SIGABA (ECM Mark II)
The American WWII rotor machine the Axis never broke — fifteen rotors, irregular stepping.
Never publicly broken by cryptanalysis in wartime service; obsolete today.
Why This Matters
SIGABA used separate control and index rotor banks to decide which cipher rotors advanced. Individual stepping patterns can repeat; the design does not promise that no pattern occurs twice. Its operational record differs from a mathematical proof of security, and standard Enigma stepping itself includes a double-step rather than being a perfect odometer.
In the public record, this system was never publicly broken by cryptanalysis in wartime service. That historical record does not establish security today; this obsolete system is presented for historical study.
SIGABA was used for important American military communications during World War II. Its five cipher rotors were controlled by two additional banks of five rotors. No successful Axis cryptanalytic reading is established in the public record cited here. Numbers describing possible settings are not automatically equivalent to bits of attack resistance.
Three banks of five rotors. The cipher bank performs the actual substitution. The control bank steps regularly, odometer-fashion. The control bank's outputs feed the index bank — five rotors whose contacts encode the question ‘which of the five cipher rotors steps next?’ The result: any given keystroke advances 1–4 of the cipher rotors in a pattern that depends on the entire current rotor state. The simulation in this exhibit models that pseudorandom advance and the resulting per-keystroke substitution.
SIGABA differs from Enigma in both stepping and signal path, including the absence of an Enigma-style reflector. Those differences matter to attacks. They do not justify saying that analysis has “nothing to grip,” or that absence of a wartime break proves a machine secure for later use.
| Concept from SIGABA (ECM Mark II) | Modern Evolution |
|---|---|
| Make the schedule itself secret | Same lesson reappears in modern stream ciphers (key schedule must be cryptographically strong) |
| No reflector → no constraint | Enigma's elegant reciprocity was also its single biggest exploitable property |
| Compartmented design teams | Friedman and Safford collaborated across services — the rare case of US inter-service cryptographic cooperation actually working |
Ciphertext: LBVIICVKBEYCICOAV
Hint: the rotor seed is the machine's own name.
| Era | WWII · 1940 |
| Trust | Historical Only |
| Origin | William F. Friedman & Frank Rowlett (US Army SIS); Laurance Safford (US Navy OP-20-G) |
| Year | Approved 1940; in service 1941–1959 |
| Key Type | 15 rotors arranged as 3 banks of 5 (cipher × 5, control × 5, index × 5) |
| Broken By | Never broken in operational service; declassified 1996 |
| Modern Lesson | Pseudorandom rotor stepping defeats the regularities that broke Enigma |