Ultra (sometimes capitalised ULTRA) was the name used by the British for intelligence resulting from decryption of encrypted German radio communications in World War II. The term eventually became the standard designation in both Britain and the United States for all intelligence from high-level cryptanalytic sources. The name arose because the code-breaking success was considered more important than the highest security classification available at the time (Most Secret) and so was regarded as being Ultra secret.

Much of the German cipher traffic was encrypted on the Enigma machine, hence the term "Ultra" has often been used almost synonymously with "Enigma decrypts". However, in terms of the intelligence value, Lorenz SZ 40/42 decrypts were more important.

Until the name "Ultra" was adopted, there were several cryptonyms for intelligence from this source, including Boniface. For some time thereafter, "Ultra" was used only for intelligence from this channel.

F.W. Winterbotham, in The Ultra Secret (1974), quotes the western Supreme Allied Commander, Dwight D. Eisenhower, as at war's end describing Ultra as having been "decisive" to Allied victory in World War II.

Sources of ULTRA intelligence

German sources

ULTRA intelligence was largely derived from German cipher traffic. These messages were mostly generated on several variants of an electro-mechanical rotor machine called "Enigma." The Enigma machine was widely thought to be in practice unbreakable in the 1920s, when a variant of the commercial Model D was first used by the German Navy. The German Army, Navy, Air Force, Nazi party, Gestapo, and German diplomats all used Enigma machines, but there were several variants (e.g., the Abwehr used a four-rotor machine without a plugboard, and Naval Enigma used different key management from that of the Army or Air Force, making its traffic far more difficult to cryptanalyse). Each variant required different cryptanalytic treatment. The commercial versions were not as secure; Dilly Knox, of GC&CS, is said to have broken one during the 1920s.

Polish reconstructions of the Enigma machine and techniques for decrypting ciphers produced on it were presented as a gift by Polish Military Intelligence to their French and British allies in Warsaw on July 26, 1939, just five weeks before the outbreak of World War II. It was not a moment too soon. Former Bletchley Park mathematician-cryptologist Gordon Welchman has written: "Ultra would never have gotten off the ground if we had not learned from the Poles, in the nick of time, the details both of the German military... Enigma machine, and of the operating procedures that were in use."