire message has been converted into cipher text. Decipherment is performed by a reversal of the procedure, that is, the cipher text is aligned across the device and the resulting plain text can be found by examining the other horizontal line of the letters thus formed, only one of which will "read".
Disk ID Letters Around The Rim Of Wheel
M-94 wheel identification matrix provided courtesy of George Mace.
Wheel # 17, when viewed properly, displays "ARMYOFTHEUS" which identifies it as an ARMY M-94 device. (Courtesy email@example.com)
A close up of the device. Made of aluminum, the device consist of 25 wheels all arranged on a common shaft. Each wheel has a number and a letter stamped on the inside face, which when arranged in a predetermined order would allow a simple encode or decode of message, providing that user of the received message had their wheels setup in same manner as sender. (Photo courtesy of firstname.lastname@example.org) The M-94 is 4.5" long with 25 numbered interlocking code wheels. One end is impressed with M-94 while the other bears the orange Signal Corps inspection stamp SC-120. (Photo via E-bay).
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