Overview M-209

an intermediate gear unit (center) meshes gears adjoining each key wheel. visible left of image paper tape , typewheel print out messages , ciphertext.



an inactive pin (red) on bottom of left key wheel (light blue) pulls guide arm (green) back. no pin blocking right guide arm, spring tilts guide arm forward.



the left guide arm prevented interacting lugs (purple) on drum (blue), while right guide arm in effective position, , push left bars lug in position.

each key wheel contains different number of letters, , correspondingly different number of pins. left right, wheels have:

26 letters, z
25 letters, z, excepting w
23 letters, x, excepting w
21 letters, u
19 letters, s
17 letters, q

this discrepancy chosen give wheel sizes coprime nature; end result wheels align same way once every 26×25×23×21×19×17 = 101,405,850 enciphered letters (also known period). each key wheel associated slanted metal guide arm activated pins in effective position. positions of pins on each key wheel comprise first part of internal keying mechanism of m-209.

behind row of 6 key wheels cylindrical drum consisting of 27 horizontal bars. each drum bar affixed 2 movable lugs; lugs can aligned of 6 key wheels, or may placed in 1 of 2 neutral positions. effective pin causes guide arm tilt forward, contacting drum. positioning of lugs comprises second part of internal keying mechanism. owing complexity of setting internal keying mechanism, altered relatively infrequently; changing internal keys once day common in practice.

when operator turns power handle, cylindrical drum makes complete revolution through 27 bars. if lug on 1 of bars contacts guide arm of active key wheel, bar slid left; lugs in neutral positions, or not contact guide arm, not affect position of bar. bars slid left comprise variable-toothed gear, in turn shifts letter encoded; shift equal number of bars protruding left. resulting ciphertext letter printed onto paper tape.

after rotation complete, retractor pushes protruding bars place. set of intermediate gears advances key wheels 1 position, , locking arm latches drum prevent second encoding until indicator disk adjusted next letter.

this system allowed offset change each enciphered letter; without facility, enciphering scheme resemble insecure caesar shift cipher.