An Enigma machine is any of a family of related electro-mechanical rotor machines used for the encryption and decryption of secret messages. The first Enigma was invented by German engineer Arthur Scherbius at the end of World War I. This model and its variants were used commercially from the early 1920s, and adopted by military and government services of several countries — most notably by Nazi Germany before and during World War II.

A range of Enigma models were produced, but the German military model, the Wehrmacht Enigma, is the version most commonly discussed. The machine has become well-known because Allied codebreakers were able to decrypt a vast number of messages which had been enciphered using the Enigma. The intelligence gleaned from this source, codenamed ULTRA by the British, was a substantial aid to the Allied war effort.

The exact influence of ULTRA on the course of the war is debated; an oft-repeated assessment is that decryption of German ciphers hastened the end of the European war by two years. Though the Enigma cipher had cryptographic weaknesses, in practice it was only in combination with other factors (procedural flaws, operator mistakes, occasional captured hardware and key tables, etc.) that those weaknesses allowed Allied cryptographers to cryptanalyze so many messages.

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