Ludwig Stummel

Stummel was notable for being the person responsible for the cryptographical security of the Enigma cipher machine, and security of Kriegsmarine radio signal security and own key processes for most of the latter half of World War II, as Group Director of Naval Intelligence, Naval War Command, OKM.[2]

Stummels senior officer at the Naval Command was Erhard Maertens. In 1940. Stummel was commanded to Maertens to conduct an investigation into a number of unexplained losses of shipping. One of these was Patrol Boat 805, which was lost under obscure circumstances in Heligoland Bight. The U-33 U-boat then captained by Hans-Wilhelm von Dresky, was sunk in February 1940 in shallow water in the Firth of Clyde. The third event occurred four days later, when the German tanker, the Altmark, a supply ship for the German cruiser Admiral Graf Spee, that was carrying British prisoners of war, was attacked in the neutral waters of the Norwegian fjord, and boarded by British sailors, who shouted, The navy's here, and subsequently freed the prisoners.[1] These three events taken together were enough for Karl Dönitz, who ordered the investigation.

Stummel spent several weeks on the investigations, speaking principally to the cryptanalysts and cipher specialists, but did not conclude that a leak had occurred in any of those cases. However, as a precaution, the Indicator for weather messages was changed, fake messages were increased in number, and the indicator for officer-grade messages was changed to the indicator for general-grade messages. In his report, Stummel stated in the conclusions, that:

• The many components of the Enigma systems offered security even if some components were lost to the enemy.
• Water soluble in protected most important documents.
• Solution could only be achieved through superimposition. But the frequent changes of keys, more frequent, he boasted, than in the enemy's systems precluded this.[1]

In early May 1943, Dönitz fired Erhard Maertens, for reasons that went beyond his fears about crypto-security and sent him to run the Kriegsmarine shipyard in Kiel.[4] Stummel, who was Maertens chief of staff, was promoted to rear admiral, to take his place. Stummel maintained that the Enigma:

had on the basis of repeated and thorough investigations, proved itself up to the present as unbreakable and military resistant.

Dönitz apparently believed him, as in June he was telling the Japanese ambassador, Hiroshi Ōshima, who was himself one of the main sources of communication intelligence for the allies during the war, that the U-boat losses were due to a new Allied Direction finding system.[4]

Despite the claims, and perhaps nagging doubt that the Enigma machine cipher was insecure, in 1944, Stummel convinced Dönitz to carry the Kriegsmarine basic cryptography principles to its logical conclusion. By subdividing the navies cryptography map into as many Enigma key nets as necessary, Stummel tried to ensure that the number of messages for a specific key was reduced, thereby increase security of own processes.[4] As the volume of traffic increased from the interwar period, when it was one key net, to separate to separate key nets as the message volume increased, to the addition of a U-boat net and many others by 1943, when traffic average 2563 radio messages a day. By then, Stummel decided to give each U-boat its own key.[4]

Individual keys were issued to some U-boat shortly after the Normandy landings on 6 June 1944. They began to be widely used by late 1944 and early 1945 and were carrying all the operational traffic of the Kriegsmarine High Command. In the early months of 1945 Dönitz told Hitler that Allied knowledge of wolfpacks came from radar and espionage. By that time Stummel had been removed from the navy High Command. However, his faith in individual keys was justified, as they were only broken for brief periods.[4]