Internal elements M-209

1 internal elements

1.1 overview
1.2 example configuration
1.3 example encoding

internal elements
overview

inside casing of m-209, more complicated picture emerges. 6 key wheels each have small movable pin aligned each letter on wheel. these pins may each positioned left or right; positioning of these pins affects operation of machine. left position ineffective, while right position effective.

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.

example configuration

prior encoding using m-209, operator must set machine according preset configuration. configuration includes settings each pin on 6 of key wheels, , position of each lug on rotating drum; these typically specified tables in secret system publication given both sender , receiver. rotational alignment of key wheels chosen sender @ random, , provided receiver via secure channel of communication.

each letter on each key wheel associated pin can set either left or right. table specifying setting of these pins might resemble following:

letters present in table given key wheel should have corresponding pin set right, or effective , position. absent letters, represented dash, set left, or ineffective , position.

the rotating drum has 27 bars, each 2 lugs. these lugs can set position 1 through 6, in case aligned corresponding key wheel, or may set 1 of 2 0 positions, in case ineffective. table indicating lug settings drum might this:

bar 1 have lugs set in 3 , 6 positions, bar 2 s lugs in 0 , 6 positions, , on. lug in 3 position, example, pushed side guide arm when active pin on key wheel 3 in effective position.

finally, external key set rotating key wheels either specific or random sequence of letters. in testing internal key settings of m-209, customary operator set key wheels aaaaaa , , proceed encoding message consisting of nothing letter a. resulting ciphertext compared long check string verify of internal settings have been performed properly. check string particular configuration is:

t n j u w u q t k c z k n u t o t b c w r m o

key wheel pins come play when reach lower part of key wheel during rotation; here may contact or release guide arm deflects lugs left. active pin offset particular amount letter being displayed on front of key wheel; when aaaaaa showing on key wheels, pins in play associated letters ponmlk , left right.

example encoding

after m-209 configured according settings above, machine ready encode. continuing example of known check string, first letter encoded . operator sets indicating disk letter , , turns power handle.

since key wheels set string aaaaaa , active pins ponmlk ; according settings above, pin p ineffective on first key wheel, pin o effective on second key wheel, n effective on third, m effective on fourth, l ineffective on fifth, , k effective on sixth. guide arms associated effective pins tilt forward , contact rotating drum; in case, guide arms 2, 3, 4, , 6 effective.

any bar on drum lug in of positions slid left, , bar participate in variable-toothed gear driving output of machine. according given settings, bars 1, 2, 3, , 5 through 21 slid left, total of 20 bars, or 20 teeth on variable-toothed gear. encoding letter use shift of 20.

the m-209 uses reciprocal substitution cipher or beaufort scheme; alphabet used in plaintext message mapped same alphabet in reverse (atbash):

if shifting not considered, becomes z , b becomes y , c becomes x , on. shifting proceeds in reverse direction; instance, plaintext p maps ciphertext k ; shifting 3 positions, left, gives ciphertext n . shift circular, when shift steps off left side, continues again on right. approach self-inversing, meaning deciphering uses same table in same way: ciphertext n entered if plaintext; maps m in ciphertext alphabet, or p after shifting 3 positions, giving original plaintext back.

continuing example above, initial letter encoded , maps z in ciphertext. shift given variable-toothed gear 20; shifting left 20 positions gives final ciphertext letter t , same first digit in check string.

at end of encoding cycle, 6 key wheels advanced 1 position. key wheels read bbbbbb , , active pins qponml . new set of guide arms interact drum, resulting in different shift next encoding operation.