* The ‘Foreign’ key-system, used by German vessels in waters such as the Indian Ocean, was never broken. Furthermore the ‘Home’ key-system no longer covered the communications of the Mediterranean surface vessels. These, from April 1941, had gone on to a new system which remained immune from decryption for another year.
* A reference to punched-card machine work employed on other stages of the process.
* Luftwaffe key-system used in Africa.
† A reference to the problem of testing the positions at which the Bombe stopped, to eliminate those which had arisen by chance.
* The ‘crib’ for 14 March came from a special message sent out both on the (broken) Home key-system, and on the U-boat system, announcing the no doubt vital news that Dönitz had been promoted to the rank of Admiral.
* They thought of the tape as reading from left to right, as shown, and so thought of it as having five ‘rows’. This is not the usual terminology, but for consistency it will be used throughout.
* Other types of teleprinter-enciphering machine systems remained unbroken.
* Nothing to do with testing, but named after its head, a Major Tester.
Bridge Passage
Aboard at a ship’s helm,
A young steersman steering with care.
Through fog on a sea-coast dolefully ringing,
An ocean-bell – O a warning bell, rock’d by the waves.
O you give good notice indeed, you bell by the sea-reefs ringing,
Ringing, ringing, to warn the ship from its wreck-place.
For as on the alert O steersman, you mind the loud admonition,
The bows turn, the freighted ship tacking speeds away under her gray sails,
The beautiful and noble ship with all her precious wealth speeds away gayly and safe.
But O the ship, the immortal ship! O ship aboard the ship!
Ship of the body, ship of the soul, voyaging, voyaging, voyaging.
While the Atlantic remained in the dark, November 1942 proved to be the worst month yet for Allied shipping. But the North African landings drew off part of the U-boat force, and the Queen Elizabeth, faster than any U-boat, made her way in safety. Alan disembarked at New York on 13 November, but according to a story he told his mother,1 was very nearly refused entry to the United States:
He had on arrival some difficulty over admission as he had been told on no account to take any papers other than those in the Diplomatic Bag which he carried. The triumvirate who confronted him on landing talked of despatching him to Ellis Island. Alan’s laconic comment was, ‘That will teach my employers to furnish me with better credentials.’ After further deliberation and passing of slips of paper, two of the triumvirate outvoted the third member and he was admitted.
Such problems were supposed to be kept under control by W. Stephenson, the Canadian millionaire who directed ‘British Security Coordination’ from Rockefeller Center. Stephenson, originally installed to liaise between the British secret service and the FBI, had made a considerable effort to advance British interests in America by undercover manipulation. Since 1941 his office had expanded to take in the more serious work of channelling Bletchley’s productions to Washington. But perhaps Alan’s tiresome habit of taking instructions literally had defeated even him.2 It was certainly a curious greeting for a person who in so many ways was bridging old and new worlds. His primary assignment took him to the enormously expanded capital city, much changed since the sleepy days of 1938, where his opposite numbers in the Navy’s cryptanalytic service, ‘Communications Supplementary Activities (Washington)’, were based.
From Bletchley’s point of view, America was the miraculous land across the rainbow bridge, possessing resources and skilled labour in quantities that desperate Britain could not supply. The CSAW was already closely connected with the most advanced sections of American industry, using Eastman Kodak, National Cash Register and IBM to plan and build its machinery. As in other ways, Hitler had the effect of adding British ideas to the massive capacity of American business. Again It was Alan Turing’s role to connect the logical and the physical.
But CSAW was certainly not without its own brains, and one of its staff was the brilliant young Yale graduate mathematician, Andrew Gleason. He and another member of the organisation, Joe Eachus, looked after Alan during his period in Washington. Once Alan was taken by Andrew Gleason to a crowded restaurant on 18th Street. They were sitting on a table for two, just a few inches from the next one, and talking of statistical problems, such as that of how best to estimate the total number of taxicabs in a town, having seen a random selection of their licence numbers. The man on the next table was very upset by hearing this technical discussion, which he took to be a breach of ‘security’, and said, ‘People shouldn’t be talking about things like that.’ Alan said, ‘Shall we continue our conversation in German?’ The man was insulted and told them in no uncertain terms how he had fought in the First World War.
They were all spy-conscious in Washington now, but such anecdotes apart, the central event of Alan’s visit was the breakthrough back into the U-boat Enigma. This was achieved without the possession of faster Bombes; it depended upon a precarious thread of luck, ingenuity, and a German blunder. It went back to the weather signals that in mid-1941 had given them an almost unfairly simple crib each day, thanks to the fact that they were transmitted both in the Enigma, and in the special meteorological cipher. But early in 1942 a change in the system had denied this method to Hut 8. Not until the U-boat capture of 30 October could it be regained. This gave them the cribs, but the difficulty remained that it would take three weeks to work through all the rotor settings, just for one day’s traffic. Here, however, they were saved by a German blunder which, in effect, threw away all the advantage that the fourth wheel offered. For the weather reports, and other routine short signals, the U-boats used their Enigma with the fourth wheel in ‘neutral’ position, thus reducing the cryptanalysts’ problem to the one they had mastered in 1941. This in itself was not fatal for Germany; the greater mistake lay in the fact that the three rotor settings used for the weather reports were also used for all the other traffic of the day. For this the analysts now needed only to work through 26 possibilities for the fourth wheel, rather than the 26 × 336 × 17576 possibilities that would otherwise have been the case. As a result of this slip, Hut 8 was able to supply decrypted messages from 13 December. It was not a sudden restoration of sight, but more like a return to the period in the spring of 1941. They had weeks in which nothing worked out. But it was sufficiently copious a flow of information for the Tracking Room in the OIC to have by 21 December a clear idea of the location of all eighty-four U-boats at large in the North Atlantic. And this time Hut 8 was not alone. In Washington, Alan Turing was indoctrinating the American analysts into all their methods. Now, when the rotor settings were discovered, they were passed back and forth across the Atlantic, the analysts beginning to communicate directly as indeed the two tracking rooms were also doing.
The decrypts flooded in, at an average rate of 3000 a day, like a newspaper filled with nothing but concise, up-to-date news about the Atlantic operations. Just as the flow began, in early December, the3 ‘irreplaceable’ Winn collapsed from ‘total mental and physical exhaustion’ and ‘what was not of immediate operational importance had to be put on one side for later study and usually, before this could be undertaken … the next crisis was upon us and the study had to be abandoned’, and so forth. But somehow the joint system managed to keep going, and in the new year became able once more to divert convoys away from known U-boats. The result was that on the other side, they could not understand why their success in sinking Allied shipping had suddenly been reduced to the level of September 1941. Indeed they were sure that U-boat positions were somehow known to the enemy. But the head of the German naval intelligence service at Naval High Command adhered to the opinion that it would be impossible for the enemy to have deciphered the signals. They continued to assume that there wa
s a spy network operating in their bases in occupied France, although nothing could have been further from the truth. And so their faith in machines and experts continued to be matched by distrust of men. There were in fact many other factors involved besides cryptanalysis – the provision of escorts and aircraft patrols, the development of radar and of counter-radar measures, and the fearsome weather of this fourth winter of the war. But the crucial change was that Allied authorities once more knew where the U-boats were.
His liaison job accomplished, Alan left Washington at the end of December. He had been working at the nerve-centre of the alliance, at its point of equilibrium. The British contribution had not yet been overtaken by the American. The Casablanca conference, from 14 to 24 January, saw Churchill as Roosevelt’s equal. For the last time, the Americans supported a British strategy to regain the Mediterranean. For the first time, Britain was to act as an American base. It was also the equilibrium point of the war. It was taking far longer than expected to clear North Africa, Montgomery missing some ultra-fine chances, with disastrous consequences all over the globe. The Russian front was still undecided. Nothing was clear, despite the demand for ‘unconditional surrender’. The crude ‘strategic bombing’ was endorsed, for want of anything better. But the Atlantic battle, agreed at Casablanca to remain the top priority, had taken a turn. For the first time, new Allied ship construction was exceeding losses.
Alan went to Saunderstown, Rhode Island, to visit Jack and Mary Crawford again, as he had done from Princeton. But Jack had died on 6 January, a few days before Alan arrived. His widow asked him to stay on a few days nonetheless. Then his direction turned too. He went down to New York City, arriving4 at the Bell Laboratories building on West Street, by the piers, on the afternoon of 19 January 1943. And for two months he soaked himself in the electronic technology of speech encipherment.
Like most organisations devoted to secret work, Bell Labs operated in a cellular fashion so that people never knew what was happening outside their own department. Alan, however, was free to move into any ‘cell’ he wished, although he had to be careful not to transfer information himself. It came through to the Bell engineers with whom he worked that his ‘clearance’ had come not from the Army or the Navy, but from the White House itself. Most of his time, however, was spent in one particular ‘cell’ which had the responsibility of trying to crack speech encipherment systems which had been proposed. He made an impression from the start, for within an hour of arrival he had solved a problem. It involved a scrambling system in which time segments were permuted, by means of nine magnetic heads simultaneously reading a magnetic tape. ‘That ought to give you 945 codes,’ said Alan when this was explained. ‘It’s only 9 × 7 × 5 × 3.’ It had taken one of their technicians a week to work it out.
For his first week, Alan acquainted himself with all the projects on which they were working, and became particularly keen on taking up one of them himself. This was the challenge presented by an RCA engineer, who had devised a system in which a speech signal would be multiplied by a key signal. It presented quite an unusual problem. On 23 January, Alan announced he had begun to think of a way of attacking it, and then he came in after the weekend convinced of its possibilities. His idea involved the use of the Vocoder.
Alan had probably already learnt about the Vocoder when in Britain, since Dollis Hill had received information on it in 1941. It was a very advanced piece of communications technology, which had been patented by the Bell engineer H. W. Dudley in 1935 and developed since then at the Bell laboratories. The idea of the Vocoder was to abstract the essential elements of speech, throwing away much of its redundancy, and conversely to reconstruct the speech signal from its essential components. One way of thinking of this process was to regard it as reducing the bandwidth, or frequency range, of a speech signal.
Any Bell Labs engineer would be familiar with the idea of reducing the frequency range of speech, since the telephone cut off sound above 4000 Hz. The resulting lack-lustre tones were still perfectly comprehensible, the point being that higher frequencies were redundant in ordinary applications. But to reduce that frequency limit much further would produce a sort of miserable grunting, which would not do at all. The Vocoder did something far more sophisticated. It collected information about the amplitude of the speech signal at each of ten frequencies up to 3000 Hz, and also took an eleventh component which coded either the fundamental pitch of the sound or (during unvoiced sounds like ssss) an absence of pitch. Each of these eleven signals required a frequency range of only 25 Hz. In this way, sufficient information was abstracted for intelligible speech to be reconstructed, and yet the total bandwidth was confined to less than 300 Hz.
Alan had already suggested that the principle of the Vocoder, taking samples at ten different frequency levels, could be applied to an attack on the time-segment permuting type of speech scrambler – perhaps with the idea of recognising the neighbouring segments automatically. His idea for applying the Vocoder to the RCA multiplying speech cipher was something much more sophisticated, and he said it would require at least a week of computational work to see if it was feasible. In his second week at Bell Labs he settled down to this work, which involved calculations with Hermite polynomials, and in his third week he had some assistance with the calculations.
But Alan was also involved in a quite separate ‘cell’, which was devoted to creating the world’s first totally unbreakable speech encipherment system. This was the most advanced work in progress at Bell Labs, and its best guarded secret. The original goal had been that of finding a way to encipher speech on the Vernam principle, so that if a one-time key were employed, the result would be as unbreakable for speech as it was for telegraph signals. With this end in view, they had attacked the quite novel problem of representing speech by the discrete 0’s and l’s used in a Vernam cipher system.
They had begun in 1941 with the Vocoder, and tried to adapt it to their purpose by approximating its eleven outputs as being either ‘on’ or ‘off’. This, however, resulted in a ‘badly mutilated’ speech signal. Accordingly they had abandoned the simple binary ‘on or off’ of the Vernam cipher, and instead approximated the Vocoder outputs not by two possible levels, but by six. The eleventh signal required a finer tuning than the others, and was allowed thirty-six levels. The effect was to encode the speech-signal as a total of twelve streams of ‘base six’ digits, like 041435243021353…. Each such stream would then be added in modular fashion* to a similar but random key sequence, and the result transmitted. At the other end the identical key would be subtracted, and the speech reconstituted. The speech signal was to be sampled for its ‘levels’ fifty times a second, which meant that the transmission was roughly equivalent to sending 300 teleprinter characters a second. They had succeeded in devising the equivalent of a one-time pad system for speech.†
The development was given the intriguing name of ‘Project X’ or the ‘X-system’. By November 1942 an experimental model had been installed at New York and tested with5 ‘a synthetic set of signals from a signal generator that had previously been sent to England’. In January 1943 they were beginning to assemble the first model intended for operational use. There were tremendous technical obstacles. Not only was the basic Vocoder already very complicated, but it required a large number of further components for taking the discrete (or ‘quantised’) levels. It also required the allocation of seventy-two different frequencies, for the twelve streams of digits were to be played like music, with a different frequency, not a different amplitude, for each possible digit. The system also required perfect synchrony between sender and receiver, and had to allow for the fading and time delays in the Atlantic ionosphere.
The result was a roomful of electronic equipment at each end of the system:
A terminal occupied over 30 of the standard 7-foot relay rack mounting bays, required about 30kW of power to operate, and needed complete air conditioning in the large room housing it. Members working on the job occasionally remarked
about the terrible conversion ratio – 30kW of power for 1 milliwatt of poor-quality speech.
But it worked, which was the main thing. For the first time, secret speech could cross the Atlantic. Alan’s inspection of the apparatus on behalf of the British government preceded a formal Anglo-American agreement on the subject. The somewhat disgruntled minutes6 of the meeting of the Chiefs of Staff Committee of the War Cabinet, on 15 February 1943, explained the position:
THE COMMITTEE had before them a Memorandum by the British Joint Communications Board on a proposal for the installation by the Americans of a highly secret apparatus for telephone communication between the United States and London.
THE COMMITTEE were informed that Major Millar, a US officer specially sent over for the purpose of installing the apparatus, had now arrived. His instructions were to place it in some building where it would be exclusively under American control, though it could be used by high officials in the British Government. There were only two other sets, one of which was being installed in the White House, and one in the War Department in Washington. No more sets could be produced for eight or nine months.
The following were the main points in the discussion:
(a) Security. It was noted that the only Englishman who had yet been able to examine the apparatus was Dr Turing of the Government Code and Cypher School. In view of the fact that conversations relating to British operations would undoubtedly take place over the secret telephone, we had a legitimate interest in finding out whether the new apparatus could really be considered one hundred per cent secure. It was thought that this could best be cleared up by the Joint Staff Mission in Washington, where ample technical talent was available.
Alan Turing: The Enigma: The Book That Inspired the Film The Imitation Game Page 40
