Enigma is the cipher being used by the nazi-german military in world war 2. Invented as a black-box design by a man named Arthur Scherbius, the enigma was believed to be secure by the germans.
A polish man named Marian Rejewski broke enigma and the polish government gave his work over to the british government before poland was invaded by germany and russia. The british built electro-mechanical computers called "bombes" to break enigma.
It is said that because of the breaking of the enigma that the cruel war was shortened by 2 years.