M-94 Cipher

The M-94 Cipher is a mechanical cipher system developed by the U.S. Army in 1922, using 25 rotating disks, each engraved with a scrambled alphabet. Messages are encoded by arranging the disks in a predetermined order and reading the ciphertext from a selected row. Each disk acts as a simple substitution cipher, but the combination of multiple disks creates a polyalphabetic substitution.

Encryption depends on the order of the disks and the starting alignment of letters. Once the plaintext is aligned on the top row of disks, the ciphertext is read from a different row, determined by the key. Decoding reverses the process by arranging the disks in the same initial order and reading the plaintext row corresponding to the key.

M-94 Cipher: Encoding

Using a default disk setup, encode the plaintext HELLO with the top row aligned to the plaintext. Suppose the key specifies reading the third row for ciphertext:

Plaintext: H  E  L  L  O
Disk Top Row: aligned to plaintext
Read Row (Key=3): the third row on each disk
Cipher Letters:
H → P
E → I
L → P
L → S
O → S

Ciphertext: P  I  P  S  S

Each letter’s substitution is determined by the corresponding disk’s third-row character. Changing the row or disk order alters the ciphertext, producing a polyalphabetic effect.

M-94 Cipher: Decoding

To decode, the recipient arranges the disks in the same initial order and aligns the ciphertext row to the row specified by the key. Then the plaintext is read from the top row of the disks:

Ciphertext: P  I  P  S  S
Disk Order: default
Aligned Row (Key=3)
Read Top Row: H  E  L  L  O

Plaintext: H  E  L  L  O

M-94 Cipher: Notes

The M-94 Cipher demonstrates early 20th-century mechanical polyalphabetic encryption. Its security relies on the secrecy of disk order and the chosen row for reading ciphertext. While straightforward to operate, it provides strong variation across messages due to the multiple disks acting together, which foreshadowed more advanced systems like the M‑209 Cipher.