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Quick Overview

Name:              M-94 / CSP-488
Type:              Substitution with Cipher Wheel
Ease of use:       Easy / Intermediate / Difficult
Breaking the code: Easy / Intermediate / Difficult

Background

The M-94 a.k.a. CSP-488 wheel cipher was used by the US Army and US Navy after World War I. The M-94 was first used by the US Army and later adopted by the US Navy as the CSP-488 cipher. It was invented by Colonel Parker Hitt in 1914 and is similar to the wheel ciphers of Thomas Jefferson and Etienne Bazeries, but invented independently. The cipher was declared insecure in 1942 and obsolete in 1943. It was then replaced by the M-209 cipher.

The cipher consists of placing 25 aluminium disks on a shaft. By rotating the disks you can scramble or unscramble messages.

The CSP-488 cylinder, partially disassembled.
Copyright © 2006 Maritime Park Association

The image above shows the CSP-488 wheel cipher partially disassembled. Note that the disks are numbered to allow ordering.

The letters on the disks where fixed and did not vary. The order of placement did vary however. Below is a table of the cipher alphabets used for the M-94 Wheel Cipher:

Disk: Cipher alphabet: 
1:ABCEIGDJFVUYMHTQKZOLRXSPWN
2:ACDEHFIJKTLMOUVYGZNPQXRWSB
3:ADKOMJUBGEPHSCZINXFYQRTVWL
4:AEDCBIFGJHLKMRUOQVPTNWYXZS
5:AFNQUKDOPITJBRHCYSLWEMZVXG
6:AGPOCIXLURNDYZHWBJSQFKVMET
7:AHXJEZBNIKPVROGSYDULCFMQTW
8:AIHPJOBWKCVFZLQERYNSUMGTDX
9:AJDSKQOIVTZEFHGYUNLPMBXWCR
10:AKELBDFJGHONMTPRQSVZUXYWIC
11:ALTMSXVQPNOHUWDIZYCGKRFBEJ
12:AMNFLHQGCUJTBYPZKXISRDVEWO
13:ANCJILDHBMKGXUZTSWQYVORPFE
14:AODWPKJVIUQHZCTXBLEGNYRSMF
15:APBVHIYKSGUENTCXOWFQDRLJZM
16:AQJNUBTGIMWZRVLXCSHDEOKFPY
17:ARMYOFTHEUSZJXDPCWGQIBKLNV
18:ASDMCNEQBOZPLGVJRKYTFUIWXH
19:ATOJYLFXNGWHVCMIRBSEKUPDZQ
20:AUTRZXQLYIOVBPESNHJWMDGFCK
21:AVNKHRGOXEYBFSJMUDQCLZWTIP
22:AWVSFDLIEBHKNRJQZGMXPUCOTY
23:AXKWREVDTUFOYHMLSIQNJCPGBZ
24:AYJPXMVKBQWUGLOSTECHNZFRID
25:AZDNBUHYFWJLVGRCQMPSOEXTKI

Note that the cipher alphabet on row 17 starts with 'ARMYOFTHEUS' or 'ARMY OF THE U.S.', a clear indicator of the intented use for the cipher.

Method

To encode a message, you would start by placing the disks on the spindle in a predefined order. Rotate the disks so that one row shows your plaintext message. Use any other row now showing as the ciphertext message. It is not important that the recipient knows which row you used. If he knows about the order of the disks and constructs the ciphertext he can rotate the entire cylinder to scan for a row that does not show gibberish.

Cryptanalysis

This type of wheel cipher was considered very strong at the time and even deemed undecipherable. Frequency analysis and crib dragging are tools that can be used on longer messages to break it. Fact remains that the cipher is relatively secure and very hard to crack. The only real weakness is the offset between the plaintext row and the ciphertext row, which is the same on all disks.

Example

Feel free to go the online tool for this cipher to try it yourself. The use is pretty straight forward.

Continue to the tool for this cipher or go back to the list of ciphers.