- Statistical summary, with comment.
For the period from January through June 1943 there are at least 39 cases in U-boat traffic of presumed or confirmed compromise of Allied naval radio communications. These cases are listed in Appendix 13 together with cases which appeared after this period. Appendix 12, taken from a British study, lists confirmed cases of cipher compromise for 1942. The enemy sources referred to in Appendix 12 were not available to the Atlantic Section of OP-20-G. Nearly all of the 39 cases for January - June 1943 whose sources have been identified involved Naval Cipher #3, tables "M" and "S," and it is probable that those whose sources were not identified likewise involved this cipher.
Tabulation for January - June 1943
(The case numbers refer to Appendix 13)
Case Total January 3 1 Case 1, British Naval Code 4 February 4 1 Case 5, Combined Cipher #3 5 March 3 1 Case 3, Combined Cipher #3 4 April 2 1 Case 3, Combined Cipher #3 3 May 7 8 (9) 15 (16) June 6 2 8 Total 25 14 (15) 39 (40)
(Appendix 13 contains 40 for this period rather than 39 with the addition of a case in May, taken from British records. The German message involved was not sent to U-boats. See case 5 for May.) After the introduction of Naval Cipher #5 in June only two cases of confirmed compromised were recorded, one in October and one in November 1943.
For a discussion of the delay in Allied counteraction the reader is referred to the chapter on German intelligence in Volume I. The present chapter's concern is with cipher compromise in relation to the Atlantic Section of U.S. Navy communications intelligence and to the information currently available to it. The tabulation above, however, incorporates a British study received after the compromise of Naval Cipher #3 had been definitely demonstrated here. In not one of the three cases listed above as confirmed for the period February through April 1943 was the compromised Allied dispatch available to the Atlantic Section. Nor were these dispatches found in the Navy Department's files.
- Fears of compromise.
The highly successful attack on convoy ON 166 in February 1943 crystallized suspicions of cipher compromise, although compromise could not be demonstrated at that time. Last minute shifts in the patrol lines of Groups Ritter and Neptun at 2322A and 2330A on 18 February showed clearly that the German Naval High Command had abandoned the idea of operating on an expected eastbound convoy (HX 226) and was rapidly reforming his lines for a westbound convoy (ON 166). Within a few minutes of these changes, at 2349A/18 a third group of U-boats, Knappen was formed to swing out to the southeast of the Neptun-Ritter line and thus cut off any possible southerly diversion of the convoy.
It was Knappen that made contact on the morning of the 20th, U-604's (Höltring) hydrophones having picked up the convoy's screws. Three diversions had been sent to ON 166 on 17-18 February in Naval Cipher #3, table "S" as the convoy attempted to clear the U-boat area by proceeding on a southerly course. In addition, several position reports had been sent by the convoy before 17 February in table "S." That U-boat Command had accurate information on the convoy can scarcely be questioned.
The disposition and shifting of the U-boat groups between 18 and 20 February suggest knowledge of the convoy's diversion rather than reckoning based on the convoy's own position reports. Of the three diversions sent, the first
one, 1001Z/17 February from CINCWA, seems the most likely suspect, not only from the point of view of the time lag, but also in view of the U-boat disposition. This first diversion would have sent the convoy through the Ritter line just to the south of its mid point. The straggler's route would have passed through Knappen's line.
- Fears communicated.
On 26 February the Atlantic Section sent a memorandum to COMINCH calling attention to the extraordinary and effective sequence of changes in the U-boat line from 18 to 20 February, and the fear of compromise was orally communicated to COMINCH by the Commanding Officer of the Atlantic Section.
- Further evidence, March 1943.
- Raubgraf - Convoy HX 229.
The Raubgraf operation in mid-March of HX 229 and SC 122, "the greatest success yet achieved against a convoy," was probably assisted in large part by a compromised diversion dispatch, sent in the world-wide table of combined cipher. U-boat traffic suggests that HX 229 was the one involved in compromise rather than SC 122, which was proceeding on approximately the same route with HX 229. After the operation was well underway Command recognized that he had two convoys, but the one first contacted and the one which Command seemed to be looking for was presumably HX 229 rather than SC 122. It will be helpful to list the critical Allied and U-boat dispatches in their chronological relation.
041704Z March Original route for HX 229, sent in Naval Cipher #3, "M" 1602Z/13 HX 229 diverted; ordered to turn due east on reaching 49°N - 48°W. (J) Sent in Naval Cipher #3, "M" (The presence of U-boats across the original route between Newfoundland and Greenland was known.)
Groups Raubgraf, Dränger, and Stürmer - HX 229 and SC 122, March 1943
1214A/14 Raubgraf ordered to form new line off Newfoundland for 15 March in expectation of a "north eastbound convoy" (HX 229's original route would have bisected this line at about 50°30'N - 47°W) 1847A/14 Before Raubgraf could reform on the line for the 15th, it was suddenly ordered to head for area 49°40'N - 42°15'W at high speed. 1920A/14 Raubgraf line ordered for 1200A/15 from 51°15'N - 42°05'W to 49°27'N - 40°55'W. "Get hold of eastbound convoy to which further groups can be detailed later."
It is difficult to account for Raubgraf's sudden shift without assuming compromise. Between the time of the order for the first Raubgraf line and the high speed heading (1214A to 1847A/14), 5 Raubgraf submarines transmitted, two to give their positions and three to report land based aircraft.
There is nothing in these reports which could have justified Command's conclusion that a northeast convoy not yet sighted was turning into an eastward convoy. It was not until 2300A/14, more than four hours after Raubgraf's "diversion," that Command had anything like a sighting from a submarine. At that time Walkerling (U-91) reported having seen smoke clouds at 2030A in 49°57 'N - 46°45 'W, but he had been bombed and forced off by aircraft before he could investigate. Walkerling remained close, for he made contact on a destroyer the following evening. It was Feiler (U-653) who finally established contact on the convoy itself on the morning of the 16th. Feiler had been detached from the group and was headed for a refueler off to the southeast.
Meanwhile, Raubgraf U-boats were going through several maneuvers involving such fine points as a 15 mile shift to the south, accompanied by such phrases as "The convoy must be found!" (0443/15) During this interim Command was consistently putting his successive reconnaissance lines a few miles too far to the north for HX 229. The lines would have caught SC 122 had that convoy not been several hours ahead of the line schedule.
The possibility of a compromised dispatch to SC 122 can not be altogether excluded, for the heading point ordered at 1847A/14 actually lay between the routes of SC 122 and HX 229, but orders to U-boats showed no awareness of two
convoys and U-boat maneuvers pointed to HX 229. German uncertainty as to the precise location of the convoy and Command's failure to arrange his U-boats with requisite precision before Feiler's accidental contact probably indicate that Command did not possess a complete recovery of the HX 229 diversion dispatch. It should be noted that neither in the case of HX 229 nor in the case of ON 166 were critical German messages sent in the Offizier setting.
- Convoy TO 2.
On 18 March U-boats in the Trinidad area were informed by Offizier message of the expected arrival of a convoy (TO 2) at Trinidad on 21 March. The convoy's position as of 2000A/13 March was given along with three points on her ocean route. Her estimated time of arrival was explained by the Germans as based on dead reckoning with a speed of 13 knots. This Offizier was read on 22 March and in a memorandum of that date the Atlantic Section called COMINCH's attention to it, stating that "the message gives an accurate description of the convoy's course . . ." This judgment was not based on the convoy's dispatches, which were not available in OP-20-G but on the convoy position estimates of COMINCH Convoy and Routing.
When access to the convoy files had been gained later on, no dispatch could be found in TO 2's file which would have accounted for the 2000A/13 convoy position given in the German Offizier. The following correspondences, however, were found in NOIC Gibraltar's Secret 2242A/10 March to USS Roper and Decatur: sent in Naval Cipher #3, table "S":
NOIC Gibraltar German Offizier Point F: 18°05'N - 43°56'W
Point G: 15°02'N - 51°55'W
Point H: 11°30'N - 60°02'W
Speed of advance: 13 knots.
"It is proceeding via
18°09'N - 44°02'W
15°09'N - 52°03'W
11°33'N - 60°09'W
(The above positions are the mid-points of German grid squares.)
It was later learned that GC&CS traced the German Offizier to two Allied dispatches, on from Flag Officer Gibraltar on 10 March (2247A) and the other from FOC WAC on 13 March
(1402), both in Naval Cipher #3, table "S". Neither of these dispatches was seen in the convoy files in COMINCH Convoy and Routing. Presumably the second of these dispatches contained the estimated position of the convoy for 2000A/13.
- Convoy UGS 6.
In February some 5 U-boats of the 740 ton class departed France under orders Seewolf. Their heading point was deciphered as off Cape May, a decipherment soon confirmed by clarification from Control which resulted from an error by one of the U-boats. From area 42°N - 45°W Seewolf U-boats were suddenly diverted to the southward where they intercepted UGS 6 on 13 March, west and a little south of Flores. That the above operation involved the compromise of UGS 6's ocean route seemed highly probable, but gaps in German traffic (noon 7 to noon 9 March and noon 11 to noon 12 March) made complete investigation impossible. Evidence tending to confirm compromise in this case turned up in January 1944, when Marbach (U-953) was informed in Offizier setting that "until March 1943, traffic proceeded to port (Casablanca) via DJ 2196 (34°03'N 08°00'W)." (1517/19 January 1944) Point "Z" on the ocean route for UGS 6 was 34°04'N - 08°01'W.
- Compromise established, May 1943. Convoy HX 237.
The Atlantic Section's wall chart on 8 May showed convoys HX 237 and SC 129 on a diversion route that would safely clear the south end of the long Rhein-Elbe patrol line whose position off Flemish Cap had been accurately fixed by decryption. At this point the current reading of traffic stopped temporarily, but U-boat contact seemed very unlikely. When B'B' short signals, with the group count known to be predominantly convoy sighting reports, were fixed by D/F the following afternoon in the convoy's path, it was clear that Rhein-Elbe submarines had made a rapid sweep to the southeast and had found the convoy, for there were no other U-boats in the general area at that time except the members of these groups. When the traffic became available, a few days later, attention was immediately concentrated on three Offizier messages. Grammatical variations of the crib "Ein erwarteter Geleitzug" (an expected convoy) were tried and the compromising information came out.
- Investigation of first Offizier.
"2307 To: (Groups) Rhein and Elbe
An expected convoy was in LD 2684 (BC 7684 = 43°57'N - 48°25'W) on 6 May 2330B. Precise course not known, but apparently eastward. Speed 9.3"
A careful study was at once undertaken but for several day yielded no satisfactory result because the Atlantic Section had not received all the pertinent Allied dispatches. Not until access to Convoy and Routing had been gained via COMINCH submarine tracking room was it possible to find the source of compromise. The examination of the convoy files showed that German cryptanalysis had a good depth with which to work, for the diversion of the convoys, complicated by a bad fog off Newfoundland Banks, had led to frequent exchanges of dispatches between shore authorities and the escorts - all in Naval Cipher #3, table "S." On 7 May, for example, there were at least 6 transmissions from escorts (5 from C2 and 1 from W6). While there was no way of determining how many dispatches the Germans had read, the first Offizier could be traced without question to:
W6 Secret 062130Z to CINCCNA (in Naval Cipher #3, table "S") HX 237's position 43°56'N - 48°27'W, course 131, speed 9.5.
Allied dispatch German Information Time: 2130Z
45°57'N (mid-point of
48°25'W German grid square)
The Germans had apparently failed to make a complete recovery and remained in ignorance of the southerly diversion until the evening of 8 May.
- Investigation of second Offizier.
"0025/9 To: (Group) Rhein
The expected convoy, according to sure report, is further south and further ahead than assumed. A patrol line must therefore be drawn up by 2000B on 9 May extending from CG 2927 (BD 7927 = 43°33'N - 34°55'W) to VA 9154 (CE 4154 = 39°45'N - 35°02'W). Maintain radio silence."
Although there could be little doubt that the second Offizier derived its information from compromise, it was not possible to identify the specific Allied dispatch in question. It was clear that German Command had discovered the southerly diversion between 2015B and 2310B on 8 May, for at 2051 the failure of the U-boats to make contact had led to an order for a sweep on course 060, speed 8, thus indicating that a northerly route was deemed possible. At 2310B the order to sweep on course 060 was canceled and U-boats were put on course 120 at top speed. U-359 (Forstner) made contact the following afternoon in 41°N - 37°W. Any one of several dispatches between CINCCNA and convoy escort, which resulted from difficulties in trying to change course, would have yielded the information, especially CINCCNA 060900Z to W6 and CINCCNA 061530Z to escorts, both in table "S."
- The third Offizier and the straggler rendezvous.
"0952/11 To: (Group) Drossel
Eastbound Clausen convoy will be in nav. sq. 9552 (44°21'N - 27°15'W) at 1600B May 11."
The Clausen referred to in the third Offizier was the Commander of U-403. This U-boat had regained contact on HX 237 the preceding day, 10 May, by following an obliging tug until it rejoined the convoy. The tug was the ship Dexterous. It is mentioned because Dexterous was in part responsible for the broadcast of straggler rendezvous for the 11th that was sent to Group Drossel in the third Offizier.
Allied dispatch German Information (CINCWA Liverpool 090901)
R/V 1400Z/11 in
44°22'N - 27°20'W
1600B in 42°21'N - 27°15'W
(mid-point of German grid square)
Escort had informed CINCWA Liverpool that Dexterous had strayed and requested that she be informed of rendezvous position, adding that what books she held was not known. CINCWA Liverpool broadcast the rendezvous positions in 090901, which was sent in Naval Code #3, Auxiliary Vessel System SP 02358/44 and marked BAMS. It was assumed here that the message must have been repeated in Naval Cipher #3, as was the custom in such cases, but no such dispatch could be found in Convoy and Routing. There were other puzzling points for which satisfactory explanations could not be obtained. (For British conclusions see Appendix 13, case 4 under May 1943.)
- Demonstration of compromised accepted. Action taken.
The demonstration of compromise was at once submitted to COMINCH. Meanwhile the British had arrived at the same conclusion and recommended certain precautions for the month of June until a new basic book (No. 5) could become effective. The insecurity of Naval Cipher #3 was attributed to:
(a) "compromise of portions of aviation base book due to heavy use over long periods.
(b) overload of "M" and "S" tables in spite of 10 day change.
(c) ease with which enemy can classify messages in Naval Cipher 3 due to distinctive combined call signs."
(ref. Ultra personal for Admiral King for First Sea Lord 072250, 072255, 072302 June 1943)
The proposed countermeasures consisted largely in weekly changes of "M" and "S" tables. In view of the continuing evidence of compromise, which increased markedly during this period, the interim cipher safeguards could not be accepted satisfactory. In consequence Naval Cipher #5 was brought into effect on 10 June.
- More compromise - to 10 June 1943.
During May and the first 10 days in June 22 cases of compromise (confirmed or presumed) appeared in U-boat traffic. Three of these cases are of particular interest to the U.S. Navy.
- COMINCH submarine notices:
COMINCH Sub Notice German Offizier Confidential
291613Z May (in part) "USS Herring on patrol within 20 miles of 54°N - 42°W. USS Haddo on patrol vicinity 51°N - 35°W"
(Positions converted from grid)
"On evening of 29 May an American submarine was on patrol within 20 miles from 53°57'N - 41°55'W. Another one in 50°57'N - 34°55'W submerged by day, on surface at night."
Comment: While the last phrase in the Offizier did not appear in COMINCH, the Germans had presumably read it before and may have assumed that the last sentence in the sub notice, which they did not read, contained this ordinary and sensible conclusion. Time interval: 45 hours.
051606Z Jun "Hake on patrol vicinity 53°20'N - 37°00'W. Haddo on patrol vicinity 51°00'N - 35°00'W. Herring 54°41'N - 28°48'W submerged by day enroute 54°45'N - 28°01'W. ETA 051800 thence to 54°01'N - 22°02'W. ETA 061800 surfaced at discretion."
"American sub Hake on 5 June patrolling area of 53°21'N - 37°05'W- sub Haddo in 51°03'N - 34°55'W."
"USA submarine Herring was 54°39'N - 28°45'W on 5 June; course not known, proceeding submerged by day."
Comment: Time interval: 71 hours.
- Flight 10 (with note on GUS 7A).
COMINCH Secret 211944Z May informed CESF, NOB Bermuda, and others in Naval Cipher #3 that 19 British LCI(L)'s were to set sail about 24 May from Norfolk to Bermuda. NOB
Bermuda was to direct from Bermuda to Gibraltar according to the ocean route which was given. In Offizier 1106B/24 May German Command ordered 16 U-boats to leave their stations in the North Atlantic and head at once for area 35°15'N - 42°05'W. The U-boats had to reach their destination by 2000B/31 May. An explanation was not forthcoming until 1832B/29 May:
"1. Action is planned against west-east convoy expected in the patrol line from 1 June to 6 June. Speed 8 - 8.5
2. Beginning 1 June an eastbound convoy is expected approximately in area of Struckmeier's position consisting of storm landing boats of 250 tons and of their attendant tankers protected by escorts . . . No operation against this. Take advantage of opportunities for shots against valuable targets (tankers). Do not report when you sight this convoy . . ."
The west-east convoy was undoubtedly Flight 10. The position of Struckmeier (U-608) should have been approximately 33°N - 43°W, since he was the third man from the south end of the line (Group Trutz), which had been ordered on 26 May to run due north and south along the 43rd meridian and from 39° to 32°N. It was to be occupied by 0800B/1 June. Position "H" for Flight 10 was given as 33°10'N - 43°15'W in the COMINCH dispatch referred to above.
Note on GUS 7A:
If it seemed peculiar for a long patrol line to expect a convoy at its southern end rather at the middle, this oddity may have been explained on 1 June (1021B) when Group Trutz was informed by Offizier that:
"Beginning noon today, count also on westbound convoy. When you sight it, operate on it."
The "westbound convoy," GUS 7A, would have passed through the northern half of the Trutz line, according to it original route would explain the peculiar formation of Trutz, as designed to catch two convoys at the same time, but would not account for the long delay in informing U-boats. CMSF 240300Z was not believed compromised, for it had been sent in ECM 38. The
possibility of another source for compromise, however could not be excluded, since at this time daily position reports were being sent in Naval Cipher #3 by various shore authorities.
- Comment: The enemy's own ULTRA intelligence during U-boat decline.
If one may judge from U-boat traffic, German ULTRA intelligence had never been better than it was just a that period when the decline in U-boat fortunes became so evident. The increase in ULTRA intelligence disseminated to U-boats during this period may and probably does represent a corresponding increase in the amount available. The way in which it was used, however, suggests a desperate and hurried attempt to give all possible information to the U-boats at sea. In trying to give his men an additional advantage, Command certainly disregarded security regulations - without compensation - for the risk he ran. In effect, he was sacrificing his best source of intelligence at a time when his fleet was incapable of using this intelligence.
The last U-boat group attempt in May to destroy an US-UK convoy (Group Mosel - HX 239) made use of a decrypted dispatch giving straggler rendezvous positions, yet the operation ended on 24 May in miserable failure. At least six U-boats were sunk, and Command had to stop the operation while the convoy was still in sight. As U-boat after U-boat was requested to "Report position at once" ("Standort sofort melden"), Command was trying to review the total situation in a series of long messages. He promised suitable changes in operational areas until such time as his boats could be provided with adequate protection against aircraft.
It was just 12 hours later that he ordered the southern heading to intercept Flight 10. German radio intelligence had surely influenced his choice of the Gibraltar lane as the place where he might find convoys less well defended. Our decryption of his plans, however, had led to the formation of the Bogue task group, which reached the Trutz patrol line before the convoys.
- Compromise of Naval Cipher #5 feared. September 1943.
The dispersal of the U-boat fleet during the summer of 1943, following the abandonment of Atlantic convoy lanes,
made it impossible to judge how effective the introduction of Naval Cipher #5 had been, since German information on convoys could not be put to any immediate operational use. Instead of convoy intelligence, U-boats were receiving relays of Allied contact and attack reports. With September's resumption of the offensive against convoys attention was again directed to the intelligence which appeared in the timing and arrangement of the patrol lines as well as in "convoy expected" messages to U-boats.
Group Leuthen was prepared for the initial attack with the familiar signs of convoy intelligence. In addition, two of Leuthen's 21 U-boats were equipped with intercept teams prepared to hear and D/F convoy voice traffic. At 1626B/16 September COMSUBs sent the following to Leuthen:
"1. Operation on westbound convoys only. Beginning 21 September, convoy "ONS" is expected; 23 September, convoy "ON." Both westbound. Possibly late.
2. When sighting eastbound convoys, no operation, but take advantage of favorable chances to attack. Report only if sub is observed by enemy. After using attack opportunities, speediest return to position in patrol line."
The convoys were readily identified as ONS 18 and ON 202, which sailed 13 and 15 September respectively. The convoys were delayed, as the Germans had anticipated, and U-boats were kept informed of their progress. On the morning of 19 September U-boats were told to expect one of the westbound convoys "beginning today." ON 202 was attacked that night (200300Z). Inasmuch as the convoys had already been at sea for four and six days respectively, it was necessary to account for the accuracy of the revised German information. There were of course the possibilities of GAF reconnaissance and of D/F on unit transmissions, but the fact that convoy escort had reported to CINCWA twice on 16 September and once on 17 September in Naval Cipher #5 compelled the Atlantic Section to fear cipher compromise, especially since the dispatch of 17 September had been sent in the world-wide table ("M").
A detailed statement of the reasons for this fear was promptly submitted to COMINCH. The British, however, were not convinced in view of the fact "that enemy at times possesses general picture of convoy cycles and nomenclature
from various low-grade sources including WTI." (GC&CS 191858 September 1943 to OP-20-G)
- German awareness of standard convoy routes.
Quite apart from subsequent shifts in anticipation of the convoys, Leuthen's original line lay across the standard route used by the majority of ONS convoys since July. There was no reason to doubt German intelligence had been keeping accurate tab on North Atlantic convoys during the period of U-boat inactivity and had an idea of the standard routes now being used. It might be noted that the standardization of convoy routes simplified the Atlantic Section's problem of deciphering the disguised German grid positions, for it had long been the practice to consult the convoy chart in the process of deducing the disguised U-boat heading points and lines, particularly in those cases which read "convoy expected."
- Evidence accumulates. SC 142 and U-220.
After the first Leuthen operation U-boat dispositions showed even more clearly that enemy coverage on convoy movements was reliable. On 22 September (1115B) a large minelaying-supply U-boat, U-220 (Barber), was warned of an eastbound convoy (SC 142) in the vicinity of 52°N - 33°W, an area through which most of the eastbound convoys had passed since July. Again, two dispatches sent on 17 September in table "S" could have accounted for the message to U-220.
- HX 257, ON 203. COMINCH communicates apprehensions to First Sea Lord.
ON 23 September Leuthen boats were informed that their next operation would be against an eastbound convoy about 27 September. The convoy was HX 257. But on the following day plans were changed and U-boats were directed to proceed eastward at high speed to intercept what was clearly ON 203. This conduct was strongly reminiscent of the period when the Germans were known to have been reading the combined cipher. All the information necessary to account for the German activity could be found in dispatches sent in either the "M" or "S" tables. Hence, even though there was no definite proof of compromise, a detailed statement of the situation was
again submitted to the appropriate authorities in the Admiralty and to COMINCH, who communicated his apprehensions to the First Sea Lord.
- Compromise of Naval Cipher #5 confirmed. October 1943 - HX 261.
On 19-20 October U-boats were enroute from the mid-North Atlantic, when Group Schlieffen's operations had proved discouraging, to form a new line (Group Siegfried) off Newfoundland. At 2239A/19 they received the following Offizier:
"Eastbound convoy will be in nav. sq. AK 97 (area 51° to 51°54'N - 29°30' to 31°00'W) at noon 20 October. No operation; count on strong aircraft. Take advantage of chances that are offered."
The convoy, HX 261, had been diverted by CINCCNA on 15 October in table "M," the new route lying well to the south of both standard routes A and B. Of the messages from escort, sent in table "S," C3's Secret 181659Z October 1943 to CINCWA was established by the Atlantic Section as the source for German information.
C3 Secret 181659 to CINCWA (table "S")
"HX 261: request BAMS for
noon R/V 19th: 50°18'N - 35°35'W
noon R/V 20th: 51°36'N - 30°50'W."
That the Germans had not given the position as precisely as they might have was understandable, for no operation had been intended. GC&CS concurred in the Atlantic Section's conclusion.
- SC 146 diversion compromised. November 1943.
By 7 November the last attempt to mass U-boats off Newfoundland was breaking up. U-boat groups had been well informed on eastbound convoys but had been powerless before aircraft. Forced to try more unorthodox tactics, Command formed Group Eisenhart in an effort to escape Allied detection. As convoys SC 146 and HX 265 proceeded on a diversion course
to the south of Eisenhart's loose and sprawling arc, Command countered by moving Eisenhart to the south and east, explaining on 12 November that:
"The shift in convoy routes which has occurred in the last few days necessitates a shift in your position." (1232/12 November 1943)
and again on 14 November that:
"The persistent avoiding action of the convoys to the south makes another removal of the line necessary." (1113/14 November 1943)
Late on the 14th (2300) U-boats received the following Offizier setting:
"1. Slow northeast bound convoy which is running approximately via Green IW 69, 5350, and Green AW 8610 is placed tonight by dead reckoning approximately in Green SP 48."
The correspondence between the German Offizier and the points contained in CINCCNA's diversion dispatch, sent in table "M", could not have been mere coincidence.
CINCCNA Secret 100606Z to
Escorts to SC 146
New Straggler's Route
Point O: 46°22'N - 32°25'W 45°36'N to 46°30'N
34°00'W to 35°30'W
Point P: 50°40'N - 25°40'W 50°24'N to 50°42'N
25°30'W to 26°o0'W
Point Q: 55°18'N - 17°10'W 55°12'N to 55°30'N
17°00'W to 17°30'W
The new stragglers' route had been repeated in BAMS 100615Z, but past experience threw the weight of suspicion on CINCCNA's dispatch in table "M." Of CINCCNA's diversion, however, the Germans could have read only the last part, the part giving the stragglers' route, since their dead reckoning position for the convoy indicated that they did not know the
diversion route of the convoy itself. Lack of agreement between "Point 0" and its German counterpart was probably explained by a garbled German intercept copy or by partial recovery. HX 265 had also been diverted by a dispatch in table "M." An Offizier sent to Eisenhart on 15 November (2113/15) mentioned that "HX convoy steering northeast is in the same area."
- Indications of Combined Cipher compromise cease in U-boat traffic.
After Eisenhart's attempt to operate on SC 146 no further cases of combined cipher compromises were seen in U-boat traffic. If the German communications intelligence organization was having difficulties at home, for which, according to a Japanese Naval Attaché report, Italian betrayal was blamed, the U-boat at sea was scarcely in a position to exploit convoy intelligence. U-boats had been told in October 1943 that locating convoys was the principal problem of the U-boat war and that the solution of this problem was the U-boats own responsibility.
To assist the U-boats an effort was made to locate convoys by radio intercept teams onboard, by relays of D/F fixes from shore, and finally by very long range aircraft. But instead of finding convoys, U-boats were being found by antisubmarine forces, while the convoys proceeded more or less in peace. Whatever the cryptanalytic situation at home may have been, U-boat traffic seemed barren of any high grade intelligence during the remainder of the war. Other than decodes of British low grade traffic, of which there is no complete record here, U-boat traffic carried only occasional decryption intelligence taken from merchant signals. (See Appendix 13)
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