February 9, 1926, the Enigma was introduced into the German Navy as Radio Key C or Radio Cipher C.The workhorse of German military communications, the Enigma was used to encrypt tens of thousands of tactical messages throughout World War II. The number of mathematical permutations for every keystroke is astronomical, yet allied forces were able to read most of the Enigma encrypted messages throughout most of the war as a result of the tireless effort of many Allied cryptologists.  Encryption by substitution, this electromechanical machine used a combination of wired rotors and plugs to change each letter as it is typed. Today, many historians believe that the Allies’ ability to read the Enigma-enciphered messages and act on that information shortened the war by as many as two years, saving thousands of Allied and Axis lives.

Source: NSA.gov

The mathematics behind Enigma (Part One)

Flaw in the Enigma Code (Part Two)