The Enigma I – a Cipher Machine for the German Wehrmacht

Enigma I cipher machine Koblenz — A detail of the Enigma I cipher machine with, from top to bottom, the rotor housing, the light field and the key board

The Enigma I cipher machine, also known as the “Wehrmacht” Enigma, was widely used by the German Military and government to send encrypted messages over radio channels before and during World War Two. Enigma’s encryption was considered so secure, that it was trusted to encipher much of Nazi Germany’s vital and top-secret communications during the war.

With Nazi Germany’s focus on fast moving mobile forces in the European theater of war and later the Eastern campaign, the army had to rely heavily on radio communications for commanding and coordinating its troops. Because radio communications could be easily intercepted, the Enigma machine provided an ideal and compact solution to protect its messages from the enemy.

Early Enigma models

In Februari 1918, the German firm Scherbius & Ritter patented the ideas of engineer Arthur Scherbius for a cipher machine. From 1923 the company began marketing the finished product under the brand name “Enigma“. While early models, like the Enigma A and B (1924) were mainly used commercially, the machine was soon adopted by military and government services of several countries under which Germany.

Use in the German Army

Enigma I cipher machine plug board — Detail of the plug board of the Enigma I Wehrmacht cipher machine

From 1926 the Reichsmarine (German Navy) adopted the Enigma as “Funkschlüssel C” (Eng: “Radio cipher C”) and the Wehrmacht used the Enigma G starting from 1928. In 1930, the army introduced the “Enigma I” which had an additional plug board that allowed to swap pairs of letters, greatly increasing the cryptographic strength compared to the earlier commercial models. At the start of WW2 all branches of the German Army had adopted the use of the Enigma machine.

By the end of war, the Enigma cipher machines had been improved several times.

Enigma I cipher machine rotor housing — Detail of the rotor housing of the Enigma I cipher machine

Operation

Enigma machine schematic drawing — Diagram of the Enigma cipher machine’s inner working

The message to be transmitted by the sender is entered into the keyboard as plain text. A chi letter lights up for each clear letter in the lamp field above the keyboard and the first reel is moved up one level. The chi letters are written down and transmitted to the recipient.
The receiver, with the rollers in the same starting position as on the sender’s machine, enters these letters on the keyboard and the plain text letters are displayed in the lamp field.

Instead of the light bulbs, a remote viewing device could also be connected or an Enigma printer could be attached, although the latter was unreliable and rarely used.

Enigma remote viewing device — A remote viewing device of the Enigma cipher machine

The Enigma clock allows the plug exchange alphabet to be changed quickly.

Breaking Enigma

Enigma I cipher machine rotors — Detail of the Enigma’s rotors

While many improvements were made to the Enigma encryption over the years, setting back some of the attempts to decrypt the code, they did not prevent Poland from cracking it as early as December 1932 and reading messages prior to and into the war. Thanks to the Poles sharing their knowledge about the Enigma code the Allies were able to intercept important German communications and use it in their advantage during the course of the war.