The compromise of Polish diplomatic, intelligence service and resistance movement communications 1930’s-1945

 In WWII Poland fought on the side of the Allies and suffered for it since it was the first country occupied by Nazi Germany. In the period 1940-45 the Polish Government in Exile and its military forces contributed to the Allied cause by taking part in multiple campaigns of war. Polish pilots fought for the RAF during the Battle of Britain, Polish troops fought in N.Africa, Italy and Western Europe and the Polish intelligence service operated in occupied Europe and even had agents inside the German High Command.

Although it is not widely known the Polish intelligence service had spy networks operating throughout Europe and the Middle East. The Poles established their own spy networks and also cooperated with foreign agencies such as Britain’s Secret Intelligence Service and Special Operations Executive, the American Office of Strategic Services and even the Japanese intelligence service. During the war the Poles supplied roughly 80.000 reports to the British intelligence services (1), including information on the German V-weapons (V-1 cruise missile and V-2 rocket) and reports from the German High Command (though the agent ‘Knopf’) (2).

The activities of the Polish government departments and intelligence agencies attracted the attention of both Germany and the USA and UK and their codebreakers were able to solve most of the Polish cipher systems.

The Combined Cipher machine - 1942-1962

Cipher machines were extensively used by both the Axis and the Allies during WWII.

The Germans had the Enigma, the Lorenz SZ 40/42 and the Siemens T-52.

The Japanese had the Type B Cipher Machine (Angooki Taipu B).

The Americans had the Converter M-134-C (SIGABA) and the Hagelin M-209.

The British had the Typex.

Another machine that became increasingly important for the Allies, in the period 1943-45, was the Combined Cipher Machine - CCM. Unfortunately, this machine has not received a lot of attention from historians because there is limited information available on its internal operation and use in the field.

Here I have attempted to present information on the CCM from various sources that are not easy to find (the timelines section has detailed sources): 

Information on French codes and ciphers of WWII

Two very interesting articles from the ‘Bulletin de l’ARCSI’:

1). Bulletin N°3 1975: Essai d'historique du Chiffre (Add. N°3) by Général Ribadeau-Dumas.

2). Bulletin N°4 1976: Essai d'historique du Chiffre (N°5) by Général Ribadeau-Dumas.

Analysis of the Czechoslovak STP cipher

Essay on the WWII Czechoslovak STP cipher by Štefan Porubský.

Research Gate link: Application and Misapplication of the Czechoslovak STP Cipher During WWII

Note that I’ve dealt with the ciphers of the Czechoslovak resistance movement in Svetova Revoluce and the codes of the Czechoslovak resistance.

Article on Chinese codes and ciphers

Interesting article from the journal Cryptologia: ‘Chinese cryptography: The Chinese Nationalist Party and intelligence management, 1927–1949’ by Ulug Kuzuoglu.

ABSTRACT

This paper is the first scholarly attempt to examine the history of Chinese cryptography and the role it played in building the intelligence network of the Chinese Nationalist Party (KMT) from 1927 to 1949. Rather than investigating the institutional structure of intelligence, I focus on Chinese characters, the primary medium that made cryptology and intelligence possible. Given that the Chinese writing system is by nature nonalphabetic and thus noncipherable, how did cryptography work in Chinese? How did the state and its scientists reengineer Chinese characters for the purposes of secret communication? This paper argues that due to the Chinese writing system itself, Chinese cryptography was bound to the use of codebooks rather than ciphers; thus, “codebook management” was central to building intelligence networks in China.

The Carlson-Goldsberry report - Compromise of State Department communications by the Finnish codebreakers in WWII

During WWII the US State Department used several cryptosystems in order to protect its radio communications from the Axis powers. For low level messages the unenciphered Gray and Brown codebooks were used.  For important messages four different codebooks (A1, B1, C1, D1) enciphered with substitution tables were available.

Their most modern and (in theory) secure system was the M-138-A strip cipher. Unfortunately for the Americans this system was compromised and diplomatic messages were read by the Germans, Finns, Japanese, Italians and Hungarians. The strip cipher carried the most important diplomatic traffic of the United States (at least until mid/late 1944) and by reading these messages the Axis powers gained insights into global US policy.

Germans, Finns and Japanese cooperated on the solution of the strip cipher. In 1941 the Japanese gave to the Germans alphabet strips and numerical keys that they had copied from a US consulate in 1939 and these were passed on by the Germans to their Finnish allies in 1942. Then in 1943 the Finns started sharing their results with Japan. 

Finnish solution of State Department cryptosystems

During WWII the Finnish signal intelligence service worked mostly on Soviet military and NKVD cryptosystems however they did have a small diplomatic section located in Mikkeli. This department had about 38 analysts, with the majority working on US codes.

Head of the department was Mary Grashorn. Other important people were Pentti Aalto (effective head of the US section) and the experts on the M-138 strip cipher Karl Erik Henriksson and Kalevi Loimaranta.

Their main wartime success was the solution of the State Department’s M-138-A cipher. The solution of this high level system gave them access to important diplomatic messages from US embassies in Europe and around the world. 

Apart from purely diplomatic traffic they were also able to read messages of other US agencies that used State department cryptosystems, such as the OSS -Office of Strategic Services Bern station, Military Attaché in Switzerland, Office of War Information representative in Switzerland, the Foreign Economic Administration, War Shipping Administration, Office of Lend-Lease Administration and the War Refugee Board.

Operation Stella Polaris

In September 1944 Finland signed an armistice with the Soviet Union. The people in charge of the Finnish signal intelligence service anticipated this move and fearing a Soviet takeover of the country had taken measures to relocate the radio service to Sweden. This operation was called Stella Polaris (Polar Star).

In late September roughly 700 people, comprising members of the intelligence services and their families were transported by ship to Sweden. The Finns had come to an agreement with the Swedish intelligence service that their people would be allowed to stay and in return the Swedes would get the Finnish crypto archives and their radio equipment. At the same time colonel Hallamaa, head of the signals intelligence service, gathered funds for the Stella Polaris group by selling the solved codes in the Finnish archives to the Americans, British and Japanese. 

The Stella Polaris operation was dependent on secrecy. However the open market for Soviet codes made the Swedish government uneasy. In the end most of the Finnish personnel chose to return to Finland, since the feared Soviet takeover did not materialize. 

The American reaction and the Carlson-Goldsberry report

According to the NSA study History of Venona (Ft. George G. Meade: Center for Cryptologic History, 1995), it was at that time that the Finns revealed to the US authorities that they had solved their diplomatic codes. On 29 September 1944 colonel Hallamaa met with L. Randolph Higgs of the US embassy in Stockholm and told him about their success.

In response two cryptanalysts were sent from the US to evaluate the compromise of US codes in more detail. They were Paavo Carlson of the Army’s Signal Security Agency and Paul E. Goldsberry of the State Department’s cipher unit. Their report dated 23 November 1944 had details on the solution of US systems.

The Carlson-Goldsberry report

Unfortunately locating this report proved to be quite a problem. Initially I searched for it in the US National Archives (both in the NSA and OSS collections) but without success.

Thankfully the NSA FOIA/MDR office has managed to locate this file and they have finally declassified it.

Google drive link

The 4-page report summarizes the information gathered by US officials from their interviews of Finnish codebreakers in 16, 18 and 21 November 1944.

From the Finnish side Erkki Pale (head of the department working on Soviet ciphers) and Kalevi Loimaranta (member of the department dealing with foreign diplomatic codes) gave a summary of their work on various cryptosystems.

The Finns admitted to solving US diplomatic systems, both codebooks and the strip cipher M-138-A. According to them an unenciphered codebook could be reconstructed in 6 months but an enciphered one was harder to solve.

Regarding the M-138-A cipher it was solved because the alphabet strips were used for long periods of time, the same strips were used by several users and the numerical keys were the same for all users. Stereotypical beginnings and endings were also exploited in assumed plaintext cryptanalytic attacks.

There was cooperation with the German codebreakers on US systems and the Finns received a lot of intercepts from them.

The Finnish codebreakers also used a number of IBM machines for statistical work.

Although the Finns stated that after the introduction of channel elimination in January 1944 they could no longer solve strip cipher traffic a memo included in the report says that their detailed knowledge of channel elimination procedures may indicate continued success with the M-138-A system.

Acknowledgments: I have to thank my friends in the US for requesting this file from the NSA FOIA/MDR office and getting it declassified.

Compromise of Greek military and diplomatic communications in WWII

At the start of WWII the Kingdom of Greece, ruled by Ioannis Metaxas  (head of the 4th of August Regime) followed a neutral foreign policy and tried to avoid taking part in the conflict. However constant Italian harassment and provocations (such as the sinking of the cruiser Elli) and the transfer of Italian army units to Albania made it clear that war could not be avoided for long.

In October 1940 Italian forces invaded Greece, in the area of Epirus, and the Greek-Italian war started. The Greek forces were able to contain the assault and the Greek counterattack forced the Italians back into Albanian territory. After the defeat of a major Italian offensive in spring 1941 the front stabilized inside Albania.

At the time Britain was overextended with obligations in Europe, Middle East and Asia. However the British armed forces made a small contribution with an RAF expeditionary corps. When more British forces started to arrive in March 1941, their involvement gave Germany an excuse to become involved in the conflict.

German forces invaded Greece in April 1941 and made rapid progress due to the fact that almost the entire Greek Army was fighting in the Epirus area. The remaining units and the small British forces transferred to Greece in March-April 1941 were unable to stop them. Then in May 1941 the Germans were also able to defeat the Greek and British forces that had retreated to the strategic island of Crete.

Cipher machines of WWII

In the period 1939-45 most countries enciphered their communications using hand methods (codebooks, transposition etc). Only a few countries used cipher machines.

The following list covers these countries and the specific models they used.

United States

SIGABA

SIGCUM/Converter M-228 T/P

SIGTOT T/P (one time tape system)

CCM - Combined Cipher Machine

M-209 (US version of the Hagelin C-38)

AT&T double tape machine

Hebern cipher machine (5 rotor version)

SIGFOY/M-325 (Enigma type)

Britain

Typex (Enigma type)

Rockex T/P (one time tape system)

CCM - Combined Cipher Machine

Poland

Lacida

Soviet Union

K-37 (Hagelin B-211 copy)

Pogoda or Pagoda (copy of US AT&T double tape machine)

M-100/101 T/P

France

Hagelin B-211

Hagelin B-211 modified

Hagelin C-38 (US M-209 version)

Hagelin C-36

Sweden

Hagelin B-211

Hagelin C-38

Norway

Hagelin C-38

Holland

Hagelin C-38

Enigma G

Portugal

Hagelin C-38

Switzerland

Enigma K

Germany

Kryha

Enigma K

Enigma G

Enigma I (plugboard machine)

Enigma M4 (4 rotor naval version)

SG (Schlüsselgerät) 41 (Hagelin type)

SZ-40/42 T/P

T-52 T/P

T-43 T/P (one time tape system)

Italy

Enigma K

Enigma G

Enigma I

OMI NISTRI

Hagelin C-38

Olivetti T/P

Japan

RED/Angōki Taipu-A

PURPLE/Angōki B-gata

JADE

CORAL

Enigma T

Finland

Hagelin C-36

Romania

Enigma G

Enigma I

Hungary

Enigma I

Slovakia

Enigma I

Bulgaria

Enigma I

Croatia

Enigma K

Spain

Enigma K

The ciphers of Czechoslovakia’s government in exile

At the end of the First World War the multiethnic Austro-Hungarian Empire collapsed and out of its ruins emerged several new countries. One of these was Czechoslovakia, containing the Czech areas of Bohemia and Moravia together with Slovakia and Carpathian Ruthenia in the east.

In the interwar period Czechoslovakia followed a foreign policy supportive of France and was part of the Little Entente. The country had a stable democracy and its industrial resources were large (based on the Skoda works) for such a small country. However there were two important problems affecting Czech national security. On the one hand the rise of Nazi Germany and its rearmament was a clear security threat. At the same time there were serious problems with the German and Slovak minorities that resented Czech rule.

Czechoslovakia contained a large number of minorities that were dissatisfied with the ruling Czech establishment. Especially the German minority made up roughly 23% of the population (according to the 1921 census) and a large part of it was concentrated in the border with Germany called Sudetenland. Many of the Sudeten Germans wanted for their areas to be unified with Germany and in the 1930’s Hitler’s Germany supported the demands of the Sudeten German Party. These claims were rejected by the Czech government of Edvard Beneš and as the Czech crisis threatened Europe with a new war a conference took place in Munich between the governments of Germany, Italy, Britain and France.  

Without support from Britain and France the Czech government was forced to cede the Sudeten territories to Germany and also lost other disputed areas to Hungary and Poland. Even though Germany had succeeded in absorbing the Sudeten areas and in weakening Czechoslovakia that did not stop Hitler’s offensive plans and in March 1939 German troops invaded and occupied the rest of the country. From then on the country was ruled by Germany and special attention was given to its heavy industry which produced weapons for the German armed forces.

During the war the Czechoslovak Government in Exile, headed by Beneš, was based in London and had regular communications with the Czech resistance and with its diplomatic missions and intelligence service stations abroad. In order to protect these communications several cryptosystems were used by the Czech crypto department.

Information on these systems is available from books and articles written recently:

1). The books Odhalena tajemstvi šifrovacich kličů minulosti, Gentlemani (ne)čtou cizi dopisy and Válka šifer. Výhry a prohry československé vojenské rozvědky (1939-1945) by Jiri Janecek have descriptions of the ciphers and the author’s conclusion was that the security afforded by these systems was limited.

2). A series of articles written for ‘Crypto World’ by Jozef Kollár describe each cipher system, explain how to encipher and decipher and also evaluate their security. The articles are:

Cipher ‘TTS’ (double transposition of the text followed by letter to figure substitution)

Cipher ’Rimska dva’ (letter to figure substitution followed by double transposition of the digits)

Cipher ‘Rimska osem’ (homophonic letter to figure substitution followed by encipherment with repeating 10-figure additive sequence)

Cipher ‘Rimska devat’ (letter to figure substitution followed by additive encipherment, repeating additive created from a passphrase)

Cipher ‘Rimska desat’ (letter to figure substitution followed by transposition then additive encipherment, repeating additive created from a passphrase, passphrase is also used to limit the cells of the transposition table)

Cipher ‘Rimska trinast’ (transposition of the text followed by polyalphabetic substitution)

Cipher ‘Eva’ (transposition of the text using a pyramid shaped transposition table)

Cipher ‘Marta’ (letter to figure substitution followed by additive encipherment, additive created via passphrase autoclave procedure)

Cipher ‘Ruzena’ (letter to figure substitution followed by additive encipherment, additive created via passphrase procedure)

Cipher ‘Utility’ (letter to figure substitution followed by transposition)

Cipher ‘Palacky’ (letter to figure substitution followed by additive encipherment, additive created via passphrase procedure, cipher digits multiplied by a constant)

Compromise of Czech ciphers

The Czech resistance movement and the Czech intelligence service caused serious problems for the German authorities with their most audacious operation being the assassination of Reinhard Heydrich, protector of Bohemia and Moravia and former head of the Reich Main Security Office. However after this episode the Germans took many security measures and were generally able to keep the resistance activities under control. Keeping the Czech areas pacified was particularly important since Czechoslovakia had a developed heavy industry sector which produced weapons for the German armed forces.

In their counterintelligence operations the Germans benefitted from having the ability to read a substantial amount of the traffic exchanged between the Czech IS in Britain and the Czech resistance in the occupied territories. This case has been covered in detail in Svetova Revoluce and the codes of the Czechoslovak resistance.

Report on the compromise of the communications of the government in exile

After the end of WWII it seems that the Czechoslovak authorities learned from POW interrogations about the compromise of their ciphers. Karol Cigáň, who worked in the Defense Ministry’s cipher department, summarized some of this information in a report written in 1989.

The report ‘Dopady lúštenia šifrovacieho systému čs. londýnskeho MNO z rokov 1940-1945 na domáci odboj’, can be found in the archive of the Museum of the Slovak National Uprising in Banská Bystrica and in the Central Military Archive at Prague.

In the report Cigan analyzed the Czechoslovak STP cipher and found it insecure. In addition he proved the compromise of Czechoslovak ciphers by examining reports from the office of the high ranking SS official Karl Hermann Frank.

A report from November 1944 had a summary of Funkwabwehr (Radio Defense) operations and it said that during the previous month 8 radio links, whose cipher procedures could be solved, were kept under observation. Of special interest was traffic between the Protectorate and London regarding the preparations for the uprising.

In the month of October a total of 488 messages were solved and 8 cipher keys derived for the STP cipher.

In pages 37-41 Cigan directly compared the Funkawbehr decodes with some of the Czechoslovak telegrams found in the country’s national archives.

For example messages exchanged between the Minister of National Defense General Ingr and Ján Golian and Jaroslav Krátký in the Protectorate and with Heliodor Píka in Moscow. 

The author’s conclusion was that the use of insecure ciphers during wartime played an important role in undermining the operations of the Czechoslovak resistance movement and these events should be acknowledged by the country’s historians. 

Acknowledgments: I have to thank Jozef Krajcovic for his help in locating the report ‘Dopady lúštenia šifrovacieho systému čs. londýnskeho MNO z rokov 1940-1945 na domáci odboj’ and Štefan Porubský for informing me of the articles from Crypto World. 

Crypto World articles on the ciphers of Czechoslovakia’s government in exile

Information on Czechoslovak ciphers of WWII is available from a series of articles written for crypto world. Jozef Krajcovic has added links to these articles at katkryptolog.

These were hand systems using mostly letter to figure substitution and transposition. 

Overview of reports on compromise of Allied communications in WWII

When researching WWII signals intelligence and codebreaking operations it is important to track down all the available sources of information, from POW interrogations and captured documents to detailed reports summarizing Allied cipher policy, decisions and compromises.

It is reasonable to assume that in the postwar era all the Allied countries tried to assess the extent of the compromise of their communications during WWII. Thus I’ve tried to track down these reports. In the following space i’m going to list the reports I know of and give a short summary of each one. I’ve used most of them in my essays but there are some that I haven’t been able to locate (yet).

USA

Military cryptosystems

1). The ‘European Axis Signal Intelligence in World War II’ volumes are available from the NSA’s website. They are dated May 1946 and have a lot of information on German cipher machines, the German codebreaking agencies and their major achievements. Unfortunately I don’t think that the EASI volumes are entirely accurate.

2). The report Special conference on M-209 security is dated 1950 and looks into the German exploitation of the US M-209 cipher machine. It is difficult to explain why the people involved looked only at the available TICOM reports and not the actual files of the German Army’s codebreaking agency Inspectorate 7/VI. These had been retrieved in 1947 from a camp in Austria.

Overall it seems to me that US reports are not as thorough as they should be.

State Department

I am not aware of any State Department communications security report written after the end of WWII. The ones available were produced during the war and submitted to the US Joint Chiefs of Staff.

1). The reports of June 1941 and November/December 1943 are available online via the NSA’s Friedman collection.

2). The report of June 1944 can be found in the US National Archives and Records Administration - collection RG 457- Entry 9032- box 1384 - 'JCS Ad hoc committee report on cryptographic security of government communications'.

The problem is that these reports only have theoretical evaluations of the State Department cryptosystems, not information on their compromise by the Axis powers.

OSS

The Office of Strategic Services was the predecessor of the CIA and during WWII its overseas stations gathered military and political intelligence on the Axis powers. For example in Bern, Switzerland the local OSS station (headed by Allen Dulles) kept in contact with members of the German Resistance.

According to an OSS veteran a report on WWII crypto security was prepared but I don’t have any details.

Soviet Union

Both in the Soviet era and today in modern Russia the files and archives of the signal intelligence organizations remain closed to researchers. This makes it very difficult to find information on Soviet cryptosystems and on whether the Soviet authorities evaluated their communications security during WWII.

According to Anatoly Klepov's “The historical truth” of Beria and Suvorov about cryptography and radio intelligence, there were evaluations of Soviet cryptosecurity in 1941, in 1943 and at the end of WWII. These investigations came to the conclusion that Soviet ciphers had not been compromised by the Germans but that they were theoretically solvable. This led to the introduction of new, more secure systems.

UK

Army

There are files dealing with British Army communications at the National Archives, collections WO 219 and WO 193. However these deal with cipher policy, not cipher compromises.

Navy

The report ADM 1/27186 ‘Review of security of naval codes and cyphers 1939-1945’ is available from my Google docs and Scribd folders. It is dated November 1945 and it was written after examining the files of the German Navy’s codebreaking agency B-Dienst. Thus it has an excellent analysis both of Royal Navy cipher policy and the compromise of RN codes and ciphers.

RAF

The report AIR 20/1531 ’R.A.F. signal communications: security’ can be found in the National Archives but the information on the compromise of RAF systems is only based on decrypted German messages, not other sources.

SOE

The Special Operations Executive was an intelligence organization created during WWII. I don’t know if a report on the compromise of SOE codes was ever prepared. The British authorities certainly had the available information, since they captured the War Diary of Inspectorate 7/VI in 1947 and this included the summary reports of Referat 12 which dealt with Allied agents’ codes.

The report HS 7/41 ‘Section II: field cyphers; appendices L, M, N and O’ has a file titled S.O.E. FIELD CIPHERS which is available from the Arcre website. This file points out theoretical and practical problems with SOE cipher security but does not have information on the compromise of said systems by enemy codebreakers. 

France

French military high level codes were solved by the Germans in the 1930’s and up to the Battle of France. Unfortunately I am not aware of any French report summarizing their cryptosystems and their compromise during the war.

Czech Republic

During WWII the communications of the Czech resistance movement were solved by the Germans. It seems that at the end of the war the Czechoslovak government found out about this case and personnel of the Defense Ministry prepared the report ‘Dopady lúštenia šifrovacieho systému čs. londýnskeho MNO z rokov 1940-1945 na domáci odboj’.

I requested this report from the Czech Defense Ministry’s history department but they were not able to locate it. Let’s hope that some Czech researcher will be able to find it.

Poland

Polish military, diplomatic, resistance movement and intelligence service codes were read by the Germans during WWII. I don’t know if there are detailed reports in the Polish government archives but the following sources have valuable information.

1). The Cryptologia article ‘From the Archives: Polish Interwar MFA's Cipher Compromised?’ by Jan Bury presents two reports written in 1940 and 1947 by the intelligence officer Major Tadeusz Szumowski, concerning the security of the Polish diplomatic Code 45.  According to Szumowski this codebook had been compromised and the Germans could read the Polish diplomatic traffic since the leadership of the Polish Foreign Ministry chose to keep using it despite his warnings. Szumowski was correct about this case.

2). Recently the book KODY WOJNY. Niemiecki wywiad elektroniczny w latach 1907–1945 a losy polskich, sowieckich, alianckich kodów i szyfrów has been published by Marian Zacharski. I don’t know if the author has information on the Polish ciphers from Polish sources but he has uncovered material from the recently released TICOM files on the extensive compromise of Polish cryptosystems during the war.

Conclusion

Compromises of communications security are usually difficult to acknowledge by the countries that suffer them. For example since the 1970’s countless books have been written about the successes of Bletchley Park, yet detailed information on the Axis solution of Allied codes only started to become available in the 2000’s when TICOM reports and other relevant documents were released to the public archives by the US and UK authorities.

I’ve listed some reports that deal with the compromise of Allied codes but I hope that researchers will be able to locate more. Specifically:

1). State Department postwar reports on WWII compromises (especially the M-138-A strip cipher case)

2). The OSS report on WWII cryptosecurity.

3). The Soviet reports of 1941, 1943 and postwar period.

4). Reports similar to ADM 1/27186 for the British Army, RAF and SOE.

5). French reports on their WWII cryptosystems.

6). The Czech report ‘Dopady lúštenia šifrovacieho systému čs. londýnskeho MNO z rokov 1940-1945 na domáci odboj’.

7). Polish government-in-exile reports on their codes and ciphers. 

The US Division Field Code

When the United States entered WWII, in December 1941, US military and civilian agencies were using several cryptologic systems in order to protect their sensitive communications. The Army and Navy only had a small number of SIGABA cipher machines so they had to rely on older systems such as the M-94/M-138 strip ciphers and on codebooks such the War Department Telegraph Code, the Military Intelligence Code and the War Department Confidential Code.

Another system prepared for the Army was the Division Field Code. This was a 4-letter codebook of approximately 10.000 groups and in the 1930’s several editions were printed by the Signal Intelligence Service (1). However the introduction of the SIGABA and especially the M-209 cipher machine made this system obsolete. Still it seems that the DFC was used on a limited scale, during 1942-44, by the USAAF and by US troops stationed in Iceland and the UK.

Examples of DFC training edition No 2:

Solution of DFC by German codebreakers

The German Army and AF signal intelligence agencies were able to exploit this outdated system and they read US military messages from Iceland, Central America, the Caribbean and Britain. Most of the work was done by field units, specifically the Army’s fixed intercept stations (Feste Nachrichten Aufklärungsstelle) Feste 9 at Bergen, Norway and Feste 3 at Euskirchen, Germany.

According to Army cryptanalyst Thomas Barthel several editions of the Division Field Code were read, some through physical compromise (2):

The DFCs (Divisional Field Codes).

(a). DFC 15

In use in autumn 42, broken in Jan 43. Traffic was intercepted on a frequency of 4080 Kos from US Army links in ICELAND (stas at REYKJAVIK, AKUREYRI and BUDAREYRI). Stas used fixed call-signs till autumn 43, and thereafter daily call -signs. This field code was current for one month only. It was a 4-letter code, non-alphabetical, with variants and use of "duds" (BLENDERN). It was broken by assuming clear routine messages were the basis of the encoded text, such as Daily Shipping Report, Weather Forecast etc.

(b) DFC 16

This was current for one month, probably in Nov 42. It was similar to  the DFC 15 above.

(c) DFC 17

This was current from Dec 42 to Feb 43. About the latter date one or two copies of the table were captured. Very good material was intercepted from ICELAND, also from 6 (?) USAAF links in Central America, Caribbean Sea etc. Traffic was broken and read nearly up to 100%.

(d) DFC 21

This succeeded the DFC 17. Results were the same.

(e) DFC 25

Current only in CARIBBEAN SEA area, and read in part.

(f) DFC 28 

This succeeded the DFC 21 in summer 43. It was used by the ICELAND links and the 28 (or 29) US Div in the South of ENGLAND. The code was read, Now and again it was reciphered by means of alphabet substitution tables ("eine Art von Buchstabentauschtafel") changing daily. This method was broken because the systematic construction of the field code was known.

(g) DFC 29

A copy of this table was captured in autumn 43. It was never used, PW did not know why.

The War Diary of the German Army’s signal intelligence agency OKH/In 7/VI shows that the DFC was called AC 6 (American Code 6) and several editions were solved in the period 1943-44. Most of the processing was left to field units, with only a few messages solved by Referat 1 (USA section) of Inspectorate 7/VI. The report of March 1943 says that the captured specimen DFC 17 could be used to solve the preceding and following versions (since they were constructed in the same way) and it showed that the code values retrieved by field units and the central department through cryptanalysis were mostly correct (3).

The Luftwaffe’s Chi Stelle was also interested in the DFC and according to Dr. Ferdinand Voegele, the Luftwaffe's chief cryptanalyst in the West, USAAF messages from the Mediterranean area were read (4).

The 29th Infantry Division and the invasion of Normandy

In 1943 the M-209 cipher machine replaced the M-94 strip cipher as the standard crypto system used at division level by the US Army, however older systems like the DFC continued to be used for training purposes. The US military forces in Britain took part in many exercises during the latter part of 1943 and early 1944, since they were preparing for the invasion of Western Europe and some of their training messages were sent on the 28th edition of the Division Field Code.

These messages were intercepted and decoded by the German Army’s KONA 5 (Signals Intelligence Regiment 5), covering Western Europe.  NAAS 5 was the cryptanalytic centre of KONA 5 and its quarterly reports (5) show that training messages from the US V Expeditionary Corps and the 29th Infantry Division were solved.

The solution of these messages allowed the Germans to identify the 29th Infantry Division and considering the unit’s rule during operation Overlord it is possible that they gave the Germans vital clues about the upcoming invasion of France.

Notes:

(1). Rowlett-1974 and Kullback-1982 NSA oral history interviews

(2). CSDIC/CMF/Y 40 – ‘First Detailed Interrogation on Report on Barthel Thomas

(3).War diary Inspectorate 7/VI - March 1943

(4). TICOM IF-175 Seabourne Report, Vol XIII, p9 and 16.

(5). E-Bericht der NAASt 5 Nr 1/44 and Nr 2/44.

Sources: Frank Rowlett NSA oral history interview - 1974, Solomon Kullback NSA oral history interview - 1982, CSDIC/CMF/Y 40 – ‘First Detailed Interrogation on Report on Barthel Thomas’, War diary Inspectorate 7/VI, War diary NAAS 5, TICOM IF-175 Seabourne Report, Vol XIII ‘Cryptanalysis within the Luftwaffe SIS’, DFC training edition No 2, Division Field Code No 4

Acknowledgments: I have to thank Rene Stein of the National Cryptologic Museum for the Rowlett and Kullback interviews and Mike Andrews for the DFC pics.