Understanding the Enigma Machine

The Enigma machine was an electro-mechanical rotor cipher device used by Nazi Germany to protect military, diplomatic, and commercial communications before and during World War II. Its apparent complexity led the Germans to believe it was unbreakable — a fatal overconfidence that ultimately contributed to the Allied victory.

Each rotor is a disk with 26 electrical contacts on each side, connected by internal wiring. When a signal enters at one position, it exits at a different position determined by the wiring. This creates a substitution cipher — but one that changes with every keypress as the rotors rotate.

Understanding Enigma requires tracing the complete path of the electrical signal. It passes through eleven stages — forward through the rotors, reflector bounce, then backwards through the rotors.

Like an odometer, the rightmost rotor advances one position with every keypress. When it reaches a specific turnover position, it kicks the middle rotor forward. The middle rotor similarly advances the left rotor.

// Stepping logic with double-step anomaly
function stepRotors(rotors) {
  const rightAtTurnover = TURNOVERS[right.id].includes(right.position);
  const middleAtTurnover = TURNOVERS[middle.id].includes(middle.position);
  
  // Right always steps
  newRight = right.position + 1;
  
  // Middle steps if: right was at turnover OR middle is at turnover
  if (rightAtTurnover || middleAtTurnover) {
    newMiddle = middle.position + 1;
  }
  
  // Left steps only if middle was at turnover
  if (middleAtTurnover) {
    newLeft = left.position + 1;
  }
}

The plugboard sits at the front of the machine. Cables connect pairs of letters, swapping them before the signal enters the rotors AND after it exits. A typical setup used 10 pairs (20 letters swapped), leaving 6 unaffected.

The reflector ensures that if A encrypts to Z, then Z encrypts back to A (with the same settings). This made Enigma convenient — the same machine and settings could both encrypt and decrypt. But it also meant:

The Allies didn't break Enigma by trying all combinations. They exploited weaknesses in both the machine and German operating procedures.