13560_937877610_927073186

PLEASE SCROLL DOWN FOR ARTICLE

This article was downloaded by: [Williams, Peter]On: 22 May 2011Access details: Sample Issue Voucher: CryptologiaAccess Details: [subscription number 937877617]Publisher Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

CryptologiaPublication details, including instructions for authors and subscription information:http://www.informaworld.com/smpp/title~content=t725304178

Ultra Reveals a Late B-Dienst Success in the AtlanticRalph Erskine

Online publication date: 17 September 2010

To cite this Article Erskine, Ralph(2010) 'Ultra Reveals a Late B-Dienst Success in the Atlantic', Cryptologia, 34: 4, 340 —358To link to this Article: DOI: 10.1080/01611194.2010.485412URL: http://dx.doi.org/10.1080/01611194.2010.485412

Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf

This article may be used for research, teaching and private study purposes. Any substantial orsystematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply ordistribution in any form to anyone is expressly forbidden.

The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae and drug dosesshould be independently verified with primary sources. The publisher shall not be liable for any loss,actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directlyor indirectly in connection with or arising out of the use of this material.

http://www.informaworld.com/smpp/title~content=t725304178

http://dx.doi.org/10.1080/01611194.2010.485412

http://www.informaworld.com/terms-and-conditions-of-access.pdf

Ultra Reveals a Late B-Dienst Success in theAtlantic

RALPH ERSKINE

Abstract This paper describes a B-Dienst success in solving signals using aBritish code used by merchant ships (the Merchant Ships’ Code (Mersigs II)) inlate 1943, despite only having a depth of two; it also relates the history of theMersigs II system.

Keywords B-Dienst, Bletchley Park, Depth, Donitz, Hitler, Kriegsmarine,merchant ships’ code, Mersigs, Naval Cypher

Introduction

Sigint was ‘‘the best source of [German] naval intelligence’’, according to Grossad-miral Karl Donitz, the Commander-in-Chief of the Kriegsmarine from 31 January1943, and the last president of the German Reich after Hitler’s death on 30 April1945. He had also retained his post as Befehlshaber der U-boote (BdU—Flag OfficerU-Boats), his title since September 1939. When Donitz was questioned in September1945, he ‘‘stated emphatically that Sigint had been very valuable to him’’.1 Unlikefour Nazi leaders who were interrogated at the same time, including ReichsmarschallHermann Goring and Joachim von Ribbentrop (the former Foreign Minister),Donitz knew the name of the officer, Fregattenkapitan Teubner,2 who had been incharge of the Kriegsmarine’s codebreaking department, the Beobachtungs-Dienst(B-Dienst – 4 Seekriegsleitung (SKL) III), for much of the war.3

Address correspondence to Ralph Erskine, c=o Parliament Buildings (Room 228),Stormont, Belfast BT4 3SW, Northern Ireland, UK.

1‘‘Report on the interrogation of five leading Germans at Nuremberg on 27 September1945’’, Ticom I-143, p. 6: National Archives of the United Kingdom, Public Record Office(PRO), Kew, Surrey, HW 40=158. For a post-war US Navy Operations Evaluation Groupreport on the value of Sigint in the Battle of the Atlantic, see [18], and for a Monte Carlomodel analysing its influence in that Battle, [17].

2Teubner was succeeded in 1943 by Kapitan Heinz Bonatz, Z.S., who was replaced byKorvettenkapitan Max Kupfer in January 1944.

3Goring knew the name of the head of his Forschungsamt, but not of the Luftwaffe’sChiStelle. Ribbentrop recalled that Hitler had a ‘‘queer, rather particular view about [Sigint]– he did not like this type of intelligence very much and said it was unreliable and often mis-leading, it was better to use one’s common-sense.’’ The other two leaders were Field MarshalWilhelm Keitel (head of the Oberkommando der Wehrmacht (OKW – the armed forces HighCommand)) and Generaloberst Alfred Jodl, the chief of the OKW’s operations staff.

Cryptologia, 34:340–358, 2010Copyright # Taylor & Francis Group, LLCISSN: 0161-1194 printDOI: 10.1080/01611194.2010.485412

340

Downloaded By: [Williams, Peter] At: 20:44 22 May 2011

As BdU, Donitz had depended heavily on Sigint from the B-Dienst whenplanning how to assign the U-boats to suitable positions for intercepting andattacking convoys.4 From February 1942 until early June 1943, BdU’s principalsource of intelligence on the Atlantic convoys had been Naval Cypher No. 3(codenamed Frankfurt by the B-Dienst,5 and colloquially called ‘‘the convoycipher’’), which was used by the British, US and Canadian navies. Despite itsname, Naval Cypher No. 3 was a four-figure codebook, reciphered by subtractortables (British figure codes in WWII generally employed subtraction instead ofaddition when enciphering codegroups) [8, p. 311, n. 73]: in Royal Navy termin-ology, ‘‘cypher’’ denoted a code used only by officers, and ‘‘code’’ one that wasused by ratings.

Group Leuthen: The U-Boats Return to the North Atlantic, Fall 1943

Disastrous losses in May 1943 had compelled BdU to withdraw the U-boatsfrom the North Atlantic until 15 September, when he formed about 20 boatsinto Group Leuthen, in a long patrol line straddling the great circle routebetween Halifax, Nova Scotia, and the United Kingdom. In order to avoid com-promising the patrol line, the Leuthen U-boat commanders had received sealedenvelopes containing reference points when they left port – signals abouttheir patrol line disguised their positions by referring to distances and bearingsfrom those points. Leuthen found and attacked convoys ON 202 (‘‘OutwardNorth’’ – Liverpool to New York) and ONS 20 (‘‘Outward North Slow’’) on20 September but did not enjoy the wildly exaggerated success claimed by theU-boat commanders, who thought that they had sunk 12 destroyers with anew homing torpedo, T5 (codenamed Zaunkonig (Wren)).6 However, signals sentto the U-boats caused the US Navy’s codebreakers in Op-20-G considerableconcern, since Leuthen had been told on 16 September to expect ONS andON convoys on 21 and 23 September, respectively.7

Op-20-G considered that signals from BdU on 16 and 23 September indicatedthat the B-Dienst was again reading ‘Naval Cypher’, although the evidence for thiswas ‘not conclusive’.8 However, the British codebreakers in Bletchley Park took theview that the signals were ‘normal policy’ messages, with the first being based onthe convoy cycle (the time gaps between convoys of the same class) and the secondnot showing any exact knowledge. It attributed BdU’s signals to a number of factors,

4There are very few books on the B-Dienst. [6] is based on German records, but dreadfullydull. It gives brief descriptions of many of the codes attacked by the B-Dienst, but has little oncodebreaking as such. Bonatz also wrote [5]. [21] appears not to have used B-Dienst records,and is rather sketchy. Chapter 14 (‘‘The Codebreaker who helped the U-boats’’) in [15] dealswith the B-Dienst.

5‘‘Frankfurt’’ was also the name applied to later editions of Naval Cypher (e.g., NavalCypher No. 6).

6The true figure was only three, with three U-boats destroyed. On the attacks by andagainst Leuthen, see [4, pp. 420–426].

7BdU KTB, 16 September 1943, IV(a): (National Archives and Records Administration,College Park (NACP), Md., microfilm T1022=4065, PG 30332.

8Op-20-G to GCCS, ICY 119, 30 Sept. 1943: NACP RG 38, Inactive Stations 1941–1945,Box 54, 3200=1 - GCCS - Cryptology (General); 1940–1943.

Ultra Reveals a Late B-Dienst Success in the Atlantic 341


including the use of distinctive convoy call signs by escorting aircraft.9 Bletchley wascorrect: BdU had indeed mainly relied on the convoy cycle and scanty data fromdead reckoning.10

However, by 23 September the B-Dienst appears to have made some progressagainst Naval Cypher (which was thenNaval Cypher No. 5),11 enabling BdU to record:

After a long time it was again possible to decrypt the route ordered for aconvoy in time. The basis for this, however, was only a minor system [theBritish Merchant Ships’ Signal Book, Volume II (known as Mersigs II,and codenamed Gallien by the B-Dienst12)]. The main system [Frankfurt]is not likely to be broken before the middle of October.13

More Sigint was to reach BdU on 27 September, this time about a stragglers’route for convoy ON 203. However, despite his hopes, Frankfurt in its variousforms never succumbed to the B-Dienst’s attacks after 10 June. Instead, he had tobe content with intelligence from Naval Code (a 4-figure book almost identical indesign to Naval Cypher - successive versions were codenamed Munchen Blau andMunchen Braun by the B-Dienst), traffic analysis and crumbs from Gallien and othersecondary systems such as Cofox or Loxo, although only Gallien was of any helpagainst the Atlantic convoys.14

Sigint from the B-Dienst provided useful intelligence about the stragglers’routes for at least 19 convoys after 27 September (Figure 1), but most attemptsto intercept them failed. Sigint from naval Enigma kept the allies informed aboutBdU’s intentions, and the convoys were diverted past the U-boat patrol lines.BdU was mystified, since he had abandoned long lines in favour of short widelydispersed lines, which kept moving to avoid detection by aircraft radar. Clutch-ing at straws, he banned the use of a radar search receiver called Wanze andeven of a short wave radio receiver (the Telefunken E 437 S), both of which wereunder suspicion for radiating emissions.15 Desperate for intelligence, heinstructed the U-boats to pick up prisoners, and to question them immediately‘‘as to how aircraft succeeded in locating submarines, whether it was by radaror by passive location methods, for instance by picking up electronic or heatradiation from the boats’’.16 Unknown to him, in a masterly piece of deception,

9GCCS to Op-20-G, POL 133, 4 October 1943: ibid.10BdU KTB, 15 September 1943, IV(3): NACP, microfilm T1022=4065, PG 30331.11BdU, Appendix 1 (Survey of the results of radio intelligence and the knowledge of value

to U-boat warfare derived from it) to KTB, 1 May 1944: NACP, microfilm T1022=4065, PG30346; [13, p. 636].

12 Mersigs II was used for some convoys and stragglers - ships that had to drop out ofconvoys, mainly due to engine problems. Strictly speaking, the code’s short title was ‘‘Mersigs,Vol. II’’, but for convenience it was generally known as ‘‘Mersigs II’’.

13BdU KTB, 23 September 1943, III(d): NACP, microfilm T1022=4065, PG 30332.14Cofox was a Small Ships Operational Code used by motor torpedo boats, motor gun

boats etc., engaged on active operations. Loxo was the Small Ships Signal Code used by smallcraft engaged on local, as opposed to operational, duties: ‘‘Review of Security of Naval Codesand Cyphers 1939–1945’’ (by Paymaster-Cdr. W. G. S. Tighe), 49 (PRO ADM 1=27186) - mythanks to Craig McKay for referring me to this file. See also ‘‘German Exploitation of‘LOXO’ and ‘COFOX’’’: ZG 341, 30 March 1945, PRO ADM 223=5. The B-Dienst couldnot solve signals using Naval Code after the end of 1943.

15BdU KTB, 5 November 1943, VI(3): NACP, microfilm T1022=3980, PG 30334.16BdU KTB, 7 November 1943, VI(2): PG 30334.

342 R. Erskine


a captured RAF prisoner had falsely claimed in August that aircraft seldom nowused radar, and instead were mainly homing on electronic emissions from theU-boats. This reinforced data from experiments carried out by the Kriegsmarineshowing that Metox, a primitive search radar receiver, emanated radiation thatcould be detected up by aircraft at considerable ranges. [16, paras. 369–371]

The Eisenhart Group

Allied air power had forced BdU to decide on 7 November to withdraw theU-boats again, this time to the Eastern Atlantic, where he hoped to locateconvoys at night with the help of long-range, radar-equipped Ju 290 aircraft.Until the Ju 290s became available, he decided to use the 30-strong Eisenhartgroup in a new disposition. With a view to avoiding aircraft radar, he dividedit into 10 sub-groups, each with three boats, in lines stretching south-easterlyfrom Cape Farewell (Greenland), although there were inevitably large gapsthrough which a convoy could pass unnoticed (Figure 2).17 B-Dienst decryptson 7, 8 and 11 November revealed that the convoy routes were now movingsouthwards.18

On 15 November 1943, the codebreakers in Hut 8 (naval Enigma) at BletchleyPark broke an Offizier (doubly enciphered) message sent by BdU at 2300 on 14

Figure 1. Convoys for which B-Dienst decrypted stragglers’ routes, 27 Sept to 5 Dec 1943(from BDU KTBs).

17BdU KTB, 9 November 1943, IV(b): PG 30334. Group Eisenhart should not be con-fused with Group Eisenbart, which operated against the Arctic convoy JW 55B about sixweeks later, on 24 December 1943.

18BdU KTB, 12 November 1943: PG 30334.



November to Eisenhart.19 The following part of the message alarmed NID 10, theAdmiralty section responsible for cipher security,

1) It is estimated that a slow convoy steering north-easterly course, which isproceeding approximately via squares Green IW 69, 5350 and Green AW 861020 willbe in approximately square Green SP 48 tonight.21

Green IW 69, 5350, MM 8610 and SP 48 were positions on the Kriegsmarinenaval grid that had been encrypted using the Adressbuch.22 When decrypted, twoof them corresponded to positions in instructions signalled to convoy SC 146 (fromHalifax, Nova Scotia, to Liverpool) by the Commander-in-Chief (CinC) CanadianNorth-West Atlantic (CinCCNA), who had sent a signal to it at 0615Z (Zulu time– Greenwich mean time) on 10 November. The CinC Western Approaches had senttwo related signals to SC 146-

at 1536Z on 12 November;at 1115Z on 14 November.

The plain text of the three signals is set out in Figure 3, in the form inwhich they would have appeared before coding, together with their text afterthe relevant Mersigs II codegroups were enciphered with subtractor table 463for inward convoys (Incon 463 – set out in Figure 4). Each Incon table was

Figure 2. Eisenhart group on 11, 14–15 November 1943 (Source: PRO ADM 234=68).

19ZTPGU 19194: DEFE 3=724.20Originally Green MM 8610 in ZTPGU 19194 – corrected to Green AW 8610 (AL 6610:

55–21N, 17–15W) by ZTPGU 19195: PRO DEFE 3=724.21BD 49 (Green IW 69): 45–36N to 46–30N, 34–00W to 35–30W; BD 3350 (Green IW

5350): 50–33N, 25–45W; BD 28 (Green SP 48): 48–18N to 49–12N, 31–00W to 32–30W.22On the Adressbuch, which was used to encrypt squares and positions in the Kriegsmarine

grid, see ‘‘U-Boat Chart Cipher, Catch-Word: ‘Address Book’’’ (NACP, RG 457, Entry 9032,Historic Cryptographic Collection, Pre-World War I Through World War II (HCC), Box 622,Nr. 1673); ‘‘General Nature of Address Book’’ (HCC, Box 622, Nr. 1674); [9].

344 R. Erskine


created for use with a single convoy, and contained only 150 ‘‘subtractives’’ (30rows of five-figure groups) from mid-January 1943 onwards. The maximumdepth available to the B-Dienst was only two on the overlapping parts ofthese three signals. The Offizier signal therefore revealed that the B-Dienstwas solving traffic in an enciphered British naval code with a time-lag ofonly 10 hours, despite there only being such a shallow depth – a remarkableachievement.23

Figure 3. Mersigs II signals for SC 146 stragglers (from PRO HW 73=1).

23The text of the three signals, in plain, code and enciphered form, is in PRO HW 73=1, asis Incon 463.



B-Dienst Solutions of Mersigs II Signals

TheB-Dienstwas greatly helped in attackingMersigs II by holding a copy that had beencaptured inMarch 1942 from a Britishmerchant ship sunk in a convoy to Russia.24 Thebasic book was therefore fully known to the B-Dienst, without any gaps. Moreover, the

Figure 4. Subtractor table, Incon 463 (Source: PRO HW 73=1).

24‘‘Review of Security of Naval Codes and Cyphers 1939–1945’’, 91.

346 R. Erskine


B-Dienst had a sound knowledge of the convoy cycle on the main Atlantic routes, andoften knew the designation of any particular convoy (eg SC (Sydney, Nova Scotia, laterHalifax, Nova Scotia, or New York, to the UK), HX (Halifax to the UK, later NewYork to the UK), and so on, together with its number), and the time when it was atsea.. The B-Dienst also knew from previous breaks into the Incon tables that the begin-nings of signals tended to be very stereotyped, with the convoy number often appearingin early groups, and other words being ‘‘stragglers’’ ‘‘route’’, ‘‘cancel’’, ‘‘substitute’’,‘‘north’’, ‘‘west’’ and so on. Simple cribs could therefore be constructed and tried witha reasonable prospect of success. In addition, characteristics in Mersigs II signalsallowed the specific Incon or Outcon (outward convoy) reciphering table to be ident-ified, and helped theB-Dienst to group the signals with a view to aligning them in depth.The B-Dienst probably used a combination of cribs and ‘‘differencing’’ to solve over-lapping parts of the signals, if there was a depth of three or more.25 However, sincethere was definitely only a depth of two on these three signals, the B-Dienst can scarcelyhave used differencing to solve them. It must therefore have used a crib, such as ‘‘SC146 next group plural stragglers route cancel (?). Substitute (?)’’ in the 14–1115Z signalto find the subtractives. The items in square brackets were letters, and could not beguessed exactly, although decrypting the signal of the 12th would have given someindication of the range of letters to be tried.

Applying that crib to the opening part of the 14–1115Z signal as interceptedgives the following results

Line 23Group astransmitted(GAT)

GCGN[indicator forstarting line]

XRPV[check

indicator] 26496 04841 77759

placode & crib 27465[SC]

03215[100þ ]

12631[46]

Subtractive 43851 07056 89380Line 24 GAT 77961 24336 70596 56105 42151placode & crib 23289

[next groupplural]

27656[straggler(s)]

00877[route]

16627[cancel]

? [letter]

Additive 90140 41982 70363 62722 ?Line 25 GAT 70266 97284 23556 44815 77328placode & crib 35972

[full stop]04206

[substitute]?

[letter]Additive 05138 91480 ?

Crib: SC 100þ 46 next group plural straggler(s) route cancel [letter – not knownor guessable] full stop substitute [letter – not known or guessable] [9 guessable groups],i.e., SC 146 stragglers’ route cancel [unknown letter]. substitute [unknown letter]

25The best study of the difference method for stripping additives from encipheredcodegroups is ‘‘Encipherment by Means of a Code Book and an Additive Key’’, by Capt.G. W. Morgan: RG 38, Crane Library, Box 63, 70–26, 63=2650=166B. ‘‘Gerry’’ Morganwas a gifted member of Brigadier John Tiltman’s research section at Bletchley Park. Morgan’sstudy was used by Op-20-GY-P-3 as ‘‘the standard text for the initial attack’’ on JN-11:‘‘History of JN-11, Japanese Fleet Auxiliary System’’, V-1: NACP, RG 38, RIPs, RIP 214.See also [1], [2, pp. 369–372, 413–415], [7, pp. 77–81] [14, pp. 440–443].



The resulting subtractives could then have been applied to the 10–0615Z signal

line etc19 [starting line,indicators onlygiven here]

URVQ[indicatorfor starting

line]

LXJN[check

indicator] . . ... . . ... . . ...

23 additive 43851 07056 8938023 GAT 21221 93862 3102223 placode 22630 14294 5836823 meaning 250 P 050 degs24 additive 90140 41982 70363 62722 ?24 GAT 47811 71635 27034 72423 5046024 placode 53339 70357 53339 90309 ?24 meaning 40mins 025 degs 40mins Q ?25 additive 05138 91480 ?25 GAT 96297 68821 2053025 placode 19941 33669 ?25 meaning 18mins 017 degs ?

to give the text ‘‘25mins P 50 degs 40mins 25 degs 40mins Q [?] 18mins 17 degs [?]’’in lines 23 to 25. Allowing for the solution of the groups indicated by question marksand the fact that, because of the Kriegsmarine grid’s construction, positions plottedon it did not necessarily provide precisely equivalent figures for latitude and longi-tude, these positions correspond closely to those for P and Q in the Offizier signalto the Eisenhart group.

ZTPGU 19194 CinCCNA 10=0615Z,as derived from crib

50–33N 25–45W P. 50–40N 25–40W55–21N 17–15W Q [55].-13N 17-[10]

W

It is not clear how the B-Dienst would have known whereabouts in the 10–0615Zsignal to apply the subtractives, since the four-letter indicators in each signal wereunique to the Incon 463 reciphering table (see the bottom right-hand corner ofFigure 5). It may be that it had to use trial and error. Although there were somesubtle variations in one of the three signals, the B-Dienst could have found mostof the overlapping text of all three messages using the methods set out above, plussome testing of different letters for groups for which there was not a specific crib.

As well as solving CinCCNA’s signal of 10 November, on the 15th the B-Dienstread a message setting out an approach position about 280 nautical miles west ofGalway (Ireland), for convoys SC 146 and HX 265. Aircraft escorting them werealso fixed by direction-finding 400miles north of the Azores.26 BdU thereforeordered U-boats to search the area in between these points, but neither convoywas located.27 On 14 November, BdU despondently concluded that

26Kriegsmarine grid square BD 82–45-09N 31–45 S.27BdU KTB, 15 November 1943, IV.C: NACP, microfilm T1022=3980, PG 30334.

348 R. Erskine


‘‘The enemy holds all the trump cards: long range air reconnaissanceconstantly covering all areas, with location methods against which westill have no effective warning equipment . . .

Until now, no air reconnaissance of our own, the U-boats can onlycarry out reconnaissance over a very small area, no new locationmethods . . . the enemy knew all our secrets and we knew none of theirs.’’28

Figure 5. Indicator table, Incon 463 (Source: PRO HW 73=1).

28BdU KTB, 14 November 1943: PG 30334.



The B-Dienst later read a Mersigs II signal sent on 4 December to convoyONS 24, altering its stragglers’ route. BdU described the alteration as ‘‘signifi-cant’’, because he had ordered the Coronel group, with about 19 boats, to movetheir patrol line between 100 and 340 nautical miles to the south,29 and the newstragglers’ route avoided this line.30 Coronel’s attempt to intercept the convoyhad to be abandoned.31 It is surprising that this did not lead BdU to questionEnigma’s security, since he had previously done so on a number of other, albeitunrelated, occasions.32

In late December a further B-Dienst decode of a Mersigs II signal encipheredwith an Outcon table was disclosed by special intelligence (Ultra). This time theB-Dienst had decoded the signal, to a British convoy bound for North Russia, inunder 24 hours.33

Admiralty Countermeasure

The Admiralty took steps to improve the Mersigs II system on 3 December 1943 byordering alterations to stragglers’ routes to be made by reference to previously estab-lished lettered positions instead of by latitude and longitude.34 This was essentially aBritish version of a scheme that the U-boats had introduced in June 1941, [9, p. 41]and could easily have been adopted much earlier. When combined with otherimprovements to British cipher security, the change was disastrous for the B-Dienst,which could no longer glean accurate information about convoy routes, even fromMersigs II. Although it had ceased to read Naval Cypher after June 1943, it had beensolving Naval Code No. 3, mainly when used with tables for auxiliary vessels, untilDecember 1943. However, even that stopped except for back traffic when NavalCode No. 4 became effective on 1 January 1944, together with the stencil subtractorsystem invented by Brigadier John Tiltman.35 From then onwards, the B-Dienst wasforced to fall back on minor codes, such as Cofox or Loxo. Gunter Hessler, theGerman historian of the Battle of the Atlantic, noted that ‘‘Never again – not even

29From approximately 56–40N, 25–20W to 55–00N, 25W to 51–10N 26W.30For the British and German signals relating to the reading of the alteration to ONS 24’s

stragglers’ route on a depth of 3, see Op-20-G, ICY 235 of 7 December 1943; GCCS POL 249of 12 December 1943(HW 73=1); also ZTPGU 19975 (PRO DEFE 3).

31BdU KTB, 5 December 43, IV(a), 7 December 43, IV(a): NACP, microfilm T1022=4067, PG 30336.

32‘‘The enemy has information on the areas of these dispositions, obtained by methods asyet undiscovered by us’’ (BdU KTB, 16 September 1941: NACP, microfilm T1022=4063, PG30297); the Tarafal Bay incident in the Cape Verde Islands, when HM SM Clyde made anunsuccessful attack on U-67 and U-111 (BdU KTB, 28 September 1941: NACP, microfilmT1022=4063, PG 30297); ‘‘This knowledge could be gained by the enemy . . .By decipheringour radio messages’’ (BdU KTB, 19 November 1941 VI(2)): PG 30300B, NACP, microfilmT1022=4063); ‘‘The impression is gained from U-boat situation reports received, that theenemy is aware of the disposition of the Group ‘Delphin’’’ (BdU KTB, 28 January 1943,IV(2)) (NACP, microfilm T1022=4064, PG 30316); ‘‘the strong suspicion that the enemyhad broken down our codes’’ (BdU KTB, 5 March 1943, VI(1)(A)(1)):NACP, microfilmT1022=4064, PG 30319).

33ZTPG 196183 of 30 December 1943 (DEFE 3=372), a decrypt of an Offizier message of19 December.

34DNI Out signal, 3 December 1943: PRO HW 40=35. The US Navy issued a similarinstruction on 4 December.

35On the stencil subtractor system, see [8, p. 311].

350 R. Erskine


for the shortest period – did we succeed in re-establishing the same standard ofdecryption that existed in 1942 and 1943.’’ [16, para. 380]

History of Mersigs II

Merchant Navy Code

The precursor of Mersigs II was the Merchant Navy Code, a 4-letter=5-figure codethat took effect on 11 January 1940. It used four different sets of reciphering tables:general, convoy (for specific convoys while they were at sea), Indship (for communi-cating with slow independent merchant ships) and Oneship (pads used for communi-cating with a specific fast independently routed merchant ship or oceangoing tanker– but not one-time pads, as such).36

In May 1940, the Germans captured several copies of the Merchant NavyCode at Bergen. This greatly facilitated the B-Dienst’s work: after fourteen daysfrom the introduction of a new general table it was usually able to read most ofthe traffic with a short time lag. The B-Dienst transmitted recovered recipheringgroups from the general tables to the radio operators on raiders such as Schiff33 (Pinguin), to assist them in reading messages they intercepted in Merchant NavyCode. Special B-Dienst group monitoring parties (B-Groups) in Kriegsmarinebattleships and cruisers (such as Gneisenau and Prinz Eugen37) had their workextended to cover traffic in Merchant Navy Code recoded by IndShip and convoyreciphering tables.

Cipher Tables for Merchant Navy Code and Mersigs II

The general and convoy reciphering tables for the Merchant Navy Code were at firstreciprocal bigram substitution tables, but were replaced by multiple alphabet tables(with 240 column alphabets (60 four-letter groups) for the convoy tables) in 1941.38

The first convoy series (Shipcon, later Outcon Tables), started on 1 June 1941 foroutward-bound United Kingdom to America convoys; the second series (InconTables) became effective for homeward-bound convoys in September 1941. Unfortu-nately, the traffic carried was greater than anticipated, which resulted in depthsenabling the B-Dienst to break into a table and read the details of stragglers’ routes;it was sometimes then able to collect data concerning the general course of the con-voy concerned. However, a lack of material meant that the keys were not broken forevery convoy.39

Mersigs II replaced the Merchant Navy Code in April 1942. It was a 5-figurecode comprising 20,000 groups (out of a possible 100,000), which suggests that, like

36Except where another reference is given, this section is based on ‘‘Review of Security ofNaval Codes and Cyphers 1939–1945’’, 89–95.

37As to B-Dienst parties on battleships, see ‘‘B-Dienstgruppen and Bord derSchlaftschiffe’’, 18 November 1941: NACP, microfilm T1022=2325, PG 34522. For a reportby the B-Group on the Prinz Eugen, see ‘‘Erfahrungsbericht der Bord-B-Dienstgruppe ‘PrinzEugen’’’, Skl=Chef MND (B), 16 June 1941, 1533=44: NACP, microfilm T1022=2325, PG34522.

38Multiple alphabet ciphers include Vigenere, Gronsfeld and Beaufort ciphers: [12, pp.108, 117].

39Ticom I-95 (Interrogation of Lt. Morgenroth on organisation of OKM=4 SKL=III), 5:PRO HW 40=182.



that code, it incorporated various garble checks.40 A page from Mersigs II is set outin Figure 6. Mersigs II was also used with the general, convoy, Indship and Oneshipreciphering tables. Where there was an adequate depth, recovering subtractives bythe difference method would have been more reliable with Mersigs II’s five-figurebook than with a four-figure codebook using all 10,000 groups, which would havetended to yield too many spurious differences.41

B-Dienst Attacks on Mersigs II

The B-Dienst’s subsection FQ (4 SKL III=FQ) attacked Mersigs II together withother minor codes: Lt. Morgenroth was in charge of the work from the end of1942 until the end of 1944. Other members of FQ working on Mersigs II includedFahnrich z.S. (Ensign=Midshipman) Baeck and Funkmaat (Petty Officer Tele-graphists) Schroeder and Hermann.42 About 60 people worked on Mersigs IIin 1944, in two shifts, from 8 am to 4 pm and 4 pm to midnight.43 In 1943 theconvoy tables for Mersigs II were changed from multip1e alphabet tables to sub-tractor tables (Outcons from 18 January 1943, and Incons from 8 April 1943),but each table only contained 150 groups, which eased the B-Dienst’s task, sinceit increased the volume of traffic, and so depths, in a given key.44 Lt. Morgenrothlater told a Ticom (Target Intelligence Committee) team that ‘‘it rather simplifiedtheir work and tended above all to speed up solutions’’.45 Since the B-Dienst heldthe Mersigs II codebook and the traffic was quite heavy, it soon broke into theIncon and Outcon tables. Realising that each table comprised only 150 groupswith any convoy, it tried to find a signal longer than 150 groups, so as to havea partial depth. The general tables for Mersigs II changed to 7,500 group subtrac-tor tables on 23 July 1943, but the B-Dienst was still able to break into the tablegeneral ‘‘very often’’.46

The B-Dienst’s success against the Mersigs II convoy tables depended on the vol-ume of signal traffic to an individual convoy, which was small if the convoy was notattacked but greater if it was, and how much overlap in a table occurred because of

40The Merchant Navy Code is said not to have used E, I and O, and H did not occur asthe first or last letter of a code group: Kaigun (Japanese Naval Attache), Berlin to Yayoi(Chief of the third section, naval general staff), Tokyo, 6 June 1942 (decrypted, 10 June1944), SJA 462 (a JNA 20 (Coral) decrypt): PRO HW 40=65. However, [14, p. 467] reproducesa page from a five-figure=four-letter ‘‘merchant ships’ code’’. Although no date is given for thecode, its groups include E, I and O. Note that groups 27656 and 63320 have identical meaningsin that code and in p 85 of Mersigs II, which is surprising.

41For a discussion of this problem, see ‘‘Weakness of Differencing in a Four-digit Code’’in ‘‘History of JN-11, Japanese Fleet Auxiliary System’’, V-D: NACP, RG 38, RIPs, RIP 214.An attempt by Op-20-G to recover four-digit additives in JN 39 (a Merchant Navy- NavyLiaison System) was ‘‘a ludicrous failure’’, since subsequent captures showed that 70 per centof the additives recovered had been wrong: ‘‘History of JN-11, Japanese Fleet AuxiliarySystem’’, V-8.

42‘‘Enzifferungspezialisten’’, Ticom I-95.43Ticom I-83 (Additional Interrogation of Lt. Morgenroth of 4 SKL=III), 2: PRO HW

40=182.44Ticom I-95, p. 5. British-American Ticom teams were charged with questioning German

Sigint personnel, and recovering their equipment and records.45ibid.46Ticom I-95.

352 R. Erskine


the way different users chose starting points. The discovery that the B-Dienst couldread traffic in the convoy tables on a depth of only two came as a very unwelcomesurprise to the Admiralty when it first became aware of this in late 1943. When largereditions with 750 groups were belatedly introduced for the Incon and Outcon tableson 13 April and 21 May 1944, respectively, they seriously hampered the B-Dienst,since it could no longer recover all the groups in a table.

The Indship series of tables remained in use throughout the war, but on 20 May1943 changed to a subtractor system with 7,500 groups in each edition. However,due to the tables’ wide distribution, their heavy use and the B-Dienst holding MersigsII, they gave it little more difficulty than the general tables until 20 April 1944, whenthe tables began to be changed monthly instead of every two months. The number ofbroken messages then ‘‘fell off considerably’’.47

Figure 6. Mersigs II codebook, page 85 (Source: Naval Historical Branch).

47‘‘Cryptographic work’’, Ticom I-95, 5.



The B-Dienst did not seriously attempt to exploit traffic in Mersigs II encipheredwith Oneship subtractor pads until late in the war. These pads had been introducedin August 1941, for use with a small five-figure Diversion Code, for communicatingwith fast independent ships, and were later extensively used for important signals tolarge troop transport ships. From 15 April 1942, Oneship pads were used withMersigs II. Although self-evident starting point indicators were employed and thepad number was given in clear at the beginning of the message until 15 November1942, the B-Dienst appears until late 1943 to have considered them to be trueone-time pads, and therefore insoluble. The B-Dienst started to penetrate the padsonly in May 1944, with the help of intelligence about how they were used, but eventhen it had scant success.

Intelligence from Mersigs II Decrypts

The intelligence gained from signals in Mersigs II became more valuable afterthe replacement of the British-U.S. edition of Naval Cypher No. 3 in June 1943.The decrypts of Mersigs II traffic even made BdU suspicious about whether theBritish were aware of the B-Dienst’s successes against British naval ciphers. Hiswar diary for 30 September 1943 records his concern that the stragglers’ route forONS 204 had been altered at 15.30 hrs, enabling it to by-pass the Rossbach group’spatrol line, which was then situated to the east of Greenland and south-west ofIceland. However, he rejected ‘‘the possibility that the cipher [i.e., Enigma] mighthave been broken’’, since ‘‘it can be assumed that the diversion was made in viewof the sighting of U-448’’ and another U-boat by the British.

On 2 October 1943, BdU followed this up with:

The following points are raised by the fact that the enemy is takingsuch unusually extensive avoiding action and constantly altering thestragglers’ routes

. . .2) Owing to Italian treachery, the enemy has captured Sigint groups

who knew of the breaking of the English ciphers. It is thereforenot impossible that the enemy is making use of this information and isdeliberately deceiving the B-Dienst and U-boat Operational Control bysending radio messages of convoy and stragglers’ routes that are notactually to be used.48

But he did not question Enigma’s security. On 4 October, he summed up hisviews on the weak state of current Sigint.

This operation [by group Rossbach against ON 204 and ONS 19] hasshown again how difficult it is to intercept a convoy using Sigint if therehas been no visual reconnaissance by our own forces. Sigint data is veryuncertain, first because bad weather may cause a convoy to fall behindschedule and secondly because one can never know for certain whetheror not the convoy route has been altered since the last intercept.

48BdU KTB, 2 October 1943: NACP, microfilm T1022=4065, PG 30333.

354 R. Erskine


Sighting reports provide the only data of value to an operation.Sigint can be of use, but only if it provides more detail than at present.49

From 3 December 1943 onwards, the mandatory use of secret lettered positionsto disguise positions in signals using the Incon, Outcon and Indship tables greatlyreduced the operational value of B-Dienst intelligence on convoys from Mersigs IItraffic. In mid-December, BdU recorded that ‘‘Information gained from interceptsources can now no longer be used operationally immediately’’. However, hededuced convoy ON 215’s position from other radio traffic. A comparison withan aircraft report of it being in AL 99 (about 51–27N, 16–45 E - 240 nautical mileswest of Ireland) showed this to be reasonably accurate. He therefore hoped to beable to find reference point positions in future, albeit after a considerable amountof work, by comparing data from decrypts with the actual position of convoys.50

But his war diary and other documents show that the Kriegsmarine did not succeed,and that the British counter-measure was completely successful.

The B-Dienst’s exploitation of traffic using the Mersigs II convoy tables virtuallyceased from 1 September 1944, when they were issued with a special ‘‘DiversionCode’’ incorporated in the tables, which only lasted for the passage of a specific con-voy. Subsequently, the B-Dienst could only derive intelligence about convoy move-ments from traffic analysis and relatively unimportant information that had beenrecoded in the general tables.

A B-Dienst cryptographic progress report of 5 January 1945 summarised theposition bleakly

The introduction of the Diversion Code on 1 September 1944 was amuch more serious complication. This consists of only two sheets, butis issued afresh for each convoy. All details on locations are given in it,and probably again in terms of latitude and longitude. Up to the present,this code has resisted all attempts at breaking with the small quantity ofmaterial available. In addition, the groups of this code cannot beincluded for purposes of comparison with others, and this hinders thebreaking of other messages.51

Conclusion

In April 1944, when investigating a security breach, the British told the US Navythat it ‘‘always assumed that S.P. 2272 series [the Mersigs II general tables] liableto be read by the enemy’’ and that British originators ‘‘fully appreciate this and wordsignals accordingly’’.52 Estimates of U-boat positions were never passed by theBritish to merchant ships, except the ‘‘Monsters’’ such as the Queen Elizabeth andQueen Mary which, being treated as Royal Navy ships, had special cipher facilities.53

49BdU KTB, 4 October 1943: ibid. This translation is based on, but not identical to, thatin [16, para. 381], q.v. on the Rossbach group’s operation.

50BdU KTB, 16 December 1943: NACP, microfilm T1022=3980, PG 30337.51Quoted in ‘‘Review of Security of Naval Codes and Cyphers 1939–1945’’, 94.52Wilson to Wenger, POL 404, 3 April 1944: PRO HW 40=62.53The ‘‘Monsters’’ were so called because from June 1942 their carrying capacity was

15,000 troops.



[3, p. 148; 20, p. 211] The Admiralty was therefore not surprised by the B-Dienst’sexploitation of traffic in the general and IndShip recoding tables. Their securityvalue had been known to be very low, since they carried loads of traffic which couldnot be satisfactorily controlled due to the impossibility of changing or distributingnew editions quickly enough.54

However, in June 1943 the Admiralty had been much too complacent whenBletchley broke a German message based on a B-Dienst decrypt of a Mersigs IIsignal.55 It thought that in view of the small amount of traffic the Germanshad more probably read the Allied signal concerned as enciphered in NavalCypher No. 3, which was about to be replaced. It took no steps to improveMersigs II’s security, contentedly declaring that the Incon and Outcon tableswere ‘‘perfectly secure for the small amount of traffic [they] would be requiredto carry’’.56 As a result, it was greatly shocked when it received proof that theB-Dienst could read Mersigs II and other super-enciphered code traffic on adepth of only two.

Commodore E. G. N. Rushbrooke, the Director of Naval Intelligence, wrote‘‘This incident [Eisenhart] reveals a new development in codebreaking’’, and Bletch-ley Park was forced to drastically revise its views on the security of all additive sys-tems except one-time pads. This demonstrates the incomplete, almost deceptive,nature of Sigint at times, especially if it is being relied on for communications secur-ity (Comsec) purposes. If BdU did not include enough detail in his signals, it wasimpossible to determine whether they relied on B-Dienst Sigint or not. Even if theydid, they did not necessarily divulge which cipher system had been penetrated. Attimes, the lack of evidence in Ultra of German codebreaking undoubtedly induceda false sense of security in the British Comsec specialists, who were too often reluc-tant to accept that naval and other ciphers had been penetrated by German code-breakers; instead, they tended to attribute German knowledge about the contentsof allied signals to espionage or some form of physical insecurity. The Kriegsmarinefell even further into a similar trap: despite carrying out a number of investigationsinto naval Enigma’s security, it always decided that since allied signals did not referto Enigma being broken, it was not being solved.57

The value to the Kriegsmarine of the B-Dienst’s solutions of Mersigs II traffic isrevealed by a B-Dienst progress report at the beginning of March 1945 about theintelligence being gained then almost exclusively from reading traffic in the generaland IndShip tables:

Merchant Navy Code [sic – Mersigs II] - 2,000 messages in this system aredecrypted monthly – and completely read; the basic book is held. Themost important operational results obtained from it are

(a) Times of arrival of Atlantic Convoys in British and US coastal waters, aswell as distribution of ships among ports of destination: This permitsfar-reaching conclusions to be drawn regarding convoy time-tables.

54‘‘Review of Security of Naval Codes and Cyphers 1939–1945’’, 95, para. 17.55ZTPGU 14839, Offizier of 30 May 1943, decrypted on 22 June 1943: PRO DEFE

3=719.56Rushbrooke, docket, 30 November 1943: HW 40=35.57Lt. Cmdr. K. W. McMahon, USNR, ‘‘German Reactions to Allied Use of Special Intel-

ligence’’ is an outstanding study of this issue; see also [10], [19].

356 R. Erskine


(b) Successes, as and when they occur, of attacks by our U-boats, as well asdamage to and losses of merchant ships at sea.

(c) Approach points for convoys and independently routed ships (Irish Sea toPort Said).

(d) Insight into the number of ships routed independently, and solution ofships’ secret call-signs, which are of assistance in traffic analysis.

(e) Weather reports from the Channel, Biscay, Mediterranean and IndianOcean.58

But good military intelligence is of little use without the forces to employ it, andboth the Kriegsmarine and the Luftwaffe were in a parlous state, and able to derivelittle advantage from intelligence from Mersigs II. Although reading messagesin Mersigs II during the second part of 1943 and early 1944 showed that largenumbers of landing craft were being transferred to the United Kingdom from theMediterranean, and their build-up in the Irish Sea and the Channel, the B-Dienstdid not provide significant advance intelligence on the D-Day landings in Normandyin June 1944.

The B-Dienst had completely failed to solve rotor cipher machines. After aninitial analysis of the traffic using Typex (the British development of commercialEnigma) and the Combined Cipher Machine (CCM – an adaptation of the USNavy’s Electric Cipher Machine and Typex), which it codenamed Ulm, it quicklydecided that any attempt to break their traffic would be pointless. As it turnedout, the B-Dienst missed a major opportunity – the CCM was insecure untilearly 1944.59 Since the CCM had become the common cipher machine used byall five British and American armed services for secure communications duringthe allied invasion of Western Europe and the B-Dienst could read neither navalcypher nor naval code after 1943, the B-Dienst’s role ended not with a bang, buta whimper.

About the Author

Ralph Erskine is a retired barrister who has written extensively on codebreaking andsignals intelligence. He is a member of the editorial boards of Cryptologia, The Jour-nal of Intelligence History and Intelforum. His latest publication is the Foreword toMavis Batey’s Dilly: The Man Who Broke Enigmas (Biteback, London, 2009).

References

1. Barker, W. G. 1979. Cryptanalysis of an Enciphered Code Problem: Where an AdditiveMethod of Encipherment Has Been Used. Laguna Hills, CA: Aegean Park Press.

2. Bauer, F. L. 2007. Decrypted Secrets: Methods and Maxims of Cryptology (4th ed.).Berlin: Springer.

3. Beesly, P. 2000. Very Special Intelligence: The Story of the Admiralty’s Operational Intel-ligence Centre, 1939–1945. London: Greenhill.

58Quoted in ‘‘Review of Security of Naval Codes and Cyphers 1939–1945’’, 96.59On the CCM and its flaw, see [11, p. 59]. There were three models of the CCM: the

ECM with an adaptor, Typex with an adaptor, and a special machine (CCM Mk. II); theywere compatible with each other when used with the same rotor set: ‘‘Operating Instructionsfor Combined Cipher Machines’’, para. 6 (HCC, Box 1423, Nr. 4668).



4. Blair, Clay. 1996. Hitler’s U-boat War: The Hunters, 1939–1942. New York: RandomHouse.

5. Bonatz, H. 1970. Die Deutsche Marine-Funkaufklarung 1914–1945. Darmstadt: Wehr undWissen.

6. Bonatz, H. 1981. Seekrieg im Ather: Die Leistungen der Marine Funkaufklarung1939–1945. Herford: Mittler.

7. Budiansky, Stephen. 2000. Battle of Wits: The Complete Story of Codebreaking in WorldWar II. New York: Free Press.

8. Erskine, R. and P. Freeman. 2003. ‘‘Brigadier John Tiltman: One of Britain’s FinestCryptologists,’’ Cryptologia, 27(4):289–318.

9. Erskine, R. 1992. ‘‘The German Naval Grid in World War II,’’ Cryptologia, 16(1):39–51.10. Erskine, R. 2001. ‘‘Enigma’s Security: What the Germans Really Knew.’’ In Action This

Day: Bletchley Park from the Breaking of the Enigma Code to the Birth of the ModernComputer, edited by Ralph Erskine and Michael Smith. London: Bantam Press.

11. Erskine, R. 2002. ‘‘The Admiralty and Cipher Machines During the Second World War:Not So Stupid After All,’’ The Journal of Intelligence History, 2(2):49–68.

12. Gaines, H. F. 1956. Cryptanalysis: A Study of Ciphers and Their Solution. Mineola, NY:Dover.

13. Hinsley, F. H., E. E. Thomas, C. F. G. Ransom, and R. C. Knight. 1981. BritishIntelligence in the Second World War: Its Influence on Strategy and Operations, 2. London:Her Majesty’s Stationery Office.

14. Kahn, D. 1967. The Codebreakers: The Story of Secret Writing. New York: Macmillan.15. Kahn, D. 1978. Hitler’s Spies: German Military Intelligence in World War II. London:

Hodder & Stoughton.16. Ministry of Defence (Navy) [Gunter Hessler]. 1989. The U-boat War in the Atlantic.

London: Her Majesty’s Stationery Office.17. McCue, B. 2005. ‘‘A Chessboard Model of the U-boat War in the Atlantic with Applica-

tions to Signals Intelligence,’’ Naval Research Logistics, 52(2):107–136.18. Operations Evaluation Group. 1952. ‘‘Evaluation of the Role of Decryption Intelligence

in the Operational Phase of the Battle of the Atlantic, OEG Report #68’’. NACP, RG457, SRH-368.

19. Ratcliff, R. A. 2006. Delusions of Intelligence: Enigma, Ultra, and the End of SecureCiphers. New York: Cambridge University Press.

20. Roskill, S. W. 1956. The War at Sea, II, The Period of Balance. London: Her Majesty’sStationery Office.

21. Showell, J. P. M. 2003. German Naval Codebreakers. Annapolis: Naval Institute Press.

358 R. Erskine


13560_937877610_927073186

Documents