German codebreakers vs Enigma

During WWII the commercial Enigma cipher machine was used by the Croat State of Ante Pavelic and the Armed forces and Diplomatic corps of Switzerland. In both cases the Germans were interested in knowing the content of the messages.

For Croatia it was the need to keep a close eye on an ally in the Balkans. Switzerland on the other hand was a neutral state but since many agents and diplomatic missions operated on Swiss soil it was only natural that the Germans would want to learn its secrets.

The Croat Enigma was used by both military and civilian authorities, so it was attacked by the  Army signals intelligence agency  Inspectorate 7/VI and  by OKW/Chi. In both cases solution rested on knowing the wiring of the rotors plus cribs (known plaintext in the cipher text).Things were easy as the Croats did not change the ringstellung often . (Ringstellung:The ring setting, the position of the rotor wiring, relative to the alphabet rings ,Source: Rijmenants, Dirk(2010) 'Enigma Message Procedures Used by the Heer,Luftwaffe and Kriegsmarine',Cryptologia)

The Swiss Enigma was harder to break as the wiring of the wheels was changed upon receiving the machines. The military traffic could not be decoded due to few messages. The diplomatic traffic however was decoded successfully by OKW/Chi and the Forschungsamt.

Let’s take a look at the available information :

Croat Enigma  :

From EASI vol4 -Army High Command Signal Intelligence Service ,p171-2

When the German government established the puppet government of Croatia in 1941, the Croats were given the commercial model of the Enigma for use by their Army and Secret Police. The traffic of these units was read by the Balkan section currently without any delay.

From TICOM I-92 ‘’Final Interrogation of Wachtmeister Otto Buggisch’’ , p2-3

2. Solution of Croat Enigma. This was not an outstanding cryptanalytic achievement. The machine used was the K model, with three wheels and no stecker. The machines were made for the Croats by the firm of KROSKY and KRUGER, Berlin, which gave the wirings promptly to OKW/WNV in about 1941 or 1942. A single key was used throughout the entire Croat Army and area, and this consisted only of a list of 100 settings for a period of a month. As far as Buggisch knew the Ringstellung stayed always at AAA, and the wheel order at 1, 2, 3. Just to make sure, the Germans paid for one of the first keys used, and with this decoded traffic were able to establish stereotypes and solve almost 100% from the first.

The solutions were done entirely by hand with wiring charts, assuming a pet beginning (one third of all messages began with "MINORS") and assuming the left hand wheels and Umkehrwalz unmoving (only one notch per wheel as in the commercial model). The Croats also had pet indicators and so would furnish depths in case this method did not work. The setting was indicated directly by a two digit number unenciphered, so that the settings wore solved almost as fast as they came, and the traffic read currently from then on. Buggisch did not recall the contents in detail. 90% of it was uninteresting; there were some interesting messages about actions against Tito

Buggisch said the Germans had considered equipping the Croats With the military Enigma, as they did for HUNGARY,  ROUMANIA , FINLAND and ITALY (and JAPAN, he thinks) in about 1942. However, they decided acainst this as they believed the corrupt CROATS would go right on selling the keys to British agents, while they, the GERMANS, would have to pay as well instead of solving free. (The possibility of a BRITISH solution obviously did not occur to Buggisch during this discussion of the K model.)

Swiss Enigma :

From EASI vol 4 -Army High Command Signal Intelligence Service , p163

80. Work on Swiss, Spanish , Portuguese ,Brazilian traffic - From 1941-1944 –the French section of In 7VI had subsections for Swiss, Spanish, Portuguese, and Brazilian traffic. As Kuehn stated, however, the volume of traffic was always extremely small and unimportant .Buggisch, one of the chief cryptanalysts of In 7/VI, worked with Kunze of the Foreign Office on the solution of the Swiss Enigma (the Commercial Enigma) although they worked out a theoretical solution of the machine, the theory was never applied at In 7/VI to Swiss traffic because the volume did not warrant the effort.Moreover, easy solution of the traffic was precluded by the fact that the Swiss did their own wiring of the Enigma wheels and changed these frequently.

This refers to the Enigma used by the Swiss army. For the diplomatic version :

From EASI vol2 – Notes on German high level cryptology and cryptanalysis ,p76 :

39. Swiss Enigma rotor wirings were solved by cribs; other Enigmas were compromised - The Swiss diplomatic Enigma ("K' type) was read regularly, probably by the Signal Intelligence Agency of the Supreme Command Armed Forces (OKW/Chi), although Dr. Huettenhain, who revealed this solution, did not state definitely that it was his agency which accomplished it. The Swiss changed their Enigma rotor wirings every three months, but the changes were not effected on the Berne-Washington link at the time they were made on the Berne-London link. As a result duplicate messages sent by the Swiss to Washington and to London during the periods of changeover provided the "break' necessary: to learn the new wirings.

The Croat enigma, used for both diplomatic and military traffic, was read regularly by Inspectorate 7/VI (In 7/VI). This was no credit to the cryptanalysts involved, however, as their problem was particularly easy:

 (1) they had obtained the rotor wirings from Konski and Krueger (Berlin) who made the rotors;

(2) there was no end-plate plugging involved;

(3) the rotor orders were not changed by the Croats;

(4) the "ringstellungen' (devices enabling the notches and alignment indicator letters to be "slid' in relation to the rotors) remained fixed; and

(5) there were only 100 initial rotor alignments used by the Croats each month.

An excellent treatise on Enigma ("K’’ type) solution was found in the files of the Foreign Office Cryptanalytic Section (Pers Z S). It involved obtaining many messages in depth, reading these messages by solving the successive (monoalphabetic) columns of superimposed text, and then applying the resultant cribs to recovering the wirings of the rotors.These methods are well-known to Anglo-American cryptanalysts.

From EASI vol7 - Goering’s Research Bureau ,p87 :

Swiss Systems.

Swiss Enigma Machine. Paetzel stated that the Swiss Enigma  was formerly broken for a while, but only when it was Improperly used. The same internal settings  were used for a long time. After the inner settings changed we did not have any more solution. At first we reconstructed  the wheels from cribs and from the fact that the inner settings remained the same. In his 1941 Report Dr Brandes of Pen ZS mentions solution of the Swiss Enigma. Apparently the FA furnished Peps ZS with a partial solution which Dr Kunze was able to complete.Thereafter there was an exchange of keys between the two agencies.

From EASI vol3 - Signal Intelligence Agency of the Supreme Command Armed Forces , p68 :

Switzerland:

There was apparently a separate desk for Swiss traffic headed by First Lt. Kunze. Traffic enciphered on the Swiss Enigma vas regularly read. Huettenhain said that the wiring was changed every three months, but the changes at Washington were not made at the same time as those in London. Cribs thus provided by the Washington Traffic for the London traffic made it possible to solve the London wirings.

From EASI vol6 - The Foreign Office cryptanalytic section ,p32 :

Switzerland. Generally speaking, all Swiss diplomatic systems were read except the Enigma cipher machine. That, too, was read for a time in 1941. The codes were chiefly three-letter books, usually bi-lingual, enciphered with a series of substitution tables. New wirings of the Enigma were solved by cribs every three months. Subsequent messages were recovered from message texts.

The American and British codebreaking agencies also attacked the Swiss Enigma using the same methods as the Germans. It seems however that the results did not justify the effort as the content of the messages was not interesting:

By the fall of 1943, American cryptanalysts were reading Swiss Enigma traffic on the Berne-Washington circuit, and by 1945 they had general access to all circuits. The intelligence product, however, was modest. Traffic usually dealt with trade, prisoners of war, the repatriation of internees, and Swiss protection of 'enemy' interests in belligerent states. While appreciating the opportunity to monitor Swiss imports for evidence that goods were transshipped to Axis countries, American intelligence ultimately concluded that 'the intelligence value of most messages in Swiss systems is low'.

[Source: Alvarez, David(1997) 'No immunity: Signals intelligence and the European neutrals, 1939-45',Intelligence and National Security ]