M-209 using (enciphering/deciphering)

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M-209 Workings Home Page M-209 Workings:

Preparations of the machine

Before enciphering or deciphering a message, the code clerk must set the keying elements of the M-209:

Internal Key

The internal key is composed by the pins and lugs setting. This setting is made in accordance with a cipher-key list. Each code clerk of the same key-net (a division for example) shares the same key list which was prepared by the signal officers of the military unit. Normally, a key list is valid for one day. The key lists of the month are usually written in a document named SOI (Signal Operation Instructions). In the following example, we use the key list included in the TM 11-380 of 1944.

External Key

The external key is composed of the six key wheels initial setting. Letters on the key wheels are selected at random by the code clerk and lined up from left to right along the white bench mark. Different letters (and them different external key) must be selected for every message.

Indicator

The ciphered message sent to the correspondant is surrounded by the indictor (one copy at the begin and another at the end of the message). Structure of this indicator can respect several alternatives rules. The aim of the indicator is to transmit cipher means used, external key and key list used. The convention described in the TM 11-380 of 1944, is very simple: the two first letters are "FW" to indicate M-209 using, then the external key and finally two letters which specified the key list used, in the example, "LP". The Indicator page describes several Indicator methods.

Encipherment

Preliminary instructions

  • Turn the enciphering-deciphering knob so that the letter "C" is up and facing the front.
  • Zeroize the letter counter. This will insure that the enciphered text will be printed in groups of five letters, and will show the exact number of letters enciphered.
  • Align the key wheels in accordance of the external key chosen, for example "PEOPLE".
  • Advance enough paper tape to allow insertion of the indicator by hand.

cipher action read
Choose plain letter: "A" Action drive knowb Read cipher letter: "W"

Procedure

Encipher the following message: 
           ATTACK  AT  DAWN
Proceed as follows:
  1. Turn the indicating disk until the first letter to be enciphered ("A") lines up the with the indicating index, and release the knob.
  2. Turn the drive knob until it locks.
  3. You can read the ciphered letter ("W") on the reproducing disk faced to the indicating index. This operation is mandatory if you run out of paper or ink pad over. Normally, the ciphered letter is automatically printed on the tape.
This procedure is repeated for all letters of the clear text.

Encipher the letter Z between the words of the clear text; i.e., between ATTACK and AT, and between AT and DAWN.

Note that the cipher text has been automatically spaced into five letter groups.

Closure instructions

If the last group contains only 1,2,3, or 4 letters, add enough X's in pencil to make it a complete 5-letter group. In our example, the last group contains only 4 letters, then we add one X to complete the group.

Now write the indicator (“FWPEO PLELP”) at the beginning of the message and repeat them in that order at the end of the message.

Here is the message to be sent : 

           FWPEO PLELP WUHDU AJRJQ TLRGX FWPEO PLELP

Decipherment

Received encrypted message : 
           FWPEO PLELP WUHDU AJRJQ TLRGX  FWPEO PLELP

Preliminary instructions

  • The code clerk must have internal settings on is M-209 which are identical to those used by the code clerk who enciphered the message. The last two letters of the indicator ("LP") specifies the key list used and then the actual internal setting used.
  • Turn the enciphering-deciphering knob so that the letter "D" is up and facing the front.
  • Zeroize the letter counter. This will show the exact number of letters deciphered.
  • Align the key wheels in accordance of the external key extracted from the indicator, in our example "PEOPLE".

decipher action tapes
Choose cipher letter: "W" Action drive knowb Tapes (cipher and plain messages)

Procedure

Decipher the following message (we have removed the indicators): 
          WUHDU AJRJQ TLRGX.
Proceed as follows:
  1. Turn the indicating disk until the first letter to be deciphered ("W") lines up the with the indicating index, and release the knob.
  2. Turn the drive knob until it locks.
  3. You can read the plain letter ("A") on the reproducing disk faced to the indicating index. This operation is mandatory if you run out of paper or ink pad over. Normally, the plain letter is printed on the tape.
This procedure is repeated for all letters of the clear text.

Note that the words of plain text are normally spaced.

Closure instructions

If the last letters of the last group are Xs, they probably correspond to null letters. But to avoid any incertitute we must decipher them.

Here is the plain message after decipherment : 

          ATTACK AT DAWN

A small exercice

If you have understand the above explanation, you can easilly decipher the following message: 
         FWQAH NKELP CJENO WFZOT IKHUU 
         XZJFU DPBDB UWZJV FWQAH NKELP

Duration of traitements

Duration of traitements:
  • Perform Indicator groups: 5 minutes
  • Ciphering (or deciphering): 10 minutes for 150 letters message
  • (un)Drafting, Paste the tape: 5 minutes
Thus one can estimate that the complete transmission of a message between two officers (transportation, encryption, broadcast, deciphering, issue) takes an half an hour in the best case and most likely at least one hour.

References