In September, 1942, British interrogators recorded the following exchange between two German prisoners of war, on the security or the Navy codes:
U-Boat radioman: We have often cracked the British code, during the Norwegian campaign, for example. But they will never crack the code we have in the Navy. It’s absolutely impossible to crack.
Nazi commando: Everyone says that or their own code.
U-Boat radioman: What? They can’t crack it.
Nazi commando: Oh, that’s just one or those silly ideas people have.
U-Boat radioman: No!
Historical research has confirmed that the UBoat radioman’s staunch faith in his code mirrored that or the German Navy High Command — a confidence as misplaced as it was unshakable.
The recent declassification or extensive intelligence materials has opened a new dimension to the study of the Battle of the Atlantic by underscoring the role or Allied signal intelligence in the defeat or German submarines.
The German Navy’s tested but reaffirmed faith in its codes, based on the “Enigma-M” machine, therefore commands particular attention.
“OUr ciphers were checked and rechecked to make sure they were unbreakable, and each time they were voted as impossible for the enemy to decipher,• recalled Admiral Karl Doenitz, the WW II Commander in Chief of Submarines. Such a determination was made by the Radio Renaissance Section of the German Navy Communications Service. Employing between 5,000 and 6,000 personnel during the war, this section monitored Allied signals and attacked the Allies naval codes. Yet it was a much smaller staff within this section which was concerned with the protection of German naval codes and whose assessments may have been more important for the war’s outcome. One such assessment made in October, 1941 appears to have been most critical.
Doenitz, sensitive to his dependence on radio communication to direct his “wolf packs”, had worried in April, apparently needlessly, that his ciphers might have been compromised. His fears were premature. But on 9 Hay, with the seizure of an “Enigma H-3” encoding machine when the U-110 was boarded by the British, along with the capture of cipher documents from German weather ships, the British were able by September to achieve a decode of German submarine messages 41 hours after their transmission. By October this was improved to 26 hours.
Aware that Allied convoys were beginning to elude his U-boat patrol lines, Doenitz in early September broached the possibility of having a compromised code to the Naval War Staff. The War Staff reassured Doenitz on 19 September that “a penetration of our cipher does not come into question.” However, “additional security measures were authorized and full investigation into cipher security was begun.”
The investigation assumed a great urgency when Doenitz on 28 September, learned that a British submarine had surprised U-67 and U-111 at a refueling rendezvous off the Cape Verde Islands. Doenitz told the Naval War Staff “that either our ciphers have been compromised or treason is involved.” On 24 October the investigation was completed and a Report sent to Doenitz. In this Report the Staff first collected the evidence which indicated a possible British reading of the German naval codes. Then each instance was examined to determine the most probable intelligence source that produced it. Four cases were selected as the basis for the examination:
- First was an intercepted British message of 6 September 1941 which had a “surprising representation” of U-boats deployed in southern Atlantic waters, when the subs were still in transit and had not broken radio silence. In analyzing this case, other British summaries of Uboat dispositions for several other dates, were compared. These summaries were attributable to allied radio direction-finding and to sightings by ships and aircraft of the Allies. And, since the three U-boats involved had radioed their positions when west of Spain and one had sunk a Dutch tanker enroute, it appeared reasonable that the British had enough information to reveal their presence and indicate their movement — without a deciphering of the German code.
- SeCond was the unexpected appearance of a British submarine at the U-boat refueling rendezvous — far from nowhere — on 27 September. The War Staff felt that the use of a single British sub against a planned refueling rendezvous, for all three boats headed into the South Atlantic, hardly indicated a “trap.” More likely, the Staff felt the British sub was on a reconnaissance of potential resupply points for Uboats headed south. The investigators did note that U-111 bad sent a message arranging the rendezvous and in it mentioned the site of the refueling — Tarafo Bay in the Cape Verde Islands. In passing, the staff mentioned that far too many naval commands enjoyed access to “Triton,” the new cipher just coming into use for the Atlantic U boats, and proposed removing most of the offices getting U-boat radio traffic.
- Third, with the capture of the German supply ship GEDANIA and a British claim to have captured a U-boat (the U-570) intact, implying its coding machine and related documents as well, the Germans were faced with the possibility of British possession of an Enigma machine — permitting them a “theoretical” simultaneous reading of encoded German messages. But the cryptographers stressed that in both cases the radio personnel had the time to destroy at least the most important code books before capture. Without these, and with the daily changes in key words, it was concluded, the cipher should still be safe. Doenitz was apprised of the 0-570’s capture by intercepts of messages from the British aircraft and armed trawler that brought the boat into Iceland. The submarine had been surrendered by an inexperienced captain and a demoralized crew with only two months of training. British press accounts indicated that the U-boat’s crew had sufficient time to destroy the most important materials. In any case, the code table on board U-570 extended only through the end of October 1941. Thus, the staff concluded that if only one of the principle elements of the coding system was not seized intact, “decryption by the enemy would only be possible at considerable cost of trial-and-error, with little probability of success.” But the capture of the U-570, the Staff felt, rendered more urgent the planned changeover to a new cipher system — Triton. Still, transmissions of German messages in the old code, despite the Staff’s optimism that it hadn’t been cracked, were readable in London by the next afternoon. Moreover, Doenitz bad some concrete evidence of this, as all the 15 allied convoys in October successfully evaded U-boat patrol lines. (The captured U-570 posed a dual problem for the British — how to exploit the intelligence opportunity to the utmost while somehow persuading the Germans that a U-boat’s surrender had not compromised their codes. Doenitz on 21 December, 1941, in a telegram to the Naval War Starr said that a “special source” had confirmed the destruction of U-570’s cipher material prior to capture. This information might have come from Uboat prisoners of war in letters sent home — in prearranged codes. The possibility, however, cannot be discounted that British intelligence allowed this information to pass or even provided it to allay German fears of compromised ciphers.)
- Fourth, a decode of a British directive of 17 September, regulating the flow of shipping on either side of St. Paul Rocks at the junction of the North and South Atlantic, seemed to indicate a knowledge of German submarine patrol patterns. But, it was felt, British knowledge of U-boat dispositions could be traced to such conventional intelligence sources as sighting reports by the Allies and neutral shipping, aerial reconnaissance of the departure and arrival routes used by German submarines, and the “excellent efficiency of British radio direction-finding,” all combined with the “most exacting and businesslike staff work” of the British. Yet the German circle with access to the codes was far too large “to preclude the possibility of betrayal.”
The Naval War Staff were, moreover, influenced in their evaluation by the known activities of the widespread Allied espionage networks. Even while their Report on the cryptographic situation was being prepared, there was a great increase of monitored radio traffic of Allied agents, whenever a German warship passed through the Kattegat. Allied espionage, moreover, was being facilitated by serious security leaks within the Wehrmacht command and communications centers. Thus, with the wide dispersion of German naval stations throughout occupied Europe, together with the unconvincing evidence that the British were actually decrypting submarine traffic, the Naval War Staff’s lessened concern with physical compromise of their Enigma system becomes more plausible.
Confidence in the new “Triton” cipher -operational on some vessels by 5 October — plus restricting access to “Triton” to only eight naval commands plus six U-boat flotillas paid off. “Triton” restored German cryptographic security and defied solution by Allied code breakers for the next year. But with “Triton’s” cracking in mid-December of 1942, the Allies regained the lasting advantage in the naval intercept war.
In early February, 1943, Doenitz again suspected a compromising of his codes and once more the Naval War Staff judged the ciphers to be safe — focussing instead on the immediate problem of coping with Allied radar. With the U-boats• spectacular success against Allied convoys in March, 1943, Doenitz felt certain that submarine messages were secure. But this time there was no cipher system available to safeguard against a miscalculation. Thus, with the shattering defeat suffered by his U-boats in May, 1943, Doenitz withdrew most of his subs from the North Atlantic and focussed on countering Allied radar, not calling into question the security of his “Triton” cipher. The constant rumor of compromised codes was felt to represent an orchestrated Allied propaganda campaign against the “morale” of U-boat crews.
By contrast the Allies examined their own naval cipher in the wake of their March defeat of Allied convoys, and accurately diagnosed it as compromised and revised it in June, 1943.
Stripped of communications security and denied cryptographic intelligence, the U-boats waged an increasingly hopeless struggle against Allied technical and material superiority in ships and aircraft.
By June, 1943 the Allies’ ULTRA intelligence could finally be applied in an offensive role as the British and Americans seized the offensive in the Atlantic.
In March, 1944, the use of signal intelligence in attacking a U-boat refueling rendezvous, worried some Allied officers that the secret of their decrypting or the Triton cipher could no longer be concealed. But again Doenitz’ fears were assuaged by the explanation of Allied radio direction-finding — and no basic changes in Uboat ciphers were introduced. The Naval War Staff’s investigation in October, 1941, had established a pattern of misjudgment or radio security that the German Navy never overcame.
This article is a digest of Dr. Timothy Mulligan’s THE GERMAN NAVY EVALUATES ITS CRYPTQGRAPHIC SECURITY, October. 1941.
“Reprinted from MILITARY AFFAIRS, April 1985, pp. 75-79, with permission. Copyright 1985 by the American Military Institute. No additional copies may be made without the express permission of the author and of the editor of MILITARY AFFAIRS.”