A riddle, usually one involving metaphor; in figurative usage, a person or thing that is mysterious, puzzling, or difficult to understand; the word is recorded from the mid 16th century, and comes via Latin from Greek ainigma, from ainissesthai ‘speak allusively’, from ainos ‘fable’. It was famously used by Winston Churchill in a broadcast of 1941 to describe Russia, ‘a riddle wrapped in a mystery inside an enigma’.
Enigma was the name of the German encoding machines used for vital strategic messages in the Second World War; with the assistance of a machine smuggled out of Germany, British cryptographers working at Bletchley Park on the project codenamed Ultra broke the German codes. The story of Enigma remained an official secret until the ban on publication was lifted in 1974. See also Colossus.