Thursday, February 2, 2012

Intercepted conversations - Bell Labs A-3 Speech scrambler and German codebreakers

One of the most interesting codebreaking successes for the German side ,during WWII ,was the solution of the American A-3 speech scrambling device. This system was used in the radio-telephone link between Washington-London throughout the war. Up to summer 1943 it was the only speech privacy system employed on this link. Then the new SIGSALY device entered service and both systems were used concurrently.

The efforts of Post Office engineer Kurt Vetterlein have been mentioned in numerous books, so that part of the story is well known. However there was also another team under the Army Ordnance, Development and Testing Group, Signal Branch - Wa Pruef 7 which successfully solved the A-3 system.
As far as I know the work of this second team has not been mentioned in any book or article.

Time to take a closer look at the work of both teams and the intelligence they got from the A-3 system:

The A-3 device
First let’s start with an overview of the Bell Telephone Laboratories A-3 device. It was a speech privacy system used during the 1930’s which utilized speech inversion .Its security was limited against a determined opponent that could devote scientific personnel and equipment for the task of decoding it.

Still it was the only system available at the time so it was used for high communications during the war. British and American officials on both sides of the Atlantic used it for discussions on economic and military matters.
The most well known intercepted discussions were between the leaders of the Anglo-American alliance, Roosevelt and Churchill. It seems that in at least one case their conversations gave the Germans vital clues on impending military actions.   

The Post Office effort:
The Post office - Deutsche Reichspost  was the organization responsible for telephone, telegraph and  wireless communications in Germany. The Reichspostminister from 1937 to 1945 was Wilhelm Ohnesorge , a convinced National Socialist with close ties to Hitler.

Ohnesorge was interested in the new radio technologies and was willing to fund research in decoding the A-3 device. How he came to know of the existence and use of this machine is not mentioned in any of the books I’ve read.
The unit that handled this operation was the Forschungsstelle der Reichspost (Research Post of the Reichspost).

After the initial decision was made, two factors made things relatively easy for the Post Office people. First was the fact that the Post Office already owned an A-3 device. The second factor was their gifted engineer Kurt Vetterlein who headed the effort to decode the A-3.
After researching the A-3’s operating procedure ,Vetterlein and his team were able to build equipment that decoded the conversations in real-time. The equipment and the team were moved to Noordwijk in Holland where the reception was excellent. Conversations were decoded from late 1941 till late 1944.

The transcripts were sent by teletype to the Sicherheitsdienst (security service of the SS ). Then these were forwarded to Himmler, Hitler and other personalities of the Third Reich.
The threat of British commando raids forced the German team to move inland to a more secure location close to Eindhoven and  in 1944 because of the advancing Allied armies they returned to Germany.

Interesting details regarding the intercepted material are given by Dr Hans Wilhelm Thost, a journalist and employee of the SD.
Thost had a strange history. In 1935 he was the London correspondent of the Völkischer Beobachter, the newspaper of the National Socialist Party. In October of that year he was ordered to leave the country. What was the reason for his expulsion? It seems that Thost may have taken part in unlawful activities like espionage .

Whatever the case he was one of the people who translated the incoming A-3 material and his interrogation TICOM  I-190 ‚‘’Extracts from report on interrogation of Dr Hans Wilhelm Thost’’ is very interesting.
According to him the Post Office minister Ohnesorge distrusted the military and did not want to give them the transcripts of the intercepted communications. That was the reason for the Reichs Post-SD connection.

Thost says that the address for Washington was Republic 2020.In his interrogation he lists the memorable calls as follows :
a). Between War Office ,London and British Army staff ,Washington. Most of the time the caller was Brigadier Leslie Dawes and in London Brigadier Owen Young. The discussions concerned British orders of American military equipment. Cover words were used for the items (‘grapefruits ,‘pineapples‘)

b).Between the Ministry of War Transport, London and British Shipping Mission, Washington. Talks concerned the allocation of shipping space. Theatres of war were referred to by cover name. (‘Arthur’s place’ , ‘John’s place’)
c). Ministry of War Transport, London and representative of same organization in Washington. Talks concerned the allocation of tanker shipping space. Thost says that there was a serious shortage of tanker ships.

d).Concerning political and diplomatic matters.
Cases include :British Embassy, Washington to Foreign Office , London , Dutch Government, London to its representative in Washington , in one or two cases Soviet ambassador Maisky to Soviet ambassador in Washington .Also  conversations between Eden in Washington and Churchill in London.

e). Concerning economic matters.(Foreign Economic Administration , United Nations Relief and Rehabilitation Administration – UNRRA)
The most interesting calls were of course those between Roosevelt and Churchill. Their conversation of  29 July 1943 alerted the Germans to the impending Italian surrender and allowed them to take swift measures against the Italian army. Other top level people were also recorded : General Mark Clark , Lord Halifax , Averell Harriman and Harry Hopkins.

Walter Schellenberg, head of SD foreign intelligence, was the recipient of the transcripts and he mentioned the Roosevelt-Churchill talks in his memoirs and in his postwar interrogations. In ‘Report on interrogation of Walter Schellenberg 27 June- 12 July 1945’, p31 he said:

Amt Vi telephone monitoring of the Trans-Atlantic telephone service between London and Washington was very successful. This monitoring was effected from Holland, and a highly complicated machinery was used for that purpose. Before the Teheran conference, SCHELLENBERG received a report of a conversation between CHURCHILL and ROOSEVELT. Most trans-Atlantic calls referred to questions of supply. Decoding of these talks was difficult as the essential words were coded twice. Oberpostrap VETTERLEIN supervised the monitoring service in Holland. This service offered great difficulties from the technical side.

In his autobiography ’The memoirs of Hitler’s spymaster’, p418 he said:
Early in 1944 we hit a bull's eye by tapping a telephone conversation between Roosevelt and Churchill which was overheard and deciphered by the giant German listening post in Holland. Though the conversation was scrambled, we unscrambled it by means of a highly complicated apparatus. It lasted almost five minutes, and disclosed a crescendo of military activity in Britain, thereby corroborating the many reports of impending invasion. Had the two statesmen known that the enemy was listening to their conversation, Roosevelt would hardly have been likely to say good-bye to Churchill with the words, 'Well, we will do our best—now I will go fishing.'

The Post Office operation was undoubtedly a great success but it was not the only effort against the A-3 device.
The Army directed its own separate operation through the Army Ordnance, Development and Testing Group, Signal Branch Group IV - Waffenpruefung Abteilung 7/IV.

Alfred Muche and the 5B machine

Now I said earlier that all the books on codebreaking mention the Vetterlein-Post office story. However that is only 50% of the puzzle.
In TICOM report I-213 ,recently declassified by the NSA , a German engineer named Muche working for the   Army Ordnance, Development and Testing Group, Signal Branch Group IV Section E  (Wa Pruef 7 /IVe) ,describes his own successful effort versus the A-3 scrambler.

The WaPruef 7/IV agency was involved in special tasks during the war. They analyzed and decoded Soviet speech scramblers and built equipment that intercepted and printed Allied multichannel radioteletype traffic. Section E, headed by Dr Loetze, did research in speech privacy systems.
Muche was an engineer with Section E . His life story is as follows :  from 1927-37 he worked for Heliowatt Werke and in 1937 moved to WaPruef 7/IVe. For the period up to 1940 he studied domestic and foreign speech privacy systems.

At the end of 1940 or the beginning of 1941 his department started the study of the encrypted transatlantic telephone link. Under Muche’s direction single sideband receivers were built and the traffic recorded at ‘Nordwyk,Holland’ [This was the same area that the Post Office used for their interception facility]. In order to build the receivers he got assistance from Prof Koomans of the Dutch PTT (Staatsbedrijf der Posterijen, Telegrafie en Telefonie)
After studying the recordings with specialized equipment the Section E party found out that ‘the speech spectrum was being split into 5 bands, inverted and translated ’ …. ‘it was found that the cipher controlling the switching repeated cyclically after 36 sections.’

Since the operating procedure of the A-3 had been solved a descrambling machine known as 5B was built. The equipment was moved from Nordwyk to Ludwigsfelde (near Berlin) in late 1942. Ludwigsfelde housed a large army intercept station.
According to Muche the 5B machine became operational in the summer of ’43 and one of the conversations between Roosevelt and Churchill foreshadowed the Sicily landings and allowed the Germans to withdraw their forces with minimal losses.

Unfortunately for the Germans the 5B machine was destroyed by aerial bombardment in late 1943. Muche then spent 8 months building an improved version and completed that task in the summer of 1944. The machine continued to intercept and decode the traffic till ’45 when the unit was forced to move. He did not know what happened to the machine at the end of the war. For his efforts he was given the Kriegsverdienstkreuz and a Speer reward of 10.000 marks.

Regarding the equipment he used the following companies are mentioned :
1.     AEG-parts for the SSB receivers

2.     Siemens - ring modulators

3.     AEG  - ‘‘star‘‘ modulators

4.     Filters for the 5B machine - Dr Vierling ( of the Feuerstein laboratory)

5.     Speech analysis equipment – Breusing Tonsystem, Berlin
Fortunately information about the disposal of the 5B machine is given in TICOM I-203 ‘ Interrogation of Herbert MARINIOK and Herbert KORN, Former Members of the Reichspost and OKW/CHI’, p4

KORN confirmed MARINIOK’s statement that X Geraet was invented by Dr. LOTZE assisted by Mr MUCHE to enable the Germans to intercept transatlantic telephone conversations. Asked about the history of the apparatus, however, he stated that he had been concerned with it only since its installation at Ludwigsfelde in August 1944. He had worked with it until April 1945 when it had been taken to the Schliersee, and he had himself been one of the group which accompanied it by truck. KORN had also been on the raft which had dropped the apparatus into the Schliersee on 1 - 2 May 1945. Contrary to MARINIOK's statement, KORN claimed that the apparatus was sunk in several sections and not in one piece. The frame, which had consisted of three large parts, was dropped in sections, and the compartments of the apparatus itself were dropped separately. KORN was certain he could  pick out the exact spot where the parts were sank, and stated that the nearest village was MIESBACH. Although he was certain that the construction of the apparatus could be seen should the parts be retrieved , KORN thought that no part would be usable because of the corrosive influence of the water.
Muche was just as successful as Vetterlein but until now his story was not known. Although the army’s effort wasted resources, by duplicating the Post Office operation, it was nevertheless successful and provided valuable information during the war.


By eavesdropping on the Allied conversations the Germans got military, diplomatic and economic intelligence. In at least one case ( Italian surrender) the information they received allowed them  to take swift  military action  and preempt the Allied plans. That event alone justified the resources spent on the A-3 both by the Post Office and the Army.
The intercepted communications between Roosevelt and Churchill are an embarrassing episode in the signals intelligence war. However the Allies knew the A-3 system was vulnerable and the SIGSALY machine which replaced it was a quantum leap in terms of security.

In theory thanks to SIGSALY the Allies had absolute security from mid ’43 onwards. However it seems that the device did not work properly till October ‘43 and only became fully operational in April ‘44. Even then officials continued to use the A-3 for most of the traffic since the only Sigsaly link could be accessed at the Cabinet War Rooms and only a few people had authorization to use it.

Overall the story of the A-3 scrambler and the German efforts against it is an interesting chapter in the history of communications security.

Sources: European Axis Signals Intelligence , NSA website , ‘’Hitler’s Spies’’ , TICOM I-190 , TICOM I-213 , TICOM I-203 ,  ‘’Nazis in pre-war London’’ , ‘’The memoirs of Hitler’s spymaster’’ , Wikipedia, ‘The woman who censored Churchill’, KV 2/95 ‘Walter Friedrich SCHELLENBERG: rose to be No. 2 in the S.D. and was close to Himmler’

I have uploaded both TICOM reports.


  1. NSA used the example "transcripts of Churchill and Roosevelt's conversations were on Hitler's desk" in training materials in the 60's, along with photos said to be of the voice scrambler system. IIRC the example was still highly classified in those days, and I felt sorry for my history major friends.

  2. Hi Christos,

    You stated:
    "Its security was limited against a determined opponent that could devote scientific personnel and equipment for the task of decoding it."

    I thought David Kahns description of the A-3's limited security was important.

    "As a not-at-all extreme example, some Bell Telephone Laboratories engineers recovered an average of 47 per cent of the words scrambled by the A-3 simply by listening to it several times. This means that almost half the intelligence leaked through. In one test, intelligibility rose to 76 per cent, or three quarters of what was said. This is enough to give an eavesdropper the gist of a conversation"
    The Codebreakers Page 558 David Kahn 1996


    1. Speech scramblers with a single and fixed inversion point are indeed breakable by a trained ear.

      Multiband frequency domain scramblers offer somewhat better protection but still leak residual legibility, however not on the orders of magnitude listed by Kahn. I believe he meant a single inversion system OR the A-3 was rather a poor implementation of the concept. Kahn [in an article on origins of spread spectrum] states the interval between code changes as 20 seconds which kind of contradicts the keystream repetition period of 36 seconds given in this article.

      BUT any non-time-domain scrambler (and some time-frequency-domain ones) gives the eavesdropper insight on things like pace, emotion and tone of the conversation. That happens because the amplitude component remains unscrambled, the silence gets encrypted as silence and vowels as vowel (no necessarily 'human' vowels).

  3. Wasnt the overall operation called "tubular bells"? Or was that an American/British operation? I have trouble finding anything on it except modern applications of the term.

    1. Do you mean operation Ivy Bells?

  4. No, this was definately a WW2 operation I am talking about. I have only seen it referenced once in my life. Of course I cannot remember the book. It was either the Black orchestra's operation on the US/Britian, or another code name for the A3.