The M-94 was a mechanical cipher device used by the U.S. Army from 1921 to 1943.
It is composed of 25 discs, each with a different alphabet stamped around its rim, revolving around an axle.
The discs are identified from “B 1” to “Z 25”, according to the letter that follows “A” in their alphabet. For example, the alphabet of disc “G 6” starts with “AG”).
This identifier, marked on one side of the discs, cannot be seen when the device is assembled..
Use
If the text is longer than 25 letters, it must first be broken in 25-letter sections then the enciphering or deciphering process is performed for each section.
Each section may be enciphered with a different key, or with the same, the latter being much less secure.
To encipher
The discs are arranged on the axle in the order specified by the key, then rotated relative to each other so the plaintext appears on a line.
By rotating the straightedge around the whole cylinder, the user can then select the ciphertext from any of the 25 other lines (though the Army and Navy prohibited using the lines directly above or below the plaintext).
To decipher
The process is the same as for enciphering.
The discs are arranged on the axle in the order specified by the key, then rotated to make the ciphertext appear on a line.
The user then rotates the entire cylinder to look for the line with intelligible text.