The Enigma machine was an electromechanical cipher device developed in the early 1920s by German engineer Arthur Scherbius Originally intended for commercial and diplomatic encryption, it was later adopted by the German military and became central to secure wartime communications during WWII. By the outbreak of the war, the device had evolved into one of the most sophisticated encryption systems available for mass deployment, with roughly 40,000 units eventually produced and distributed across all branches of the German military. The cipher's eventual decryption was a multinational effort. Polish cryptographers made early breakthroughs and developed tools such as the bomba kryptologiczna. These efforts were expanded upon at Bletchley Park by Allied cryptographers including Alan Turing, leading to the design of the British Bombe. Cracking Enigma was a pivotal intelligence victory that helped shorten the war.
Rotors were the core components of the Enigma machine's encryption system. Each rotor contained a scrambled wiring of the alphabet, internally hardwired in a complex tangle that mapped one letter to another. When a key was pressed, the electrical signal would pass through a series of these rotors, undergoing a different substitution at each stage. Only the rightmost rotor (Rotor I) advanced with every keypress. The middle rotor advanced only when a notch on the first rotor allowed a ratchet and pawl mechanism to engage. Similarly, the leftmost rotor advanced only when triggered by the notch on the second. This mechanism led to a characteristic called "double stepping", where the middle rotor would sometimes rotate on two consecutive keypresses. The mechanical design ensured that the encryption changed continuously, creating a polyalphabetic cipher with an enormous number of possible configurations, especially when multiple rotors were combined with varying initial positions and ring settings.
The reflector returns the signal back through the rotors along a different path. It ensures encryption is reciprocal - the same settings will both encrypt and decrypt. This design choice, however, meant a letter could never encrypt to itself. This may seem to enhance security, but it actually reduces the search space and was a feature exploited by codebreakers.
The plugboard (Steckerbrett) allowed pairs of letters to be swapped before and after rotor encryption, vastly increasing the possible configurations. Operators connected cables between letter pairs, adding another configurable layer of complexity to the cipher.