Ultra

   The British code name “Ultra” was given to intelligence derived from breaking the codes of Germany’s Enigma machine. Sensitive top secret information decoded from the intercepted messages was referred to as “Ultra top secret.” With the exception of the atom bomb, Ultra was the greatest secret of the war: it provided the Western Allies with critical naval, air, and army information. Ultra intelligence also included detailed information on the German military’s order of battle and military planning on the Eastern Front. The problem for the British government was how to pass this material to the Soviet Union without revealing its source.

   The British government passed Ultra material to Moscow through liaison between the two countries’ intelligence services without revealing that it came from signals intelligence. (American and British generals were misinformed as well; they were told that the material with the fictional code name “Boniface” came from a spy ring inside Germany.) The British may also have passed diluted versions of the material to Moscow through a spy ring operating in Switzerland. None of this saved the Ultra secret from Moscow: Soviet agent John Carincross provided his Soviet case officers with thousands of raw Ultra messages during the war. The Soviet military considered this material “very valuable” in preparing to counter the last German strategic offensive of the war at Kursk in 1943. Carincross’s treachery also allowed Moscow to check the material they were receiving from London and to understand British capabilities to break codes, including their own. The British and American public found out about the Ultra secret 30 years after the Soviets did.