The Fourth Rotor

Introduction

By 1941 Bletchley Park was reading much of the German Navy's traffic. Then, in February 1942, the U-boat fleet introduced a four-rotor Enigma.

The extra rotor, called the Greek wheel, sat on the left and was paired with a thin reflector. The Bombe had been built for three rotors; it took months before new Bombes and captured key material broke the blackout.

Key ideaThe German Navy added a fourth rotor in 1942, multiplying the key space and defeating British decryption for months.