Sunday, October 1, 2017

The compromise of the Swiss diplomatic Enigma K cipher machine in WWII

In the course of WWII the Allied and Axis codebreakers attacked not only the communications of their enemies but also those of the neutral powers, such as Switzerland, Spain, Portugal, Turkey, Ireland, the Vatican State and others (1).
Switzerland was a traditionally neutral country but during the war it had close economic relations with Germany and it also acted as an intermediary in negotiations between the warring nations. Important international organizations like the Red Cross and the Bank of International Settlements were based in Switzerland.

Naturally both the Allies and the Germans were interested in the communications of the Swiss government.

Swiss diplomatic codes and ciphers

The Swiss Foreign Ministry used several cryptologic systems for securing its radio messages. According to US reports (2) several codebooks were used, both enciphered and unenciphered. These systems were of low cryptographic complexity but had an interesting characteristic in that the same codebooks were available in three languages.
French, German and Italian were the recognized official languages of Switzerland. The codebooks of the Swiss foreign ministry had versions in French, German and English.
Apart from codebooks the Swiss also used a number of commercial Enigma cipher machines at their most important embassies.

The Swiss Enigma K cipher machine

Since the 1920’s the Enigma cipher machine was sold to governments and companies that wanted to protect their messages from eavesdroppers.

The latest version of the commercial Enigma machine was Enigma K. In WWII this device was used by the Swiss diplomatic service and armed forces.

The device worked according to the Enigma principle with a scrambler unit containing an entry plate, 3 cipher wheels and a reflector. Each of the cipher wheels had a tyre, marked either with the letters of the alphabet or with the numbers 1-26, settable in any position relative to the core wheel, which contained the wiring. The tyre had a turnover notch on its left side which affected the stepping motion of the device.

The position of the tyre relative to the core was controlled by a clip called Ringstellung (ring setting) and it was part of the cipher key, together with the position of the 3 cipher wheels. 

The commercial version was different from the version used by the German Armed Forces in that it lacked a plugboard (stecker). Thus in German reports it was called unsteckered Enigma.

In 1938 the Swiss government purchased 14 Enigma D cipher machines, together with radio equipment. The next order was in 1939 for another 65 machines and in 1940 they received 186 Enigma K machines in two batches in May and July ’40. The Enigma cipher machines were used by the Swiss Army, Air Force and the Foreign Ministry (3).

Military version

The majority of the Enigma machines were used by the Swiss Armed Forces. Apparently the Swiss were aware of the Enigma weaknesses so they modified their machines.
The wheels were rewired and the stepping motion of the device was altered (4).
In regular Enigma machines the movement of the rotors was predictable due to their having only one notch. The fast rotor moved with every key depression, the middle rotor moved once every 26 key depressions and the slow rotor (the left one) moved only once every 676 key strokes (26x26).

The Swiss military modified their Enigmas so that the middle rotor moved with every key depression, instead of the one on the right.

During WWII it seems that these security measures paid off since there is no indication that either the Allies or the Axis were able to solve Swiss military Enigma traffic.

US effort

The US and UK effort was concentrated on the Swiss diplomatic Enigma traffic, thus it does not seem like they were able to solve any military traffic.

The report ‘European Axis Signal Intelligence in World War II’, vol1 (dated May 1946) says in the ‘Results of European Axis cryptanalysis’ - Switzerland that the Enigma traffic SZD-1 was solved but not SZD-2 and SZD-3.

SZD and SZD-1 were diplomatic traffic and it is possible that SZD-2 and SZD-3 were the US designations for Swiss military traffic.

The special research history SRH-361 ‘History of the Signal Security Agency volume two - The general cryptanalytic problems’ mentions, in chapters VII and XVI, the Swiss diplomatic Enigma but not the military version.

Thus there is no indication that the Anglo-Americans solved the military traffic.

German effort

During WWII the German Army made extensive use of signals intelligence and codebreaking in its operations against enemy forces. German commanders relied on signals intelligence in order to ascertain the enemy’s order of battle and track the movements of units.

The German Army’s signal intelligence agency operated a number of fixed intercept stations and also had mobile units assigned to Army Groups. These units were called KONA (Kommandeur der Nachrichtenaufklärung) - Signals Intelligence Regiment and each had an evaluation centre, a stationary intercept company, two long range signal intelligence companies and two close range signal intelligence.

The KONA units did not have the ability to solve complicated Allied cryptosystems. Instead they focused on exploiting low/mid level ciphers and even in this capacity they were assisted by material sent to them by the central cryptanalytic department. This was the German Army High Command’s Inspectorate 7/VI

Inspectorate 7/VI had separate departments for the main Allied countries, for cipher security, cipher research and for mechanical cryptanalysis (using punch card machines and more specialized equipment).

Swiss ciphers were worked on by Referat 3 (France, Switzerland, Spain, Portugal), headed by Sonderführer Hans Wolfgang Kühn. In the period 1941-42 this department solved Swiss hand ciphers and did some research on the Swiss military Enigma (5).

The War Diary of Inspectorate 7/VI shows that in 1941 Swiss traffic was intercepted and worked on by the fixed intercept station Strasbourg (Festen Horchstelle Strassburg). Some hand ciphers were solved but by late ’41 it was clear that the Enigma machine was entering service and that it would replace the old cipher procedures.

Report of November 1941:

Referat 3


Der Spruchanfall der Schweiz ist sehr gering geworden. Alle Anzeichen deuten darauf hin, dass die Schweiz das Schwergewicht ihrer Verschlüsselungsmethoden auf die Maschine verlegt hat. Aus Chi-Spruch inhalten geht hervor, dass die 'Enigma' in Verwendung ist. (Vergleiche hierzu die VN-Meldungen: 1/41 Spruch)

Maschinensprüche liegen in geringer Anzahl bereits vor und werden ständig beobachtet bis eine in Arbeit nahme möglich wird.

In late 1941 and early 1942 there were several meetings with officials of the Foreign Ministry’s deciphering department Pers Z in order to discuss the Swiss Enigma problem.

In October 1941 Kühn (head of Referat 3) and dr Steinberg (member of the mathematical research department) met the Pers Z’s dr Kunze and discussed the Swiss Enigma procedure. The Inspectorate 7/VI officials wanted to clarify if the military version of the Swiss Enigma used the same wheel wirings as the diplomatic one. However due to the limited intercepted traffic it was not possible to solve this issue.

Sonderführer Kühn and dr Kunze met again in January and March 1942. The March ’42 report says that an Enigma machine with Swiss wheel wirings was loaned to the department for a short time.

Dr Buggisch, an Army cryptanalyst who specialized on cipher machines, examined the Swiss Army messages and worked out a theoretical method of solution which however depended on knowing the wheel wirings (6).

Despite these efforts the Swiss military Enigma was not solved and from August 1942 Swiss radio traffic was monitored but not actually worked on.

Diplomatic version

According to US and German reports (7) the diplomatic Enigma was used on the links Bern-Washington, London, Berlin, Rome.

The diplomatic Enigma machines were rewired by the Swiss but their stepping system was not modified.

During WWII both the Anglo-Americans and the German codebreakers were able to solve Swiss Enigma diplomatic traffic.

US/UK effort

The codebreakers of the US Army Security Agency devoted most of their resources against German and Japanese ciphers but they did not neglect to solve the cryptosystems of neutral countries.

The postwar report 'Achievements of the Signal Security Agency in World War II’ (dated February 1946) says in page 31 that ‘The traffic of the Swiss Government provided cryptanalytic problems of moderate difficulty and owing to the fact that the Swiss served as representatives of belligerents in many countries, Swiss traffic was an important source of information’.

Swiss crypto systems were worked on by a sub unit of the Romance Language Code Recovery section, created in December 1942. The Swiss unit was joined with the French Code Recovery unit in March 1943 but in August 1944 it was made independent again. The unit worked on the Swiss codebooks while the Enigma traffic was solved by the machine cipher section and the results passed to the Swiss unit for further processing. The Swiss Enigma was designated system SZD and work on it started in December 1942, with the first translations issued in July 1943 (8).

The US codebreakers cooperated closely with their British counterparts on the systems of neutral countries, including Switzerland. The British had better coverage of European radio traffic and had been working on these systems for a long time.

Regarding the Swiss Enigma traffic the British had exclusive coverage of the link Bern-London and the Americans of Bern-Washington (9).

According to US reports (10) messages were either in French, German or English and numbers were sandwiched between X and Y with the figures 1234567890 substituted by the letters QWERTZUIOP respectively.

Up to late 1942 the internal settings (wheel order and ring settings) were valid for a week and the same key was used for the links Bern-Washington-London.

The cipher machine employed only 3 wheels which the Anglo-Americans called ‘Blue’, ‘Red’ and ‘Green’. The wheels however were rewired frequently. One set was used for the period August ’42 - 6 April ’43 then new wirings for the period 7 April ’43 - 31 December ’43 and the last one mentioned in the report covers the period January ’44 – October’44. These wirings were received by the British codebreakers (11). 

Originally the indicator (showing the starting position of the rotors) was sent in the clear but from August 1942 it was enciphered. The cipher clerk chose a random position for the wheels and enciphered the ring setting on it to produce the message’s setting.

In 1943 the cipher procedure was changed and a large set of numbered keys were used with the internal key (wheel order and ring settings) being determined by the serial number of the message. The indicator procedure remained the same, with the cipher clerk choosing a random setting for the wheels and enciphering the ring setting on it to get the message’s key. Different numbered keys were introduced for each link.

From February 1944 some messages were doubly enciphered. The first indicator worked in the manner already described previously. Then the cipher clerk chose another random 4-letter indicator, set the wheels on it and enciphered the text one more time, including the first indicator. The second indicator was sent in the clear as the first group and repeated anywhere within the first ten groups of text.

The messages were sent in 5-letter groups with the first 4 letters being the indicator. Some messages had the following coded designations: Saturn, Wega, Merkur, Helos, Nira, Urania. These were indicators of content with Wega referring to shipping and transport matters, Saturn dealing with trade and Merkur with finance.

Example of Swiss telegram (12):

Solution of the Swiss Enigma depended on the use of stereotyped beginnings and on operator mistakes. The Enigma settings were recovered by using ‘cribs’ (suspected plaintext in the ciphertext) and sometimes ‘cillies’ (mistakes/non random choices by the cipher clerks) (13).

Some of the cribs used on the link Bern-Washington were: ‘Von Wanger fuer transport’, ‘Fuer transport’, ‘Pour transport’, ‘Transport’, ‘Wanger’, ‘Surcommerce’, ‘Fuer surcommerce’, ‘Handel’, ‘Ihr X’, ‘Unser X’, ‘Votre X’, ‘Fuer Wanger’, ‘Fortsetzung’.

IBM punch card equipment was used to speed up the solution.

Occasionally messages could be solved by using the indicators. As has been mentioned previously each message had a 4-letter indicator, chosen by the cipher clerk. After setting the wheels at the letters of the indicator the operator then enciphered the ring setting on the machine in order to get the message key. The 4 letters of the external indicator were supposed to be chosen at random, however sometimes the cipher clerks would choose the setting which they found in their machine after setting up the ring setting clips. This was usually one or two positions forward of the clip setting.

These non random indicators could be exploited to solve the Enigma:

The Swiss SIGABA

After recovering the internal settings of the device and the message key it was possible to decode the intercepted traffic.

Instead of buying a commercial Enigma machine and rewiring it to Swiss specifications the US codebreakers modified one of their SIGABA cipher machines, thus turning it into a Swiss Enigma clone.

Content of the messages

In general Swiss diplomatic traffic was judged to be of low intelligence value. Most messages dealt with Swiss trade, activities on behalf of the Red Cross, prisoners of war, Swiss representation of interests of other countries, conditions of neutrals in warring countries etc. Messages judged to be valuable were those that dealt with Swiss trade, Swiss representation of the interests of third countries and those concerning abuse of the Swiss diplomatic pouch.

Out of all the Swiss crypto systems the Enigma cipher was the most important and in 1943 out of 906 Bern-Washington intercepts 266 were published in reports (14).

Effects of improved security procedures

In 1943 the introduction of a different rotor arrangement for each pair of messages complicated the solution of Swiss Enigma traffic. From then on the US codebreakers would have to identify the rotor order, the ring settings and the starting position of the rotors for each two messages.

It seems that due to the limited value of the Swiss messages and the significant resources needed for regular solution of the individual key settings by late 1943 the Swiss Enigma problem was downgraded in terms of importance and the traffic was mostly used for training purposes. The keys to the Bern-London traffic were received from the British (15).

German effort

Foreign diplomatic codes and ciphers were worked on by three different German agencies, the German High Command’s deciphering department – OKW/Chi, the Foreign Ministry’s deciphering department Pers Z and the Air Ministry’s Research Department - Reichsluftfahrtministerium Forschungsamt.

OKW/Chi effort

At the High Command’s deciphering department - OKW/Chi, Swiss diplomatic systems were worked on by a subsection of main Department V. Depending on the source this was either Section 5 (France, Switzerland), headed by dr Helmuth Mueller or Section 2 (Switzerland), headed by dr Peters (16).

According to dr Erich Hüttenhain, chief cryptanalyst of OKW/Chi, the Swiss Enigma machine was solved by his unit. The wirings of the wheels changed every 3 months but they were not changed on all the links simultaneously. The machines on the link Bern-Washington continued to use the old wirings for some time thus these messages could be solved and they provided ‘cribs’ which could be used to solve the Bern-London traffic and recover the new wirings (17).

The TICOM report I-45 ‘OKW/Chi Cryptanalytic research on Enigma, Hagelin and Cipher Teleprinter machines’ describes three methods for solving the commercial Enigma machine.

1). By using ‘depths’ (messages enciphered on the same wheel settings):
If 20 to 25 messages of the same setting are available then the solution of these messages can be done in an elementary manner ie, the columns of the encoded texts written under one another in depth are solved as a Spaltencasar. In this the reciprocity of the substitutions is made use of to a great extent. In the solution procedure no other characteristic of the machine is used. This is also valid for the elementary solution of Stocker Enigma. After this elementary solution of the encoded texts the determination of the machine setting presents no difficulties.’

2). By using a ‘crib’ (suspected plaintext in the ciphertext) and taking advantage of the regular stepping of the Enigma. In the example given the crib ‘gabinetto alt’ is used:

3). By using the E-Leiste (E-List) method. This method was based on comparison of the frequency of the letter E in clear text and in the examined cipher text. According to the report this was only a theoretical solution and it was not used in practice since the ‘crib’ method sufficed:

With the K-machine six different wheel orders are possible. The adjustable Umkehr wheel can be set in twenty-six different positions. The periods of the three moveable wheels is about 17,000 steps, There are therefore 6 x 26 = 156 different periods of 17,000 long respectively possible. If in each of the 156 different periods the clear letter e is encoded 17,000 times, then 156 rows of encoded elements results, each 17,000 long. All these rows of encoded elements are designated e-Leiste.

The clear letter e appears in German with a frequency of 18%. If a German clear text encode with the K-machine is moved through the e-Leiste and if in each position the corresponding encoded elements are counted, then the correct phase position will have the maximum cases of correspondence. In this the Ringstellung need not be considered. The e-Leiste need only be prepared once. The comparison of the encoded text with the e-Leiste would have to be carried out on a machine. In order to come to a positive conclusion in a reasonable time, then several machines would have to be used at the same time, even if one machine was capable of making 10,000 comparisons per second.

In GERMANY a practical solution with the aid of the e-Leist was not carried out, as in, practice the method of solution from a part compromise was always possible.’

Pers Z effort

At the Foreign Ministry’s deciphering department Swiss systems were worked on by a group headed by Senior Specialist dr Wilhelm Brandes. This section, which dealt with French, Dutch, Belgian, Swiss and Romanian ciphers, successfully solved several Swiss codebooks and the Enigma machine.

In TICOM report I-22 ‘Interrogation of German Cryptographers of Pers Z S Department of the Auswaertiges Amt’ (18) there is some information regarding the Swiss Enigma.

In page 14 Dr Rudolf Schauffler (head of Pers Z) said that ‘The commercial type Enigma used by the Swiss was sometimes solved by stereotyped beginnings and known settings. The Swiss used to include in their messages the machine settings for the next message’.

In page 20 it says that ‘Dr. Brandes was unable to state the exact dates when the Swiss Eniqma was read but said that it was read completely for a considerable time. [Comment: the phrasing of his statement implied that there was also a time when it was partially readable].

These statements can be confirmed by the Pers Z file ‘Bericht der Belgisch-Französisch-Schweizerischen Gruppe Stand 31.12.1941’ (19) since it contains reports that mention the Swiss Enigma traffic.

The report of Group Brandes for 1940 says that most of the Swiss diplomatic traffic was sent using letter codebooks. However from the end of May 1940 traffic between Bern-Berlin and Bern-London had been sent using the Enigma machine. According to the report ‘a solution should be possible with ample material and sufficient personnel’.

According to the report for 1941 the Swiss Enigma was solved thanks to a partial solution provided by the Forschungsamt. In order to process this traffic two Enigma machines were purchased and rewired according to the Swiss specifications and the results passed on to the FA. In some cases the inner settings of the device were given in the telegrams. The machine was used on the links Bern - Berlin, London, Washington.

Apart from the Forschungsamt’s assistance there was also exchange of information between Pers Z and Inspectorate 7/VI on the Swiss Enigma. A detailed report on the solution of the commercial Enigma was found in the Pers Z files (20). This was written by Inspectorate 7/VI mathematician dr Rudolf Kochendörffer (21). It involved obtaining many messages in depth, reading these messages by solving the successive (monoalphabetlc) columns of superimposed text and then applying the resultant cribs to recovering the wirings of the rotors.

Forschungsamt effort

At the Air Ministry’s Research Department Swiss systems were worked on by Abteilung 8, Branch A, Section 3 (Holland, Switzerland, Luxembourg, Abyssinia). The department had about 30-40 workers (22).

According to dr Martin Paetzel (deputy director of Main Department IV - Decipherment) ‘their main machine success was with the Swiss Enigma as long as the same machine setting was used over a longish period’ (23).

More details about the Forschungsamt solution of the Swiss Enigma are given by Bruno Kröger in TICOM reports DF-240 and DF-241 (24). Kröger was the FA’s cipher machine expert and during the war he solved several foreign cipher machines.

The Swiss Enigma was first solved as a polyalphabetic substitution cipher, by processing several messages sent on the same key. The solution of these ‘depths’ led to the recovery of the wheel wirings and the further exploitation of the traffic. When the wheels were rewired it was possible to recover the new settings by using assumed plain text-cipher text cryptanalytic attacks. It took 5-6 workers about 1-6 weeks to recover the wiring of the first rotor and then they could quickly recover the wiring of the remaining two rotors.

Eventually the use of enciphered indicators and individual internal keys for each message (or pair of messages) made it too costly to work on this traffic, so the FA had to give up on it. According to Kröger this decision was made in early 1944.

Postwar developments – The new cipher machine and dr Kröger’s confession

At the Swiss Army’s Cipher Bureau (headed by Captain Arthur Alder, a professor of mathematics at the University of Bern) a new cipher machine was designed in the period 1941-43,  for use by the country’s armed forces and diplomatic authorities (25).

The device was based on the Enigma principle with a scrambler unit containing wired rotors and a reflector. However the new cipher machine, called NEMA, had a much more complex stepping system than standard Enigmas. The device had 10 rotors, out which 4 were the alphabet rotors, 1 was a reflector that could move during encipherment and 5 stepping wheels controlling the motion of the device.

The NEMA (Neue Maschine) was much more secure than a commercial Enigma machine and it entered service in 1947.

In 1948 a letter was sent to the Swiss government. The letter was written by dr Kröger, the Forschungsamt’s cipher machine expert, and in it he described how the Swiss Enigma was solved during the war. His conclusion was that the commercial Enigma could not satisfy the current security requirements. Kröger then offered his services to the Swiss government (26).


(2). European Axis Signal Intelligence in World War II, vol1 table ‘Results of European Axis cryptanalysis’ and US report ‘Swiss Cryptographic Systems’ (found in NARA - RG 457 - NR3254 'Foreign Cryptographic Systems, 1942-1945')

(3). Cryptologia article: ‘Enigma variations: an extended family of machines

(4). Cryptologia article: ‘Enigma variations: an extended family of machines

(5). Kriegstagebuch Inspectorate 7/VI - German Foreign Ministry’s Political Archive - TICOM collection – files Nr 2.755-2.757

(6). TICOM I-176 ‘Homework by Wachtmeister Dr. Otto Buggisch of OKH/Chi and OKW/Chi’, p3

(7).  US report ‘Swiss Cryptographic Systems’  and German Foreign Ministry’s Political Archive - TICOM collection - file Nr. 2.050 - Berichte Gruppe Frankreich, Belgien, Holland, Schweiz, Rumänien 1939-1942

(8). SRH-361 ‘History of the Signal Security Agency volume two - The general cryptanalytic problems’, chapters VII ‘The Swiss systems’ and XVI ‘The machine cipher section’.

(9). US report ‘Swiss Cryptographic Systems’, p3

(10). NARA - RG 457 - Entry 9032 - files NR3820 ‘Swiss diplomatic machine cipher SZD’, NR3821 ‘Swiss random letter traffic’, ‘SZD various notes’, NR3254 ‘Swiss Cryptographic Systems

(11). NR3820 ‘Swiss diplomatic machine cipher SZD’, p23

(12). NR3820 ‘Swiss diplomatic machine cipher SZD’, p6

(13). NR3820 ‘Swiss diplomatic machine cipher SZD’, p3-5

(14). NR3254 Swiss Cryptographic Systems’, p4-5

(15). ‘SZD various notes’, NR3254 Swiss Cryptographic Systems’ and SRH-361 ‘History of the Signal Security Agency volume two - The general cryptanalytic problems’, pages 237-238.

(16). TICOM I-123, p3 and TICOM I-150, p2

(17). TICOM I-31 'Detailed interrogations of Dr. Hüttenhain, formerly head of research section of OKW/Chi, at Flensburg on 18-21 June 1945’, p14

(18). TICOM I-22, paragraphs 113, 160 and 163

(19). German Foreign Ministry’s Political Archive - TICOM collection - file Nr. 2.050 - Berichte Gruppe Frankreich, Belgien, Holland, Schweiz, Rumänien 1939-1942

(20). ‘European Axis Signal Intelligence in World War II’ vol 2, p76

(22). TICOM I-54, p2 and TICOM DF-241 ‘Part 1’, p10

(24). TICOM DF-240-B ‘Analysis of the Enigma cipher machine type K’, DF-240 ’Parts III and IV’, p14-15 and DF-241 ‘Part I’, p23

Acknowledgements: I have to thank Frode Weierud for sharing the reports ‘Swiss diplomatic machine cipher SZD’, ‘Swiss random letter traffic’, ‘SZD various notes’.

Additional information:

The E-List method mentioned in TICOM report I-45 was also known to the US codebreakers. Their solution is described in the report ‘Suggested general solution for the commercial Enigma, where only the end plate and wheel wirings are known’ (available from the NSA’s Friedman Collection)

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