We [were] astonished that especially Mr. Roosevelt was speaking so freely.
—Kurt Vetterlein, Deutsche Reichspost
Churchill’s question written on a chit attached to a memorandum about the suspect security of the transatlantic radiotelephone brought his secretary up short: “What is the radio telephone, and when do we use it? Do I ever use it? W.S.C.” Below, John Martin wrote in reply, “You have used the radio telephone in talking across the Atlantic and to Cairo. J.M.M. 4 September 1942.”1
The British were right to be concerned. From the autumn of 1941 until the middle of July 1943, the Germans intercepted, decrypted, and listened to almost every word spoken via the radiotelephone’s supposed protection, the A3 encryption system. The intelligence acquired included potentially every transatlantic telephone call between Churchill and Franklin D. Roosevelt during that nearly two-year period. The translated intelligence product was distributed to high-level German military and political leaders including Hitler.2
Rapid, secure communication by text and voice across the barrier of the Atlantic Ocean and between far-flung headquarters everywhere else was vital to Anglo-American coordination. Equally vital to the Axis was protected communication between Berlin and Germany’s deployed forces, embassies, and Axis allies.
The story of Ultra, the Allies’ decryption and exploitation of German text messages encrypted on Enigma machines and broadcast over the air, has been told often. Also known is the story of Magic, the breaking and reading of Japanese encrypted messages. Diplomatic reporting from Japan’s ambassador in Berlin back to Tokyo was a rich source of information about Germany for the Allies. The corresponding British and American electromechanical text encryption systems were superior to the Germans’ Enigma machine for sending text messages and were not broken by German cryptanalysts.3
Penetration into encrypted voice communication was another matter. There the Germans had gained the advantage. By responding to better secure their voice communication, the Allies shut off a valuable source of intelligence to the Germans eleven months before D-Day, a time during which the ability to talk by telephone securely and frequently across the Atlantic to plan and prepare for the invasion was vital to the Allies.
From the start of the war until July 1943, there was only one secure voice option available for calls by radiotelephone between Washington and London or elsewhere by the Allies. That was the commercial A3 system, employing 1920s technology.4 The A3 was maintained in New York by the American Telephone and Telegraph Company and in London by the British Post Office for all transatlantic calls. Observing the A3 with interest was a young Deutsche Reichspost radio engineer, Kurt Vetterlein. Vetterlein believed that he could surmount the technical challenges to break the A3 system.5
To encipher spoken words, A3 depended on synchronizing keys at the send and receive ends with a precise time signal. Vetterlein and his colleagues built an apparatus of equivalent synchronized timing accuracy that could beat the A3’s pattern of enciphering, which shifted every twenty seconds on a thirty-six-step cycle. The Germans’ cryptanalytic key had to be changed every few seconds, even in the absence of an actual voice transmission, in order to stay synchronized with the A3 signal.6 The apparatus worked.
Vetterlein was placed in charge of a small radio intercept unit and established in a commandeered youth hostel on the North Sea coast of the Netherlands, near Nordwijk. From there, and later from inland Eindhoven after coastal commando raids elsewhere raised concern, Vetterlein’s unit continuously monitored the only radio frequency for A3 between London and Washington. Vetterlein’s unit intercepted, recorded, decoded, and translated as many as sixty calls per day, but never fewer than thirty calls, made on the A3 link, which Churchill and Roosevelt used frequently.7
Listening to oral communication brought added value to intelligence.8 Although sound was distorted in this early voice encryption system, interception of the spoken word could yield intelligence inferences from reactions not evident in text messages. For example, was the speaker making certain points with emphasis or, conversely, by hesitating?9 Roosevelt and Churchill used code names, but through the distortion, German interpreters still could recognize their distinctive voices.10
Field-translated intercepts were sent by land line and courier to the SS in Berlin, then distributed to the Oberkommando der Wehrmacht (High Command of the Armed Forces), the Foreign Ministry, and senior Nazi leaders including Hitler. After Vetterlein’s unit made some mistakes in translation, all of the raw intercepts were sent to Berlin for translation.11
From at least as early as December 1941, the Allies knew that the security of information transmitted with the A3 system’s encryption was questionable at best. They went to work to develop a genuinely secure replacement. Doing so at the cutting edge of the day’s technology, however, would take time. In the interim, the Allies sought to instill security consciousness in A3 users. They established censors in Washington and London to monitor A3 calls with authority to interrupt if classified conversation discipline broke down.12 At the start of an A3 call, a “minder” would caution users on each end that the enemy was listening.
To the German intercept operators, that precautionary warning in itself became a “tell.” Among Allied censors, there was natural reluctance to wag a finger at very senior A3 users. Thus the absence of a warning statement at the beginning of a call alerted the Germans that someone important was about to come on the line. Interviewed by historian David Kahn in 1967, Vetterlein recalled, “We were smiling about this.” The question on Winston Churchill’s chit attached to a memorandum on communication security shows that as late as September 1942, the prime minister was insufficiently aware of the threat.13 As for the senior Washington participant, Vetterlein recalled to Kahn, “We [were] astonished that especially Mr. Roosevelt was speaking so freely.”14
Communication security officers despaired that the response from A3 users at lower levels was to attempt to “talk around” sensitive information by using initials and euphemisms such as “the Big Man.” This never is a good practice when under attack by a skilled service that could place each call into the context of an intelligence mosaic. In a note to all British cabinet ministers, the reality was put bluntly: “As far, therefore, as the enemy are concerned all radiotelephony transmissions should be looked on as having no more secrecy than if they were in ordinary speech, and we can be tolerably certain that they are all overheard by the enemy, who, we know, watch our wireless traffic.”15
The Americans had entered the war with a bloody lesson in cost of not having a form of telephonic communication that engendered security confidence. Holding in hand, on the morning of December 7, 1941, the warning from American codebreakers that a Japanese attack in the Pacific was imminent, General Marshall also was confronted by his personal doubts about the security of the A3 voice encryption system. How to warn Hawaii, five time zones behind Washington, without risking compromise of the precious secret of American access into Japanese encrypted messages? The A3 radiotelephone would be faster but not sufficiently secure, Marshall decided. He instead sent the threat warning to Pacific commanders by coded radiotelegraph. Pearl Harbor received Marshall’s warning while already under Japanese attack.16
Stung by the failure to warn of the Japanese surprise attack, the U.S. Army immediately began to look for a better voice encryption system. Since 1936, the Bell Telephone Laboratories had been exploring conversion of analogue voice into digital data. The promise in Bell’s results led to an Army contract in 1942.17
By the beginning of 1943, building on its own research and early work by the brilliant British computing pioneer Alan Turing, Bell had produced a new system involving forty racks of equipment and weighing fifty-five tons. Essential to its functionality, the new system that digitized human speech for encryption, transmission, and decryption incorporated multiple fundamentally new technological developments. The patents for these would be kept secret for thirty-four years.18 At a secret Pentagon ceremony to inaugurate its use between Washington and London, the president of Bell Telephone Laboratories, Dr. O. E. Buckley, gave a summary description for laypeople of what had been accomplished:
Speech has been converted into low frequency signals that are not speech but contain a specification or description of it. Those signals have been coded by a system that defies decoding by any but the intended recipient. The coded signals have been transmitted over a radio circuit in such a way that an interceptor cannot even distinguish the presence or absence of the signals. At the receiving end, the signals have been decoded and restored and then used to generate speech nearly enough like that which gave them birth that it may be clearly understood.19
The system’s cryptographic key was wideband thermal noise converted to a frequency shift keyed (FSK) audio tone in multiple frequencies and recorded on vinyl phonograph discs. These were shipped to system operators and, when used, synchronized to a precise time signal. The turntables on which the discs were played were, themselves, exceptionally constant in their rotation.20
Highly secure, this forerunner of the voice encryption systems used today was given the nonsense cover name Sigsaly.21 The impenetrable system’s only external signal was a buzzing suggestive of the frenetically undulating musical theme of a then-popular radio mystery program. That prompted another informal name for Sigsaly among its operators, the Green Hornet.
The Americans were extremely protective of both Sigsaly’s revolutionary technology and the secret of its existence. That contributed to Sigsaly arriving in London as though from Mars. On February 15, 1943, Lt. Gen. Sir Hastings Ismay informed Churchill:
A United States officer has just arrived in London with instructions to install an apparatus of an entirely new kind for ensuring speech secrecy over the radio-telephone. The apparatus is an American invention, of which three are in existence. . . . We know little or nothing about it, except that it requires three rooms to house it and six men to operate it. . . . The only Englishman who has so far been allowed to see it is Dr. Turing of the Government Code and Cypher School. The Chiefs of Staff are not sure that he is sufficiently qualified on all aspects to be able to give a final opinion. . . . The fact that the Americans desire to retain complete control of this apparatus, and to prevent our experts from becoming familiar with it, is perhaps strange. Nevertheless, the Chiefs of Staff do not recommend that any objection should be raised by us at this stage.22
Churchill, by 1943, was fully appreciative of the German voice communication interception threat and, at least on this topic, unmoved by staff wariness of Americans. Taking up his red ink pen, at the bottom of Ismay’s February 15 memorandum, the prime minister simply wrote, “Good.”
Installation of Sigsaly proved to be time-consuming. Not until July 15, five months later, was a Sigsaly system, operated by the U.S. Army Signal Corps, up and running in London and Washington. The London system was located in a U.S. communications complex in the basement annex to Selfridge’s Department Store. A secure line ran to Churchill’s War Cabinet Office a mile away. Another Sigsaly was set up in the Pentagon in Washington with secure landlines to the White House and the State Department.23 The Germans soon noticed a significant drop in the number of A3 calls and deduced that another secure voice system must have been put into use, but they never identified the purpose of Sigsaly’s signal, let alone decrypted its traffic.24
Sigsaly apparently did not bring a complete end to use of the insecure A3 system by Churchill and Roosevelt. The evening of July 28, 1943, Vetterlein’s unit intercepted and decoded a Churchill-to-FDR scrambled telephone call on a line between War Cabinet rooms and the White House. Their connection was the compromised A3, not the new secure radiotelephone link.25 Churchill and FDR discussed the coup that had ousted Mussolini and how to approach the new Italian government. Alerted by the intercept, the Oberkommando der Wehrmacht concluded prematurely that secret Allied negotiations with the Italians already were under way, but took steps to move German troops, which proved prudent.26
The relative advantage in protection of their high-level communication had tilted in favor of the Allies. Suspected but unconfirmed by the Germans was that their own enciphered and broadcasted text communication had been compromised and was being read by the Allies. While Enigma hemorrhaged German secrets, equivalent Allied enciphered text was not readable by the Germans. Now Allied radiotelephone calls had gone dark to Berlin. Sigsaly was a boon for secure voice communication between Allied military leaders physically separated by the Atlantic at a critical time, the eleven months remaining before D-Day and the drive from the West to liberate Europe that would follow.
Denied the most valuable intelligence to be drawn from decryption of intercepted radiotelephone transmissions, German situation awareness had become fatally compromised. In order to gain actionable knowledge of Allied intentions in the West, the Germans were even more dependent on tactical reconnaissance and their agents infiltrated into Britain. Reconnaissance could be and was being fooled. All of their agents had been captured and turned against the Germans in a vast, integrated deception that focused increasingly on preparing the battlefield for D-Day.