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Chapter VII

German Fears of the Compromise of Their Naval Cipher


  1. German precautions have been against physical compromise.
  2. Allied radar blamed for U-boat losses.
  3. Minor scare, August 1943.
  4. U-boat prisoners of war warning, September 1943.
  5. Possible rumors in the fleet.
  6. Sinking of the Schliemann and Brake, Indian Ocean cipher crisis 12 March 1944.
  7. Sinking of U-IT-22.
  8. Examination of U-boat rendezvous, September 1943 - March 1944.
  9. Cipher changes introduced at once.
  10. These changes not effective against cryptanalytic attack.
  11. Confidence in U-boat cipher.
  12. Case of Herwartz, 30 March 1944, as example of enemy's problem. Allied D/F net.
  13. U-boat special ciphers.
  14. Increasing fear of physical compromise.

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  1. German precautions have been against physical compromise.
  2. German Naval High Command's ignorance of, or knowledge of, the extent to which their ciphers were being read has been one of the vital concerns of the Atlantic Section. This concern has been, of course, fully shared by COMINCH. After each direct offensive use of decryption intelligence, principally in attacks on rendezvous at sea, German traffic has been scrutinized for any evidence that serious misgivings were aroused. While no information is available on German research, it is known from traffic that fears of compromise were entertained, particularly in the spring of 1944, and that precautions were introduced in the form of very special message settings until certain major changes could be made effective. The German analysis of cipher weaknesses was apparently incomplete and erroneous, for the countermeasures adopted during 1943-1944 seen designed to defeat physical compromise rather than the Allied cryptanalytical attack.

  3. Allied radar blamed for U-boat losses.
  4. Other than a statement in U-boat traffic of January 1943 that British naval units had been assigned to patrol Neumann's (U-117) refueling rendezvous area, there was little in the early traffic read to suggest the possibility of undue German concern for the security of their cipher. And the case of Neumann was more serious as compromise of a British cipher. By May 1943, sufficient anti-submarine forces were available in the Atlantic to begin a general offensive sweep and from this time on decryption intelligence was steadily used in tracking down and destroying U-boats.

    Nevertheless, a general cipher alarm was not sounded in traffic for nearly a year. Cipher changes were introduced on occasions but these were routine measures. The combination of Allied superiority in radar with wide coverage of Atlantic antisubmarine aircraft patrols, both land based and carrier borne, probably diverted attention from communications security. Whatever the speculations at headquarters, the decline in U-boat successes was officially explained by the advances in

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    Allied radar. This explanation was repeatedly offered to the U-boats at sea, with interim orders on conduct and with liberal promises of countermeasures.

  1. Minor scare, August 1943.
  2. In addition to the serious disruption of U-boat plans by sinkings at successive mid-Atlantic refueling rendezvous' in June, July, and August (by August the existing fleet of supply U-boats had been practically destroyed), fully half of the U-boats which did reach distant operational areas in Caribbean and Brazilian waters in the summer of 1943 failed to return. In August 1943, the harassing of U-boat attempts to make emergency rendezvous' off Brazil brought a reaction from COMSUBs that made the Atlantic Section anxious.

    The rendezvous' had been necessitated by a surprise aircraft attack on U-604 (Hltring) which left the U-boat and its crew in a serious difficulty. U-185 (Maus) was ordered to the rescue after U-591 (Ziesmer) failed to respond to requests. (U-591 had already gone down.) On 1 August, Maus radioed that he would meet Hltring in 09°45'S - 29°21'W at 1400B on the 23rd. Although difficulty was experienced with traffic during this period and keys not always currently recovered, this particular message was read before the end of the same day. On the 4th, German B-Service gave COMSUBs his first knowledge that something had gone wrong.

    "B-Service reports: U-boat motionless on surface in area 09°45'S - 30°15'W. If rendezvous has taken place Maus and Hltring are to report positions." (0058/4 August 1943)

    The rendezvous had not been effected due to successive interruptions from aircraft. New assignments were made and a third U-boat, U-172 (Emmermann), was brought into the rescue. On the 11th the three submarines got together but another aircraft intervened before Emmermann could assist Maus with the crew of the scuttled U-604. On the 12th COMSUBs instructed Emmermann to:

    "suggest rendezvous with Maus further to the north for the evening of the 12th using

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    ENIGMA Offizier and disguised grid squares. Carry out transshipment of half of Hltring's complement as early as possible." (0012/12 August 1943)

    That Emmermann should have been superficially instructed to use Offizier settings was clearly a measure of security. The reading of Emmermann's rendezvous proposal was delayed for eight days. Suffice it to say, Maus was sunk on his way home at the Kuppisch (U-847) refueling rendezvous, whose location was known through decryption three days before its scheduled time.1

  1. U-boat prisoners of war warning, September 1943.
  2. A further suggestion concerning German reactions to the ever present anti-submarine forces came in September 1943, from a code message within a letter written by prisoner of war Hans Werner Kraus, ex-Commander of U-199.2 "July 8 Bomber Rio Betrayal in Radio Control since message content meeting point known." Although the implications of compromise seem fairly clear, the message is obscure in detail. Kraus had been sunk on 31 July off Rio de Janeiro at time when he was trying to arrange a rendezvous with Guggenberger (U-513), who had already been sunk.

  3. Possible rumors in the fleet.
  4. The decision early in 1944 to supply U-boat base Penang with cipher data was accompanied with a warning of responsibility for security: "Breaking into the cipher aids would mean the greatest danger for the U-boat war." (1539/10 February 1944) Such a statement was not exceptional in itself, but it was noted with unusual interest here because of the increasing number of molested rendezvous'. Furthermore, talk

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    of cipher compromise was apparently making the rounds among U-boat crews. A prisoner (ex-Commander U-231, Wenzel) informed OP-16-Z interrogators in February that German Command feared cipher compromise as a result of the destroyer losses in the Biscay battle of December.3

    The prisoner's statements were false on several points that could be checked against traffic. He implied, for example, that after the destroyer battle, German Command had instituted a sudden cipher change and that U-231 had been forced to use the Reserve Hand Cipher until the matter of the new keys was explained. No such traffic was intercepted. The prisoner of war story was possibly indicative, however, of rumors and gossip in the fleet. The distress of the U-boat arm would naturally have stimulated every sort of tale. In August 1944 prisoner of war coded messages protested that everything was known to the enemy and that there was treason in the highest places.

  1. Sinking of the Schliemann and Brake, Indian Ocean cipher crisis 12 March 1944.
  2. The only cipher crisis actually known through U-boat traffic occurred in March 1944 as a result of the Brake's sinking. The Brake's loss came just one month after that of Charlotte Schliemannn and completely upset the refueling plans so necessary for combined operational and freight runs between Japanese waters and the homeland. Junker (U-532), returning after many months in the Indian Ocean, had been the last to see the Charlotte Schliemann. When, after a month of waiting, he saw the Brake go down before he had been completely provisioned, exasperation alone could have prompted his report to COMSUBs: "Presumably provisionings have been systematically compromised." (1809/12 March 1944)4

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  1. Sinking of U-IT-22.
  2. To make matters worse, German Naval High Command had that very morning received warning from B-Service that the Spahr-Wunderlich rendezvous 480 miles south of Cape Town, was being tampered with. Spahr (U-178) had been successfully refueled by Schliemann before her sinking and was to receive certain materials, including radar search receivers, from U-IT-22 (Wunderlich), an ex-Italian submarine carrying cargo to the Orient. By evening of the 12th COMSUBs had reason to believe that the U-IT-22 was no more, for Spahr on surfacing reported a large oil slick where Wunderlich should have been. Within 48 hours emergency cipher arrangements were being transmitted to U-boats.

  3. Examination of U-boat rendezvous, September 1943 - March 1944.
  4. The rendezvous disturbances which occasioned this crisis were of course due to decryption. In an attempt to appreciate the problem as it might conceivably confront German Naval High Command, the Atlantic Section of OP-20-G examined the rendezvous traffic record for the period of September 1943 to March 1944, inclusive. Participating U-boats reported the presence of Allied forces in 21 of the 48 major rendezvous' actually held during this period. The attendance of Allied units at German meetings seemed sufficiently irregular to make any strong generalizations unlikely, especially since the Germans were prepared to believe in the ubiquity of Allied patrols - except possibly in the southern Indian Ocean. Had it not been for the peculiar circumstances surrounding the loss of two such important ships as Schliemann and Brake within so short a period of time and in such a remote area, fears of compromise might have been remained dormant or might have been rendered inactive by force of other possible explanations.

  5. Cipher changes introduced at once.
  6. It must have seemed that there was no place in the oceans where the German Navy could rendezvous with assurance. To meet this intolerable situation German Naval High Command introduced an awkward cipher procedure making use of the names of crew members identified by their service numbers. The first of these special settings, called "Maske," was sent on 14 March to Studt (U-488), a refueler,

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    and the three submarines who were to refuel from him. At the same time their rendezvous position which had been ordered in a regular Offizier message of 10 March was canceled. The new rendezvous was to be assigned in a "Maske" message, which in consequence could be read by the Commanders of those four submarines and those four only.

    On 16 March a similar procedure ("Schatten") was followed for a U-boat rendezvous in the Indian Ocean. In all, eight such special settings were sent out to various groups of U-boats during the next four weeks until all U-boats at sea had been reached. Aside from changing rendezvous positions, the purpose of the special settings was the safe dissemination of a new keyword order ("Bellatrix alpha") for the ENIGMA machine.

  1. These changes not effective against cryptanalytic attack.5
  2. Inasmuch as the special settings and the "Bellatrix alpha" procedure did not seem designed to defeat cryptanalytic attack, it was believed that German Naval High Command was primarily concerned with leakage of information from one or more of the following sources:

    1. Physical compromise of U-boat ENIGMA with regular and Offizier settings.
    2. Officer prisoners of war being interrogated immediately upon capture and revealing information about rendezvous.
    3. Enlisted personnel having access to Offizier settings, "Bellatrix" system, and Offizier information with the attendant danger of (b) above.6

    Support for this view seemed implied in such statements to U-boat Commanders as the following: "Strictest secrecy. Only officers to have access to information. No exceptions." A

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    British appreciation of the situation, based on German traffic not available here, pointed out that certain German naval authorities in occupied areas were known to be increasingly anxious about physical compromise and were endeavoring to enforce the most rigid security measures.

    "German naval authorities in the Adriatic and Aegean are becoming increasingly security conscious. They are afraid of leakages of all sorts, and above all of skilled interpretation of their activities by persons in Allied service. These suspicions, which were strengthened by the decode of Jugo-slav partisan traffic informing Allied authorities of the move of Admiral Adriatic to Abbasia, will be further fortified by the sinking of the Dietrichsen. Although at the present time German naval authorities are following a false track their noses as so very keen that anything untoward might put them on the true scent with dangerous repercussions."

  1. Confidence in U-boat cipher.
  2. On the whole German confidence in the U-boat cipher seems to have been consistently strong. The March alarm died down. New editions of certain cipher publications were put into effect but no radical changes were made, unless the U-boat special ciphers can be regarded as such.7

  3. Case of Herwartz, 30 March 1944, as example of enemy's problem. Allied D/F net.
  4. That the Germans were using their own communications intelligence results in an examination of the security problem was evident from an exchange of messages with Herwartz (U-843) during the period of the cipher alarm. Herwartz had been provisioned at the "Maske" rendezvous and was proceeding into the South Atlantic enroute Indian Ocean. Two members of the refueling party (Leupold U-1059 and Brans U-801) had been sunk in the rendezvous area and by 0815/30 COMSUBs was requesting immediate reports of position. At 1839/30 he asked Herwartz to report his position as of 1300/30 and to

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    state "if, when, and where you were observed by enemy before that date."

    It was clear that German communications intelligence had decrypted a submarine estimate for 1200Z/30 (1300 German time) sent by Commander Fourth Fleet in SP 2272 (29). Unfortunately the estimate had been taken from a COMINCH situation report based on special intelligence, and considerable concern was felt lest this slip should further stimulate German apprehension. Herwartz's reply (2221/8 April) showed that the submarine estimate had been very accurate and that it could not possibly be accounted for by any sightings of which he was aware. He was not conscious of having been observed at any time by Allied forces but thought it worthwhile to add sightings by neutrals on 6 and 12 March (Spanish and Portuguese).

    German attention was surely directed to Herwartz's message of 2149/24 March, which was indeed the source of the submarine estimate, reckoning from the position Herwartz himself gave for the time of transmission. The Germans had to consider, however, that the submarine plot lay mid-way between Brazil and West Africa and could have been arrived at from a good D/F on the 2149/24 transmission. COMSUBs had frequently warned his U-boats of Allied skill in D/F-ing and presumably could not in this case eliminate D/F as the source of Allied information.

  1. U-boat special ciphers.
  2. The closing months of the war brought a new challenge to Allied cryptanalysts in the form of special ciphers peculiar to each U-boat. Knowledge that U-boats were being equipped with individual ciphers was not at hand until 6 June, when the sabotage of interior land lines forced traffic between U-boat Command and flotillas into emergency radio channels. As far as known, these special ciphers were used increasingly during late December 1,944 and January 1945, but at no time eliminated the continuing reliance of the ENIGMA general and Offizier.

    It was believed at the time that the introduction of the individual ciphers was an outgrowth of the forebodings which followed the Brake's loss in March 1944 and that they represented a further safeguard against physical compromise,

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    an eventuality whose likelihood would increase with an invasion of the Continent.

    Two special ciphers were finally broken in April 1945 as the result of re-encipherments in the regular naval cipher. It was discovered that the breaking of each special cipher was equivalent in difficulty to breaking a new month of normal ENIGMA traffic.

  1. Increasing fear of physical compromise.
  2. Fear of physical compromise naturally increased as U-boats undertook shallow water campaigns while the land front went to pieces. In September 1944, a Russian attempt to salvage the U-250 (Schmidt) made probable the compromise of secret and most secret memoranda on board. It was necessary to issue warnings to all U-boats that:

    "Loss of U-boats in shallow waters gives the enemy the possibility of diving for cipher material and data.

    1. Make sure that cipher data are so kept that water can actually come into contact with the red print.
    2. When ENIGMA cipher machine is not being used, disconnect the steckers, take out the wheels and disarrange them. Keep everything concealed in separate places. See further Standing War Order Number 246.
    3. See to it that keyword orders are known to three officers only . . .
    4. Lack of attention (to these points) may have unforeseeable results for the U-boat war."

    (Offizier 2107/2237/28 December 1944)

    In addition to diving, the Germans feared boarding parties:

    "So great is the enemy's interest in new devices. ENIGMA cipher machines and cipher aids of U-boats, that he attempts in every possible way to board U-boats . . ."
    (2108/9 January 1945)

    On the eve of surrender,

    A U-boat was . . . attacked off the Norwegian coast by gunfire from 2 English S-boats

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    which were first thought to be our own R-boats . . . Attempt was made to ram the U-boat and board it, presumably in order to capture classified material." (Experience message #236, 1255/27 April 1945)

    In April the compromise of the Reserve Hand Procedure for U-boats was announced, presumably as a result of shore station losses.

    At last, on 8 May, it was announced that U-boat cipher keys had been handed over to the enemy. There was still no evidence in naval traffic that the Germans had believed a cryptanalytic compromise possible.

    Concluding note:

    From German Naval Attach traffic for spring 1944 it is now known that Command conducted an exhaustive investigation of the Schliemann-Brake sinkings. The resulting analysis of their own transmissions in the rendezvous area, together with observations by radio personnel of receiver radiation and the known efficiency of Allied radar, apparently led to the conclusion (or implication) that the Allies could have located the ships by D/F and analysis alone. (PPA 63, 25 April 1944)


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1. The Atlantic Section's concerns were well founded. The entry in the War Diary of the Naval War Staff for 10 August 1943 states "A report from an agent in the USA states that for several months, the American's have allegedly been able to pick up our submarine radio traffic. Although this is very improbable, a most careful check and investigation is again being made." War Diary of the German Naval War Staff (Operations Division), Volume 48, Part A (August 1943), 135.

2. Code messages from prisoners of war were furnished by OP-16-Z.

3. Presumably physical compromise.

4. See note at end of chapter.

5. See Commander Roeder memorandum to G-1 of 23 March 1944.

6. Prisoners of war had in fact accurately discussed the Bellatrix procedure. The information was not really necessary for the Allied cryptanalytic attack and made the Atlantic communication intelligence people worry about their own security problems primarily for fear that word of these interrogations results might get back to the enemy.

7. See paragraph 13.

Transcribed and formatted for HTML by Rick Pitz, HyperWar Foundation