Gjøvik University College Cryptology 1

3.1. Ciphering process
When the clear text letter is set on a knop with an indicator disk on the left side of the machine, the operator turns the power handle of the right side, the cage makes a complete rotation through all of the 27 bars. If an operative guide arm gets in contact with a lug on a bar, the bar slides to the left. All the bars that now are in their left position comprise a cog in the gear, which turns the letter to be enciphered. The letter shift is equal to the number of bars in their left position.
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The Hagelin M-209 cipher machine

The cipher text letter is then printed on a paper tape. In the case that the machine runs out of paper tape, the type wheel works as an indicator disk which makes the operator able to read the enciphered or deciphered letter directly. After one rotation, all the slid bars are retracted to their original position by a retractor, all the key wheels are advanced one position by another set of gears, and a locking arm locks the cage to prevent one more enciphering until the indicator disk is set to the next letter. This means that if the operator is enciphering or deciphering two equal letters successively he needs to turn the disk back and forth. A skilled operator can run the machine at 15 to 30 letters a minute.

The cipher used in the M-209 is a variant of the Beaufort cipher, which means that the ciphering process is an involution, it uses the same algorithm for both enciphering and deciphering.

Encipher: = ( − ) 26

Decipher: = ( − ) 26

What makes the M-209’s ciphering different from the Beaufort cipher, is the system of the cage and the key wheels, which makes the offset change for each enciphered letter.

The machine has a button on the right side for choosing Encipher (C) or Decipher (D), but this is only for printing adjustment. Encipher prints the letters in groups of five letters, and deciphering prints the words with the correct spacing. The letter “z” is used for as the spacer, so the word “analyze” would come out as “analy e”.