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USA
Strip cipher device
M-94 was a
manual polyalphabetic substitution cipher device
for tactical messages, developed around 1917 1
by US Army major Joseph O. Mauborgne, and manufactured by several companies,
including Doehler, Reeve and Alcoa.
It was introduced to the US Army in 1921
and is based on a 1795 invention by (then) US President
Thomas Jefferson.
It offers limited cryptographic security.
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The M-94 consists of 25 circular discs, each with a different mixed alphabet,
placed on a common spindle. For this reason,
the device is also known as a revolving discs cipher.
It belongs to the class of strip ciphers [2].
Each disc has the 26 letters of the
Latin alphabet engraved in a different (randomized) order at its
circumference, and is identified by the letter that follows the 'A'.
M-94 was manufactured by several companies.
The exact procurement figures are unknown, but ASA cryptologist
William Friedman
reported in 1943, that a total of 9432 units had been made.
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Friedman had always found the M-94 too insecure and had been looking for
several years for a suitable replacement. Finally, in 1942, the M-94
was replaced by the M-209 pin-wheel machine,
also known as CSP-1500,
developed by the Swede Boris Hagelin.
Surprisingly, in 1946, the Swiss
Army introduced introduced a device similar to the M-94,
known as Front-Chiffriergerät or
FG.
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Somewhere between September 1916 and August 1917 [2].
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The device consists of a 6.2 mm spindle in which 25 alphabet discs are installed
in a prescibed order. Each disc is engraved with a differently
scrambled alphabet, identified by a letter (e.g. R) and a number (e.g. 17)
engraved in its right side. The order of the discs is part of the KEY. At the left end
of the spindle is a thick end plate to which a horizontal ruler is attached.
At the right end of the spindle is a thin endplate and a knurled retaining nut
to hold the disc stack together.
The user first installs the discs in the order precribed by the current key.
He then aligns the discs in such a way that the desired text is readable on
the line just above the ruler. He then randomly picks one of the other
horizontal lines to be used as the ciphertext.
This is done by loosening the retaining nut and pulling out the spindle from
the left, after which the thick end plate at the left (with the ruler) can
be moved to a different position.
This text above the ruler is then sent to the recipient by means of courier
or radio (i.e. using morse code signals).
To decipher the text, the recipient has to do the reverse. He first sets up his
M-94 in such a way that the ciphertext is readable just above the ruler. He then
turns the ruler (in the same way as above) to find the only line which makes
sense in English. This is the plaintext.
In reality, the operation is slightly
more complicated, as the order of the discs was usually derived from a key word.
Ideally, the keyword (or phrase) should be 25 characters long (i.e. the number of
discs), but when it is shorter, it is simply repeated.
This procedure is fully described in Section V of the US Army Basic Field Manual -
Signal Communication [B].
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The M-94 has 25 different discs, each of which is identified by a letter (e.g. 'B')
and a number (e.g. '1') engraved in the right side of the disc.
Each disc is engraved with a full 26-letter scrambled alphabet (A-Z).
The letter following the 'A' can also be used as an identifier. It is equal to the
letter engraved at the right side.
The right side of the disc has a rectangular cam that mates with one of the 26
notches of the disc to its right. It prevents the discs from spinning freely.
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| ID | Disc | Alphabet | |
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| B | 1 | ABCEIGDJFVUYMHTQKZOLRXSPWN | |
| C | 2 | ACDEHFIJKTLMOUVYGZNPQXRWSB | |
| D | 3 | ADKOMJUBGEPHSCZINXFYQRTVWL | |
| E | 4 | AEDCBIFGJHLKMRUOQVPTNWYXZS | |
| F | 5 | AFNQUKDOPITJBRHCUSLWEMZVXG | |
| G | 6 | AGPOCIXLURNDYZHWBJSQFKVMET | |
| H | 7 | AHXJEZBNIKPVROGSYDULCFMQTW | |
| I | 8 | AIHPJOBWKCVFZLQERYNSUMGTDX | |
| J | 9 | AJDSKQOIVTZEFHGYUNLPMBXWCR | |
| K | 10 | AKELBDFJGHONMTPRQSVZUXYWIC | |
| L | 11 | ALTMSXVQPNOHUWDIZYCGKRFBEJ | |
| M | 12 | AMNFLHQGCUJTBYPZKXISRDVEWO | |
| N | 13 | ANCJILDHBMKGXUZTSWQYVORPFE | |
| O | 14 | AODWPKJVIUQHZCTXBLEGNYRSMF | |
| P | 15 | APBVHIYKSGUENTCXOWFQDRLJZM | |
| Q | 16 | AQJNUBTGIMWZRVLXCSHDEOKFPY | |
| R | 17 | ARMYOFTHEUSZJXDPCWGQIBKLNV | ← ARMY OF THE US |
| S | 18 | ASDMCNEQBOZPLGVJRKYTFUIWXH | |
| T | 19 | ATOJYLFXNGWHVCMIRBSEKUPDZQ | |
| U | 20 | AUTRZXQLYIOVBPESNHJWMDGFCK | |
| V | 21 | AVNKHRGOXEYBFSJMUDQCLZWTIP | |
| W | 22 | AWVSFDLIEBHKNRJQZGMXPUCOTY | |
| X | 23 | AXKWREVDTUFOYHMLSIQNJCPGBZ | |
| Y | 24 | AYJPXMVKBQWUGLOSTECHNZFRID | ← FRIDAY |
| Z | 25 | AZDNBUKYFWJLVGRCQMPSOEXTKI | |
| | | ↑ | |
| | | ID (letter following the 'A') | |
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The principle behind the M-94 seems to have been (re)invented at several
moments in history [2]:
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- Thomas Jefferson · 1795
Thomas Jefferson (1743-1826) was the 3rd President of the United States
(1781-1809) [7]. Although he is probably not the first inventor,
he developed the so-called Jefferson disk around 1795, which shows great
resemblance to the M-94.
It consists of 36 wooden discs with scrambled alphabets,
mounted on an iron spindle [8].
➤ More
- Etienne Bazeries · 1891
In 1891, Commandant Etienne Bazeries – a noted cipher expert in the French Army –
independently developed a similar device. It consiststed of 20 discs, each with
a scrambled alphabet with 25 letters each.
Bazeries also added the use of a key word (or phrase) for setting the order
of the discs. This principle is also used with the M-94.
- Parker Hitt · 1913
The next invertor was US Army Captain (later: Colonel) Parker Hitt
of the 19th infantry, who was an instructor at the Army Signal School at
Fort Leavenworth, Kansas. When his students were able to easily break the
Signal Corps Cipher Disk, he decided to develop a safer and simpler system for
field use. It consiststed of 10 discs each with a regular and a scrambled
alphabet. His idea later evolved into a flat strip cipher.
- Joseph O. Mauborgne · 1917
Based on Hitt's earlier work, Major (later: Major General) Joseph O. Mauborgne
— Chief of the Engineering and Research Division at Fort Leavenworth, Kansas –
developed the device that later became known as the M-94. He scrambled the
alphabets more thoroughly, so that the least number of repetitions occurred.
He also increased the number of discs to 25.
Mauborgne created 25 test messages that were sent to Riverbank Laboratories
in Geneva (Illinois, USA) and to the Cipher Bureau (MI-8), which were never
solved. 1 His device was therefore accepted as the Army Cipher Device, along
with his special alphabets.
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This was because the messages were very short and contained uncommon
words. Today, this kind of cipher is easily solved.
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Device Army Cipher Device Purpose Tactical text encryption in the field Principle Revolving discs cipher (strip cipher) Designator M-94, CSP-488, CSP-493 Manufacturer see below Users see below Discs 25 (B-Z) Letters 26 (A-Z) Dimensions 120 mm × 36.5 mm Ø Weight 144 g Quantity 9432 ~
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| | Year | Qty | Manufacturer |
| • | 1921 | 2000 | Doehler Die Casting Company, New York |
| • | 1928 | 1094 | Reeve Electrical Company,New York |
| • | ? | 6338 | Alcoa |
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| | Designator | Year | User |
| • | M-94 | 1921 > | US Army |
| • | CSP-488 | 1928 > | US Navy |
| • | CSP-493 | 1939 > | US Coast Guard |
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- Niall McLaughlin, M-94 cipher device - THANKS !
Charlotte (USA), March 2026.
- ASA Historian AS-13, The History of Army Strip Cipher Devices 1
SRH-366. July 1934 - October 1947.
Army Security Agency, Washington (USA), November 1948. pp. 8-26.
- Jerry Proc's crypto pages, M-94 (CSP-488)
Retrieved May 2018.
- Wikipedia, M-94
Retrieved May 2018.
- Louis Kruh, The Evolution of Communications Security Devices
The Army Communicator, Winter 1980. pp. 48-54.
- Christos T., US Military Strip Ciphers
Christos military and intelligence corner, 23 October 2012.
- Wikipedia, Thomas Jefferson
Accessed 6 April 2026.
- Wikipedia, Jefferson disk
Accessed 6 April 2026.
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Declassified 2 March 1987 by Director NSA, per Sec. 3, E.O. 12356.
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