Editing Ultra (cryptography) (section)

==Safeguarding of sources==
The Allies were seriously concerned with the prospect of the Axis command finding out that they had broken into the Enigma traffic. The British were more disciplined about such measures than the Americans, and this difference was a source of friction between them.{{sfn|Winterbotham|1974|pp=86–91}}<ref>{{citation | title = Bletchley park archives: October 1943 : Not all our own way | url = http://www.bletchleypark.org.uk/content/archive/oct1943.rhtm | access-date = 9 February 2011 | archive-date = 2 April 2013 | archive-url = https://web.archive.org/web/20130402205420/http://www.bletchleypark.org.uk/content/archive/oct1943.rhtm | url-status = dead }}</ref>

To disguise the source of the intelligence for the Allied attacks on Axis supply ships bound for North Africa, "spotter" submarines and aircraft were sent to search for Axis ships. These searchers or their radio transmissions were observed by the Axis forces, who concluded their ships were being found by conventional reconnaissance. They suspected that there were some 400 Allied submarines in the Mediterranean and a huge fleet of reconnaissance aircraft on [[Malta]]. In fact, there were only 25 submarines and at times as few as three aircraft.{{sfn|Hinsley|1993a}}

This procedure also helped conceal the intelligence source from Allied personnel, who might give away the secret by careless talk, or under interrogation if captured. Along with the search mission that would find the Axis ships, two or three additional search missions would be sent out to other areas, so that crews would not begin to wonder why a single mission found the Axis ships every time.

Other deceptive means were used. On one occasion, a convoy of five ships sailed from [[Naples]] to North Africa with essential supplies at a critical moment in the North African fighting. There was no time to have the ships properly spotted beforehand. The decision to attack solely on Ultra intelligence went directly to Churchill. The ships were all sunk by an attack "out of the blue", arousing German suspicions of a security breach. To distract the Germans from the idea of a signals breach (such as Ultra), the Allies sent a radio message to a fictitious spy in Naples, congratulating him for this success. According to some sources the Germans decrypted this message and believed it.{{sfn|Momsen|2007}}

In the Battle of the Atlantic,  the precautions were taken to the extreme. In most cases where the Allies knew from intercepts the location of a U-boat in mid-Atlantic, the U-boat was not attacked immediately, until a "cover story" could be arranged. For example, a search plane might be "fortunate enough" to sight the U-boat, thus explaining the Allied attack.

Some Germans had suspicions that all was not right with Enigma. Admiral [[Karl Dönitz]] received reports of "impossible" encounters between U-boats and enemy vessels which made him suspect some compromise of his communications. In one instance, three U-boats met at a tiny island in the [[Caribbean Sea]], and a British destroyer promptly showed up. The U-boats escaped and reported what had happened. Dönitz immediately asked for a review of Enigma's security. The analysis suggested that the signals problem, if there was one, was not due to the Enigma itself. Dönitz had the settings book changed anyway, blacking out Bletchley Park for a period. However, the evidence was never enough to truly convince him that Naval Enigma was being read by the Allies. The more so, since ''[[B-Dienst]]'', his own codebreaking group, had partially broken Royal Navy traffic (including its convoy codes early in the war),{{sfn|Mallmann-Showell|2003}} and supplied enough information to support the idea that the Allies were unable to read Naval Enigma.{{efn|Coincidentally, German success in this respect almost exactly matched in time an Allied blackout from Naval Enigma.}}

By 1945, most German Enigma traffic could be decrypted within a day or two, yet the Germans remained confident of its security.{{sfn|Ferris|2005|p=165}}