M-209 - Internal Elements - Example Configuration

Example Configuration

Prior to encoding anything using the M-209, the operator must set the machine according to a preset configuration. This configuration includes the settings for each pin on all six of the key wheels, and the position of each lug on the rotating drum; these were typically specified by tables in a secret system publication given to both sender and receiver. The rotational alignment of the key wheels could be chosen by the sender at random, and provided to the receiver via a secure channel of communication.

Each letter on each key wheel is associated with a pin that can be set either to the left or right. A table specifying the setting of these pins might resemble the following:

Wheel	Pin settings
1	AB-D---HI-K-MN----ST-VW---
2	A--DE-G--JKL--O--RS-U-X--
3	AB----GH-J-LMN---RSTU-X
4	--C-EF-HI---MN-P--STU
5	-B-DEF-HI---MN-P--S
6	AB-D---H--K--NO-Q

Letters that are present in the table for a given key wheel should have their corresponding pin set to the right, or "effective", position. Absent letters, represented by a dash, are set to the left, or "ineffective", position.

The rotating drum has 27 bars, each with two lugs. These lugs can be set to any position 1 through 6, in which case they are aligned with the corresponding key wheel, or they may be set to one of two "0" positions, in which case they are ineffective. A table indicating the lug settings for the drum might look like this:

Bar	1	2	3	4	5	6	7	8	9
Lugs	3-6	0-6	1-6	1-5	4-5	0-4	0-4	0-4	0-4
Bar	10	11	12	13	14	15	16	17	18
Lugs	2-0	2-0	2-0	2-0	2-0	2-0	2-0	2-0	2-0
Bar	19	20	21	22	23	24	25	26	27
Lugs	2-0	2-5	2-5	0-5	0-5	0-5	0-5	0-5	0-5

Bar 1 would have its lugs set in the "3" and "6" positions, bar 2's lugs in the "0" and "6" positions, and so on. Any lug in the "3" position, for example, will be pushed to the side by a guide arm when the currently active pin on key wheel 3 is in an "effective" position.

Finally, the external key is set by rotating the key wheels to either a specific or random sequence of letters. In testing the internal key settings of the M-209, it is customary for the operator to set the key wheels to "AAAAAA", and proceed with encoding a message consisting of nothing but the letter "A." The resulting ciphertext is then compared with a long check string to verify that all of the internal settings have been performed properly. The check string for this particular configuration is:

T N J U W A U Q T K C Z K N U T O T B C W A R M I O

Key wheel pins come into play when they reach the lower part of the key wheel during rotation; it is here that they may contact or release the guide arm that deflects the lugs to the left. The active pin is offset by a particular amount from the letter currently being displayed on the front of the key wheel; when "AAAAAA" is showing on the key wheels, the pins that are in play are those associated with the letters "PONMLK", from left to right.