How the Enigma Works

"It is a well-known fact that, now and forever, for purposes of both war and peace, politicians and military men, industrial producers and commercial houses are compelled to keep their correspondence secret. This is a point increasing in importance from day to day. Decisive battles have been lost, by land and by water, in the air and in debating with each other, because the adversary had a better method of keeping his correspondence secret, or a good apparatus being capable of considerably quickening the speed, and multiplying the number of information sent and received. Hand-writing systems will prove insufficient in times of great excitement and hurried proceedings."
~ The Glow-Lamp Ciphering and Deciphering Machine: Enigma

US patent for the Enigma Machine.

A diagram of the Enigma machine. (US Bombe Report)

A diagram of the Enigma's Wheel. (US Bombe Report)

Invented by German engineer Arthur Scherbius, the Enigma machine requires identical keys to decrypt and encrypt a message, of which there are billions. The machine started off as a commercial device but was greatly upgraded for use by the German Army. This revolutionized Germany's cryptographic security, making the Germans confident in sending vital information with Enigma encryption.

The process of creating Enigma messages,

As per the Enigma General Procedure

"Unless other cipher procedures are especially prescribed, all secret communications are to be enciphered on the Enigma. The abbreviated plain text is enciphered directly."
"The message must be as brief as possible, but nevertheless clearly understandable."

Brevity was essential to Enigma messages, as they were transmitted over the radio via Morse Code, and these transmissions had only 50 radio groups of 5 characters each.

"Set in the Cipher of the Day on the Enigma (Schluessel M) according to Cipher Table."
"Prepare the message for sending : a) Addresses and signatures -- only in general radio procedure. b) Time group -- date end serial number to complete it. c) Group count -- radio groups including radio indicator groups."

Additional information was also added to Enigma messages for additional clarity about the message. This part of the transmission was not enciphered.

"For reasons of cipher security, sending Officer and Staff messages without enciphering them according to Enigma General is absolutely forbidden."

The Enigma Machine

The process of creating Enigma messages,

As per the Enigma General Procedure