Gjøvik University College Cryptology 1

3 The M-209

The M-209 is very small, about the size of a lunchbox, measuring only 83 x 140 x 178 mm, and weighing a little over 3 kg including its rugged case. The small size, its low weight, and the fact that it is non-electrical, made the M-209 well suited for tactical use on the battlefield. It is no problem for the machine to survive hard shocks, dust, sand, tropic humidity or arctic cold [Kahn]. In addition the machine is capable of performing both enciphering and deciphering, which is a huge plus compared to many other ciphering machines.

The most important parts in the machine is the cage, the key wheels and the guide-arms which bind them together.
The drum bar cage, also known as the lug cage, but mostly called just “the cage”, consists of 27 horizontal bars between two disks, which forms a revolving cylinder. Each bar has two movable lugs which can be placed in eight positions. Two neutral, and six positions which interacts with the key wheels. Depended on the lug and key wheel pin setting, the individual bars can slide to the left, and add a cog tooth to the variable gear. The number of bars in their left position is the same as the offset between the clear text letter and the cipher text letter.

8

The Hagelin M-209 cipher machine

Figur 5: M-209s internals. Indicator disk, type wheel, key-wheels with pins, cage with lugs, guide arms etc. is shown


Six key wheels, which each controls one guide-arm, have a different number of letters on its rim, and a pin under each letter. The number of letters from left to right are 26, 25, 23, 21, 19 and

17. 
    These numbers are chosen to be coprime, which means their greatest common divisor is 1. This causes the wheels to align after26 ∙ 25 ∙ 23 ∙ 21 ∙ 19 ∙ 17 = 101 405 850 enciphered letters, this is also known as a period.
    

Each letter on each wheel has a movable pin. Setting the pin to the right means that the letter position is enabled, and to the left for disabled. An enabled letter position will in turn set the wheel’s guide arm in its operative position, which determines whether lugs in the cage is contacted. If a lug is in contact with a guide arm, its bar slide to the left, and interact with the cogs in the variable gear, as previously mentioned.

The wheels have the following letters:
Wheel 1: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Wheel 2: ABCDEFGHIJKLMNOPQRSTUVXYZ
Wheel 3: ABCDEFGHIJKLMNOPQRSTUVX
Wheel 4: ABCDEFGHIJKLMNOPQRSTU
Wheel 5: ABCDEFGHIJKLMNOPQRS
Wheel 6: ABCDEFGHIJKLMNOPQ