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The M-94 was a piece of cryptographic equipment used by the United States army, consisting of several lettered discs arranged as a cylinder. It was also employed by the US Navy, under the name CSP 488.
The device was conceived by Colonel Parker Hitt and then developed by Major Joseph Mauborgne in 1917; based on a system invented by Thomas Jefferson and Etienne Bazeries. Officially adopted in 1922, it remained in use until 1945, when it was replaced by more complex and secure electromechanical rotor machines, particularly the M-209.
The device consisted of 25 aluminium discs attached to a four-and-a-half inch long rod, each disc containing the 26 letters of the Roman alphabet in scrambled order around its circumference (with the exception of the 17th disc, which began with the letters "ARMY OF THE US"). Each wheel had a different arrangement of the alphabet, and was stamped with an identifying number and letter; wheels were identified according to the letter following "A" on that wheel, from "B 1" to "Z 25". The wheels could be assembled on the rod in any order; the ordering used during encoding comprised the key. There were 25! (25 factorial) = 15,511,210,043,330,985,984,000,000 (more than 15 septillion) possible keys, which can be expressed as about an 84-bit key size.
Messages were encrypted 25 letters at a time. Turning the discs individually, the operator aligned the letters in the message horizontally. Then, any one of the remaining lines around the circumference of the cylinder was sent as the ciphertext. To decrypt, the wheels were turned until one line matched a 25 letter block of ciphertext. The plaintext would then appear on one of the other lines, which could be visually located easily, as it would be the only one likely to "read."
The principle upon which the M-94/CSP-488 is based was first invented by Thomas Jefferson in 1795 in his "wheel cypher" but did not become well known, and was independently invented by Etienne Bazeries a century later.
In an extension of the same general principle, the M-138-A strip cipher machine used by the US Navy and US State Department through World War II featured 100 flat strips. Each strip had a scrambled alphabet, repeated twice, printed on a metal strip that could be slid back and forth in a frame; with 30 being selected for each cipher session.
Like most classical ciphers, strip ciphers can be easily cracked if there is enough intercepted ciphertext. However, this takes time and specialized skills, so the M-94 was still good enough during World War II for its intended use as a "tactical cipher"; in a similar way to the more modern DRYAD and BATCO.