Appearance and dimensions
- The device is made from aluminum and weighs about 100g.
- The text is die-stamped and filled with black enamel.
- The shaft is about 4.5″ (120mm) long and 1/4″ (6.3mm) in diameter. 1″ is threaded on the right end.
- The discs are 1.5″ (38mm) in diameter and about 3/16″ (4.8mm) thick. They have 26 notches on the left side which allow them to interlock with a projection on the disc on their left.
- The guide rule is attached to he leftmost disc. It has a notch every 5 discs, to indicate a new 5-letter group.
- The rightmost disc, or name plate, is stamped “CIPHER DEVICE / M-94 / SIGNAL CORPS / U.S. ARMY” or “CIPHER DEVICE / TYPE M-94 / SIGNAL CORPS / U.S. ARMY”.
- The Navy used an identical device called the CSP-488. The only difference with the M-94 appears to be the name plate, which is stamped “US NAVY / CSP 488”.
Images courtesy of the National Cryptologic Museum
Alphabets
The 25 alphabets are:
B 1: ABCEIGDJFVUYMHTQKZOLRXSPWN
C 2: ACDEHFIJKTLMOUVYGZNPQXRWSB
D 3: ADKOMJUBGEPHSCZINXFYQRTVWL
E 4: AEDCBIFGJHLKMRUOQVPTNWYXZS
F 5: AFNQUKDOPITJBRHCYSLWEMZVXG
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
S 18: ASDMCNEQBOZPLGVJRKYTFUIWXH
T 19: ATOJYLFXNGWHVCMIRBSEKUPDZQ
U 20: AUTRZXQLYIOVBPESNHJWMDGFCK
V 21: AVNKHRGOXEYBFSJMUDQCLZWTIP
W 22: AWVSFDLIEBHKNRJQZGMXPUCOTY
X 23: AXKWREVDTUFOYHMLSIQNJCPGBZ
Y 24: AYJPXMVKBQWUGLOSTECHNZFRID
Z 25: AZDNBUHYFWJLVGRCQMPSOEXTKI
Notice how R 17’s alphabet starts with “ARMYOFTHEUS”, and Y 24’s contains “FRIDAY”.
The Army and Navy devices used the same alphabets, so Navy device CSP-488 bore the text “ARMYOFTHEUS”.
Cryptology
The M-94 was an easy and convenient device to use. It was secure enough for field use that did not require the message to remain secret for long; the Germans actually broke it during WWII.
If one key is used to encrypt the totality of a long message, the fact that every 25th letter (the 1st and the 26th, the 2nd and the 27th, etc.) is taken from the same alphabet is a vulnerability.
Keyspace
The keyspace is actually quite large: 25! (or 1.55 x 1025) for the order of the discs.
One may also consider that, with the discs in the proper order, the correct line or offset must also be selected on the cylinder. The keyspace is then 25! x 26, or 4.03 x 1026.
Note: this is only valid for one 25-letter section; the total keyspace is larger when more sections are considered as the offset can change even if the key is the same.