Editing Cryptanalysis of the Enigma (section)

===German suspicions===
The German navy was concerned that Enigma could be compromised. They printed key schedules in water-soluble inks so that they could not be salvaged.<ref name="Kahn 1991 201">{{harvnb|Kahn|1991|p=201}}</ref> They policed their operators and disciplined them when they made errors that could compromise the cipher.<ref>{{harvnb|Kahn|1991|pp=45–46}}</ref> The navy minimised its exposure. For example, ships that might be captured or run aground did not carry Enigma machines. When ships were lost in circumstances where the enemy might salvage them, the Germans investigated.<ref>{{harvnb|Kahn|1991|p=199}}</ref> After investigating some losses in 1940, Germany changed some message indicators.<ref>{{harvnb|Kahn|1991|p=200}}</ref>

In April 1940, the British [[Battles of Narvik|sank eight German destroyers in Norway]]. The Germans concluded that it was unlikely that the British were reading Enigma.<ref name="Kahn 1991 201"/>

In May 1941, the British deciphered some messages that gave the location of some supply ships for the [[German battleship Bismarck|battleship ''Bismarck'']] and the [[German cruiser Prinz Eugen|cruiser ''Prinz Eugen'']]. As part of the ''[[Operation Rheinübung]]'' commerce raid, the Germans had assigned five tankers, two supply ships, and two scouts to support the warships. After the ''Bismarck'' was sunk, the British directed its forces to sink the supporting ships ''Belchen'', ''Esso Hamburg'', ''Egerland'', and some others. The Admiralty specifically did not target the tanker ''Gedania'' and the scout ''Gonzenheim'', figuring that sinking so many ships within one week would indicate to Germany that Britain was reading Enigma. However, by chance, British forces found those two ships and sank them.<ref>{{harvnb|Kahn|1991|pp=201–202}}</ref> The Germans investigated, but concluded Enigma had not been breached by either seizures or brute-force cryptanalysis. Nevertheless, the Germans took some steps to make Enigma more secure. Grid locations (an encoded latitude and longitude) were further disguised using digraph tables and a numeric offset.<ref>{{harvnb|Kahn|1991|pp=204–205}}</ref> The U-boats were given their own network, ''Triton'', to minimise the chance of a cryptanalytic attack.

In August 1941, the British captured {{GS|U-570||2}}. The Germans concluded the crew would have destroyed the important documents, so the cipher was safe. Even if the British had captured the materials intact and could read Enigma, the British would lose that ability when the keys changed on 1 November.<ref name="Kahn 1991 206">{{harvnb|Kahn|1991|p=206}}</ref>

Although Germany realised that convoys were avoiding its [[wolfpack (naval tactic)|wolfpacks]], it did not attribute that ability to reading Enigma traffic. Instead, [[Karl Dönitz|Dönitz]] thought that Britain was using radar and direction finding.<ref name="Kahn 1991 206"/> The ''Kriegsmarine'' continued to increase the number of networks to avoid superimposition attacks on Enigma. At the beginning of 1943, the ''Kriegsmarine'' had 13 networks.<ref>{{harvnb|Kahn|1991|p=209}}</ref>

The ''Kriegsmarine'' also improved the Enigma. On 1 February 1942, it started using the four-rotor Enigma.<ref>{{harvnb|Kahn|1991|p=210}}</ref> The improved security meant that convoys no longer had as much information about the whereabouts of wolfpacks, and were therefore less able to avoid areas where they would be attacked. The increased success of wolfpack attacks following the strengthening of the encryption might have given the Germans a clue that the previous Enigma codes had been broken. However, that recognition did not happen because other things changed at the same time: the United States had entered the war, and Dönitz had sent U-boats to raid the US East Coast, where there were many easy targets.<ref>{{harvnb|Kahn|1991|pp=210–211}}</ref>

In early 1943, Dönitz was worried that the Allies were reading Enigma. Germany's own cryptanalysis of Allied communications showed surprising accuracy in its estimates of wolfpack sizes. It was concluded, however, that Allied direction finding was the source. The Germans also recovered a [[cavity magnetron]], used to generate radar waves, from a downed British bomber. The conclusion was that the Enigma was secure. The Germans were still suspicious, so each submarine got its own key net in June 1944.<ref>{{harvnb|Kahn|1991|pp=260–262}}</ref>

By 1945, almost all German Enigma traffic (Wehrmacht military; comprising the [[German Army (Wehrmacht)|Heer]], Kriegsmarine, and Luftwaffe; and German intelligence and security services like the Abwehr, SD, etc.) could be decrypted within a day or two, yet the Germans remained confident of its security.<ref>{{Harvnb|Ferris|2005|p=165}}</ref> They openly discussed their plans and movements, handing the Allies huge amounts of information, not all of which was used effectively. For example, Rommel's actions at [[Battle of Kasserine Pass|Kasserine Pass]] were clearly foreshadowed in decrypted Enigma traffic, but the Americans did not properly appreciate the information.{{citation needed|date=August 2012}}

After the war, Allied [[TICOM]] project teams found and detained a considerable number of German cryptographic personnel.{{sfn|Rezabek|2017}} Among the things learned was that German cryptographers, at least, understood very well that Enigma messages might be read; they knew Enigma was not unbreakable.{{sfn|Huttenhain|Fricke|1945|pp=4,5}} They just found it impossible to imagine anyone going to the immense effort required.<ref>{{Harvnb|Bamford|2001|p=17}}</ref> When Abwehr personnel who had worked on [[Fish (cryptography)|Fish cryptography]] and Russian traffic were interned at [[Rosenheim]] around May 1945, they were not at all surprised that Enigma had been broken,{{cn|date=January 2025}}{{dubious|date=January 2025}}<!--The revelation about the existence of Enigma was not made until 1973--> only that someone had mustered all the resources in time to actually do it. Admiral [[Karl Dönitz|Dönitz]] had been advised that a cryptanalytic attack was the least likely of all security problems.{{citation needed|date=January 2020}}