NEMA — Swiss Rotor Machine
Switzerland’s new machine: separate the wiring from the wheels that decide when it moves
Obsolete Swiss military rotor machine. The demo fixes one historical wheel configuration; the 17,576-keypress state period is not its key space.
Fixed military configuration: A/12, B/13, C/14, D/15 (left to right); red wheel 22/1. Each run starts at these windows.
Historical wiring and military stepping; a fixed configuration, not all NEMA variants. The demo uppercases letters, removes accents, and discards spaces, punctuation, and digits.
| Step | In | Out | Windows |
|---|
Why This Matters
NEMA shows that a rotor cipher has two designs to understand: the permutations inside its electrical wheels and the mechanism that moves them. Switzerland gave those jobs separate wheels. The resulting machine is more instructive than an Enigma with an extra rotor: its stepping decisions come from two mechanical drive systems.
NEMA, short for NEue MAschine (“new machine”), replaced the Swiss Army’s commercial Enigma K after its traffic had proved vulnerable. Development began during WWII; the Swiss Army issued NEMA operating instructions dated 30 April 1947. Zellweger manufactured the machine in Uster. Training and war-reserve versions used different stepping wheels; diplomatic machines were another variant.
The 1947 date identifies the postwar machine, rather than its wartime design work. The military configuration in this exhibit is deliberately fixed so that visitors can follow every wheel position.
Current enters a static entry wheel, passes through four cipher rotors, reaches a rotating reflector, and returns through the rotors in reverse. There is no plugboard. The reflector makes the operation reciprocal: reset to the same key and run the ciphertext to recover the plaintext.
Each cipher wheel has a separate mechanical stepping wheel. Notches determine which neighbouring wheels can move, and a second track on the red drive wheel gates a slower transmission. Decisions are made from the old positions; eligible wheels then step backwards before the letter is transformed. The reflector can step too.
Fixed wheels (left → right): A/12 B/13 C/14 D/15 Red wheel: 22/1 Start windows: MQQPZQSOEV Input: AAAAAAAAAAAAAAAAAAAAAAAAAAA Output: HRSBVOOTZUCRWLMGRMGVWYWOVNF
This example is checked against the published rmsk2 NEMA test, whose reference ciphertext came from Geoff Sullivan’s simulator. Decryption must start at MQQPZQSOEV, not at the final windows printed by the demo.
The military mechanism has a maximum state period of 17,576 keypresses. A reflector also prevents a letter from encrypting to itself. Those structures constrain a cryptanalyst’s search even though the number of possible initial settings is much larger than the period.
This exhibit does not assert a documented operational break of Swiss NEMA traffic. It presents an obsolete machine and makes no claim of modern security. A message, key schedule, and operating procedure must be evaluated together.
The demo implements the historical A–D wiring, the military 12–15 stepping wheels, red wheel 22/1, moving reflector, and ten lower-row window positions. It omits alternative wheel selections, training and diplomatic variants, number-shift conventions, and message-key procedures. It is a reconstruction for study, not a tool for protecting messages.
| Exhibit | 161 of 167 |
| Origin | Swiss Army · manufactured by Zellweger, Uster |
| Year | 1947 |
| Family | Electromechanical rotor cipher |
| Key | Wheel selection, pairing, and ten starting positions |
| Trust | Historical Only |