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FG   Front-Chiffriergerät
Polyalphabetic substitution cipher - under construction


FG was a poly­alphabetic manual cipher device for tactical messages — generally used during a war at the front lines — manufactured around 1946 1 by Zellweger in Uster (Switzerland), for use by the Swiss Army [2]. The device is based on the Jefferson disk, that was invented in 1795 by Thomas Jefferson [4]. It is believed that FG is the abbreviation of Front-Chiffriergerät [2] (Front Cipher Device) or Feld-Gerät (Field Device) [6]. In 1949, it was succeeded by the FG-46 system.

The device consists of a metal cylinder that is 170 mm long and has a diameter of 38 mm. It weights approx. 325 grams and has two cut-away windows through which 25 movable discs, mounted onto a common spindle, are visible.

Each disc contains a (different) scrambled Latin alphabet, and can be set to any of 26 positions. The discs are identified by a single letter of the alphabet that is engraved at its side (A-Z). 2 The discs are mounted onto the spindle in groups of five, with blank metal discs used as separators, allowing coding of five 5-letter groups at once.
  
FG hand cipher device (Front-Chiffriergerät)

The device has a knurled knob at either end. It can be separated by pulling the leftmost knop away from the cylinder. This allows the alphabet discs to be removed from the spindle and re-installed in any of 15,511,210,043,330,985,984,000,000 possible orders (25!) 3 . The order in which the discs are installed from left-to-right, is known as the secret KEY. Up to 25 letters of the message can be encoded or decoded in each pass, making it a very time-consuming operation.

The FG-device is based on the so-called Jefferson Disk, that was invented in 1795 by the 3rd US President Thomas Jefferson. It was independently inventend a century later by Étienne Bazeries, and became known as the Bazeries Cylinder. In the US, the device later evolved into the M-94 (Navy: CSP-488) that was used from 1922 to 1945 [5]. The FG is in fact an improved M-94. It is remarkable that it was introduced in the Swiss Army by the time it was abandonned by the US.

The FG-unit shown here was developed and introduced around 1946. It is believed that only a small number of them were manufactured and used [1]. The device was succeeded by a similar system — known as FG-46 — that entered service in 1949 and was used until at least 1973 [3]. The FG-system was discarded in 1978 and has since been declassified. The FG-device was used alongside the Swiss Enigma machine, and the Swiss Nema which was also made by Zellweger.

  1. According to Rudolf Ritter in [2], the FG-device was built in 1944, but we believe that he errs, as the company name — Zellweger AG, Apparate- und Maschinenfabriken Uster — was first used in 1946.
  2. Only 25 letters of the alphabet are used for identification of the discs. The letter 'I' is omitted.
  3. 25!, or 25 factorial, is the mathematical notation for 25 x 24 x 23 x 22 ... x 2 x 1. This means that there are more than 15 septillion possible keys.

Leather storage bag FG device in leather storage bag Taking the FG device from the leather storage bag FG hand cipher device (Front-Chiffriergerät) FG device with setting tool Setting the alphabet discs Alphabet disc 'Z' Manufacturing model/serial number plate
A
×
A
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Leather storage bag
A
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FG device in leather storage bag
A
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Taking the FG device from the leather storage bag
A
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FG hand cipher device (Front-Chiffriergerät)
A
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FG device with setting tool
A
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Setting the alphabet discs
A
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Alphabet disc 'Z'
A
8 / 8
Manufacturing model/serial number plate

Features
The diagram below provides an overview of the various features of the FG-device. At the center is the metal cylinder with the two cut-out windows. At the left is the spindle with the 25 cipher discs, one of which (Z) has been removed here. Each group of five discs is separated by a metal spacer. Once the order of the discs has been fixed, the spindle with the discs should be inserted into the large open end of the cylinder. The disc setting device is stored inside the hollow spindle. When the device is not in use, it is put in the storage position 1 and stored inside the leather bag.



  1. In the storage position, the discs are inserted in the order of the alphabet (A-Z), with the letter 'A' of each disc lined up with the lower edge of the window.

Operation
First, the device has to be set to the daily (secret) KEY that has been distributed in advance. The KEY consists of a string of 25 unique letters of the Latin alphabet (A-Z minus the letter 'I'). In the example below, we have used the default storage position, which is the order of the alphabet:

A B C D E F G H J K L M N O P Q R S T U V W X Y Z

The core is now removed from the cylinder, after which the 25 alphabet discs are removed from the spindle. Next they are mounted to the spindle in the order dictated by the daily key (shown above). The core is then refitted inside the cylinder. Assume we want to send the message:

ATTACK FRIDAY MORNING AT DAWN

We now rotate the discs so that the message appears at the lower edge of one of the windows of the cylinder. This can be done with the special tool that is stored inside the core of the spindle, and can be pulled-out from the right of the device. It can also be done manually by first moving all discs to the far right, then setting the leftmost discs and shifting it to the left. Next, set the second disc and move it to the left. Continue until the entire message is visible at the lower edge:

Plaintext message 'attack friday morning at dawn'

Now turn the device over by rotating it 180 degrees, so that the other window is at the front, and read the ciphertext message from the lower edge of that window. At the given KEY, this results in:

Ciphertext message 'ejjri wgvjm hcpbe ylyac ogzij'

Write the ciphertext message out on a message sheet in 5-letter groups. If the message is longer than 25 letters, repeat the above procedure until the entire message is processed. Once the message is encrypted completely, pass the message sheet to the radio or telegraph operator and send it to the recipient. The method is reciprocal, so the recipient follows the same procedure.

EJJRI WGVJM HCPBE YLYAC OGZIJ

In the above procedure, there is a fixed relationship between the plain­text letter and the cipher­text letter of each disc. They are always exactly 180° apart (i.e. opposite each other), in order to make the method reciprocal. This is considered a weakness of the system. A side effect of this principle is that a letter will never be encoded into itself, just like on the Enigma cipher machine.

Improved security
The security of the cipher can be improved by rotating the spindle one or more positions — using the knob at the left — before reading-off the ciphertext, but that means that the rotational offset must be part of the key, so that it can be corrected at the receiving end. If, in the example above, the spindle is rotated clockwise by one position (i.e. offset = 1), the ciphertext will become:

QWFND CXWZI SSJXH SAOIY HMFYE

At the receiving end, the operator sets up the above ciphertext at the lower edge of the window and then turns the spindle counterclockwise (in this case by 1 position), before reading-off the plaintext at the other side of the cylinder. Theoretically, the above procedure can be improved further, by rotating the spindle by one or more positions before writing down each ciphertext letter, but as far as we know this was not part of the operational procedure of the Swiss Army.

Pulling-out the setting tool Rotating the spindle Core (spindle with discs) removed from the cylinder Spindle with discs Mounting an alphabet disc on the spindle Disc 'Z' placed on the spindle stack Clear text message: ATACK FRIDAY MORNING AT DAWN Cipher Text Message
B
×
B
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Pulling-out the setting tool
B
2 / 8
Rotating the spindle
B
3 / 8
Core (spindle with discs) removed from the cylinder
B
4 / 8
Spindle with discs
B
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Mounting an alphabet disc on the spindle
B
6 / 8
Disc 'Z' placed on the spindle stack
B
7 / 8
Clear text message: ATACK FRIDAY MORNING AT DAWN
B
8 / 8
Cipher Text Message

Limitations
The FG device described here offers only limited security and can be broken with pencil-and-paper methods if sufficient traffic has been intercepted. As the device was only used for tactical field messages — which will lose their value after several hours — it was thought to provide reasonable security though. The device further has the following disadvantages:

  • Cumbersome
    The device works fine for short messages, but encrypting and decrypting long messages is very time consuming [2]. As the alphabet on each disc is in a scrambled order, it takes time to find the required letter. This is a typical property of mixed-alphabet systems.

  • Message not cleared
    Once a message has been coded and sent, the last 25 letters of the message remain 'stored' in the device. If an enemy captures the device, he might be able to read part of the message by trying all 26 positions of the spindle. This problem was solved in the later FG-46 device, by returning the alphabets to their initial position when the device is closed.

  • KEY not cleared
    The most serious limitation however, is that the KEY (i.e. the order of the discs) is left behind in the device between sessions. There is no method to purge, or zeroize, the KEY. A captured device allows the enemy to decipher all traffic that has been sent on it. When the device was not used for an extended period of time, it was advised to place the discs in the storage position (i.e. the order of the alphabet).

  • Fixed offset
    In the original procedure, there was a fixed offset between the plaintext and ciphertext letter of each disc. The two were at opposite sides, which means that the offset was 13 positions. Although this makes the cipher reciprocal, it introduces a weakness. With the American M-94, on which the FG is based, an arbitrary offset was used instead.
Some of the limitations were overcome in later versions of the device, such as the FG-46 that was introduced a few years later. Other problems could be solved simply by improving the operational procedures. But some limitations remained, as they are inherent to the principle of this cipher.


Alphabets
The table below shows the order of the alphabet on each disc, starting with the letter 'A' and following the disc clockwise when viewing the spindle from the left side of the device.

Disc Alphabet     |     Reverse
A AUMCILFJXDHRQEPSVYOKNGZBWT
B AGZKBNXIOWJCUFQLVRHEYSMTDP
C AYGKHTEDQPZLUICMBFJWVNORXS
D AKQLFODBJGZHNRCUWETXIMBPSY
E ABTNJQPFMOYSCLEVHXRUGZWKDI
F AMVZSUOGPXICWDTHYEQRFBNLJK
G AWSBRNQZVIFKMJLOUTCPEHXGDY
H AGHOUQCFRMKYBXJPZEINWVLSDT
J ARZJEHCFYBPTDSXOIGQKUNMLWV
K AWRUPEFGTLDCZQNJXHOBYKIMVS
L APTIEXUOCKVFSHYNLWJQBDMRZG
M AVQLFIZXKJHBYERTUWPOGDNMSC
N AJPQULWGKVROBEHMFSIDCTZYXN
O ALPNJMFHUTIZOYWQGSDRVCEKXB
P ATOCNHEQDNFWCMJYUVSRKIPLGZ
Q AJGSYRMTQXEDUPCHKNFLIOVWZB
R ALWQETDMZRHGCJIPSXUFCONBKY
S ADXLHCVZGNUEMQJTWPBRIOYSKF
T ACNWLOZVQFJYEGBSXDMTRPHKUI
U AEZORMJNDUPHYCWILBTVSFKGXQ
V AJCZMPVNBTGUYWXIDQKFRHOESL
W AVIHPLXQZNYMGTCKEBOFUJRSWD
X AMBRCVDSJTWUFZGELHKXQINYPO
Y AJEYIGFOPRZLCSMBDWVTQXNUKH
Z AHEJGZLUVISCRQDBNPTOYMXKFW

Total number of combinations
25! 15,511,210,043,330,985,984,000,000


References
  1. Günter Hütter, Front-Chiffriergerät (FG)
    Photographed with kind permission. August 2017.

  2. Rudolf J. Ritter, Das Fernmeldematerial der Scheizerischen Armee seit 1875, 10. Folge
    Communications equipment of the Swiss Army since 1875, Volume 10 (German).
    Bern (Switzerland), Autumn 2002. pp. 29-30.

  3. HAMFU, Front-Chiffriergerät 46 (FG 46)
    Retrieved September 2017.

  4. Wikipedia, Jefferson disk
    Retrieved August 2012.

  5. Wikipedia, M-94
    Retrieved September 2017.

  6. Walter Schmid, Personal correspondence
    October 2017.
Further information
Spindle with discs
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© Crypto Museum. Created: Friday 01 September 2017. Last changed: Sunday, 05 November 2017 - 08:34 CET.
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