M-94

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."

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