It was 8 May 1945, Victory in Europe Day. With the German military’s unconditional surrender, the European part of World War II came to an end. Alan Turing and his assistant Donald Bayley celebrated victory in their quiet English way, by taking a long walk together. They had been working side by side for more than a year in a secret electronics laboratory, deep in the English countryside. Bayley, a young electrical engineer, knew little about his boss’s other life as a code breaker, only that Turing would set off on his bicycle every now and then to another secret establishment about 10 miles away along rural lanes, Bletchley Park. As Bayley and the rest of the world would later learn, Bletchley Park was the headquarters of a vast, unprecedented code-breaking operation.
When they sat down for a rest in a clearing in the woods, Bayley said, “Well, the war’s over now—it’s peacetime, so you can tell us all.”
Donald Bayley (1921-2020) graduated with a degree in electrical engineering, and was commissioned into the Royal Electrical and Mechanical Engineers. There, he was selected to work with Alan Turing on the Delilah project. In later life he designed the teletypewriter-based “Piccolo” systemfor secret diplomatic radio communications, adopted by the British Foreign and Commonwealth Office and used worldwide for decades.Bonhams
“Don’t be so bloody silly,” Turing replied.
“That was the end of that conversation,” Bayley recalled 67 years later.
Turing’s incredible code-breaking work is now no longer secret. What’s more, he is renowned both as a founding father of computer science and as a pioneering figure in artificial intelligence. He is not so well-known, however, for his work in electrical engineering. This may be about to change.
In November 2023, a large cache of his wartime papers—nicknamed the “Bayley papers”—was
auctioned in London for almost half a million U.S. dollars. The previously unknown cache contains many sheets in Turing’s own handwriting, telling of his top-secret “Delilah” engineering project from 1943 to 1945. Delilah was Turing’s portable voice-encryption system, named after the biblical deceiver of men. There is also material written by Bayley, often in the form of notes he took while Turing was speaking. It is thanks to Bayley that the papers survived: He kept them until he died in 2020, 66 years after Turing passed away.
When the British Government learned about the sale of these papers at auction, it acted swiftly to put a ban on their export, declaring them to be “an important part of our national story,” and saying “It is right that a UK buyer has the opportunity to purchase these papers.” I was lucky enough to get access to the collection prior to the November sale, when the auction house asked for my assistance in identifying some of the technical material. The Bayley papers shine new light on Turing the engineer.
At the time, Turing was traveling from the abstract to the concrete. The papers offer intriguing snapshots of his journey from his prewar focus on mathematical logic and number theory, into a new world of circuits, electronics, and engineering math.
Alan Turing’s Delilah Project
During the war, Turing realized that cryptology’s new frontier was going to be the encryption of speech. The existing wartime cipher machines—such as the Japanese “
Purple” machine, the British Typex, and the Germans’ famous Enigma and teletypewriter-based SZ42—were all for encrypting typewritten text. Text, though, is scarcely the most convenient way for commanders to communicate, and secure voice communication was on the military wish list.
Bell Labs’ pioneering
SIGSALY speech-encryption system was constructed in New York City, under a U.S. Army contract, during 1942 and 1943. It was gigantic, weighing over 50 thousand kilograms and filling a room. Turing was familiar with SIGSALY and wanted to miniaturize speech encryption. The result, Delilah, consisted of three small units, each roughly the size of a shoebox. Weighing just 39 kg, including its power pack, Delilah would be at home in a truck, a trench, or a large backpack.
Bell Labs’ top secret installation of the SIGSALY voice-encryption system was a room-size machine that weighed over 50,000 kilograms.NSA
In 1943, Turing set up bench space in a Nissen hut and worked on Delilah in secret. The hut was at Hanslope Park, a military-run establishment in the middle of nowhere, England. Today, Hanslope Park is still an ultrasecret intelligence site known as His Majesty’s Government Communications Centre. In the Turing tradition, HMGCC engineers supply today’s British intelligence agents with specialized hardware and software.
Turing seems to have enjoyed the two years he spent at Hanslope Park working on Delilah. He made an old cottage his home and took meals in the Army mess. The commanding officer recalled that he “soon settled down and became one of us.” In 1944, Turing acquired his young assistant, Bayley, who had recently graduated from the University of Birmingham with a bachelor’s degree in electrical engineering. The two became good friends, working together on Delilah until the autumn of 1945. Bayley called Turing simply “Prof,” as everyone did in the Bletchley-Hanslope orbit.
“I admired the originality of his mind,” Bayley told me when I interviewed him in the 1990s. “He taught me a great deal, for which I have always been grateful.”
In return, Bayley taught Turing bench skills. When he first arrived at Hanslope Park, Bayley found Turing wiring together circuits that resembled a “spider’s nest,” he said. He took Turing firmly by the hand and dragged him through breadboarding boot camp.
Alan Turing and his assistant Donald Bayley created this working prototype of their voice-encryption system, called Delilah.The National Archives, London
A year later, as the European war ground to a close, Turing and Bayley got a prototype system up and running. This “did all that could be expected of it,” Bayley said. He described the Delilah system as “one of the first to be based on rigorous cryptographic principles.”
How Turing’s Voice-Encryption System Worked
Turing drew inspiration for the voice-encryption system from existing cipher machines for text. Teletypewriter-based cipher machines such as the Germans’ sophisticated SZ42—broken by Turing and his colleagues at Bletchley Park—worked differently from the better known Enigma machine. Enigma was usually used for messages transmitted over radio in Morse code. It encrypted the letters
A through Z by lighting up corresponding letters on a panel, called the lampboard, whose electrical connections with the keyboard were continually changing. The SZ42, by contrast, was attached to a regular teletypewriter that used a 5-bit telegraph code and could handle not just letters, but also numbers and a range of punctuation. Morse code was not involved. (This 5-bit telegraph code was a forerunner of ASCII and Unicode and is still used by some ham radio operators.)
The SZ42 encrypted the teletypewriter’s output by adding a sequence of obscuring telegraph characters, called key (the singular form “key” was used by the codebreakers and codemakers as a mass noun, like “footwear” or “output”), to the plain message. For example, if the German plaintext was ANGREIFEN UM NUL NUL UHR (Attack at zero hundred hours), and the obscuring characters that were being used to encrypt these three words (and also the space between them) were Y/RABV8WOUJL/H9VF3JX/D5Z, then the cipher machine would first add “Y” to “A”—that is to say, add the 5-bit code of the first letter of the key to the 5-bit code of the first letter of the plaintext—and then added “/” to “N”, then “R” to “G”, and so on. Under the SZ42’s rules for character addition (which were hardwired into the machine), these 24 additions would produce PNTDOOLLHANC9OAND9NK9CK5, which was the encrypted message. This principle of generating the obscuring key and then adding it to the plain message was the concept that Turing extended to the new territory of speech encryption.
The Delilah voice-encryption machine contained a key unit that generated the pseudorandom numbers used to obscure messages. This blueprint of the key unit features 8 multivibrators (labeled “v1,” “v2,” and so forth).The National Archives, London
Inside the SZ42, the key was produced by a key generator, consisting of a system of 12 wheels. As the wheels turned, they churned out a continual stream of seemingly random characters. The wheels in the receiver’s machine were synchronized with the sender’s, and so produced the same characters—Y/RABV8WOUJL/H9VF3JX/D5Z in our example. The receiving machine subtracted the key from the incoming ciphertext PNTDOOLLHANC9OAND9NK9CK5, revealing the plaintext ANGREIFEN9UM9NUL9NUL9UHR (a space was always typed as “9”).
Applying a similar principle, Delilah added the obscuring key to spoken words. In Delilah’s case, the key was a stream of pseudorandom numbers—that is, random-seeming numbers that were not truly random. Delilah’s key generator contained five rotating wheels and some fancy electronics concocted by Turing. As with the SZ42, the receiver’s key generator had to be synchronized with the sender’s, so that both machines produced identical key. In their once highly secret but
now declassified report, Turing and Bayley commented that the problem of synchronizing the two key generators had presented them with “formidable difficulties.” But they overcame these and other problems, and eventually demonstrated Delilah using a recording of a speech given by Winston Churchill, successfully encrypting, transmitting, and decrypting it.
This loose-leaf sheet shows a circuit used by Turing in an experiment to measure the cut-off voltage at a triode tube, most likely in connection with the avalanche-effect basic to a multivibrator. Multivibrators were an essential component of Delilah’s key-generation module. Bonhams
The encryption-decryption process began with discretizing the audio signal, which today we’d call analog-to-digital conversion. This produced a sequence of individual numbers, each corresponding to the signal’s voltage at a particular point in time. Then numbers from Delilah’s key were added to these numbers. During the addition, any digits that needed to be carried over to the next column were left out of the calculation—called “noncarrying” addition, this helped scramble the message. The resulting sequence of numbers was the encrypted form of the speech signal. This was transmitted automatically to a second Delilah at the receiving end. The receiving Delilah subtracted the key from the incoming transmission, and then converted the resulting numbers to voltages to reproduce the original speech.
The result was “whistly” and full of background noise, but usually intelligible—although if things went wrong, there could be “a sudden crack like a rifle shot,” Turing and Bayley reported cheerfully.
But the war was winding down, and the military was not attracted to the system. Work on the Delilah project stopped not long after the war ended, when Turing was hired by the British National Physical Laboratory to design and develop an electronic computer. Delilah “had little potential for further development,” Bayley said and “was soon forgotten.” Yet it offered a very high level of security, and was the first successful demonstration of a compact portable device for voice encryption.
What’s more, Turing’s two years of immersion in electrical engineering stood him in good stead, as he moved on to designing electronic computers.
Turing’s Lab Notebook
The two years Turing spent on Delilah produced the Bayley papers. The papers comprise a laboratory notebook, a considerable quantity of loose sheets (some organized into bundles), and—the jewel of the collection—a looseleaf ring binder bulging with pages.
The greenish-gray quarto-size lab notebook, much of it in Turing’s handwriting, details months of work. The first experiment Turing recorded involved measuring a pulse emitted by a
multivibrator, which is a circuit that can be triggered to produce a single voltage pulse or a chain of pulses. In the experiment, the pulse was fed into an oscilloscope and its shape examined. Multivibrators were crucial components of Turing’s all-important key generator, and the next page of the notebook, labeled “Measurement of ‘Heaviside function,’ ” shows the voltages measured in part of the same multivibrator circuit.
A key item in the Bayley papers is this lab notebook, whose first 24 pages are in Turing’s handwriting. These detail Turing’s work on the Delilah project prior to Bayley’s arrival in March 1944.Bonhams
Today, there is intense interest in the use of multivibrators in cryptography. Turing’s key generator, the most original part of Delilah, contained eight multivibrator circuits, along with the five-wheel assembly mentioned previously. In effect the multivibrators were eight more very complicated “wheels,” and there was additional circuitry for enhancing the random appearance of the numbers the multivibrators produced.
Subsequent experiments recorded in the lab book tested the performance of all the main parts of Delilah—the pulse modulator, the harmonic analyzer, the key generator, the signal and oscillator circuits, and the radio frequency and aerial circuits. Turing worked alone for approximately the first six months of the project, before Bayley’s arrival in March 1944, and the notebook is in Turing’s handwriting up to and including the testing of the key generator. After this, the job of recording experiments passed to Bayley.
The Bandwidth Theorem
Two loose pages, in Turing’s handwriting, explain the so-called bandwidth theorem, now known as the Nyquist-Shannon sampling theorem. This was likely written out for Bayley’s benefit. Bonhams
Among the piles of loose sheets covered with Turing’s riotously untidy handwriting, one page is headed “Bandwidth Theorem.” Delilah was in effect an application of a bandwidth theorem that today is known as the Nyquist-Shannon
sampling theorem. Turing’s proof of the theorem is scrawled over two sheets. Most probably he wrote the proof out for Bayley’s benefit. The theorem—which expresses what the sampling rate needs to be if sound waves are to be reproduced accurately—governed Delilah’s conversion of sound waves into numbers, done by sampling vocal frequencies several thousand times a second.
At Bell Labs, Claude Shannon had written a paper sketching previous work on the theorem and then proving his own formulation of it. Shannon wrote the paper in 1940, although it was not published until 1949. Turing worked at Bell Labs for a time in 1943, in connection with SIGSALY, before returning to England and embarking on Delilah. It seems likely that he and Shannon would have discussed sampling rates.
Turing’s “Red Form” Notes
During the war, Hanslope Park housed a large radio-monitoring section. Shifts of operators continuously searched the airwaves for enemy messages. Enigma transmissions, in Morse code, were identified by their stereotypical military format, while the distinctive warble of the SZ42’s radioteletype signals was instantly recognizable. After latching onto a transmission, an operator filled out an Army-issue form (preprinted in bright red ink). The frequency, the time of interception, and the letters of ciphertext were noted down. This “red form” was then rushed to the code breakers at Bletchley Park.
Writing paper was in short supply in wartime Britain, and Turing used the blank reverse sides of these “red form” sheets, designed for radio operators to note down information about intercepted signals.Bonhams
Writing paper was in short supply in wartime Britain. Turing evidently helped himself to large handfuls of red forms, scrawling out screeds of notes about Delilah on the blank reverse sides. In one bundle of red forms, numbered by Turing at the corners, he considered a resistance-capacitance network into which a “pulse of area A at time 0” is input. He calculated the charge as the pulse passes through the network, and then calculated the “output volts with pulse of that area.” The following sheets are covered with integral equations involving time, resistance, and charge. Then a scribbled diagram appears, in which a wavelike pulse is analyzed into discrete “steps”—a prelude to several pages of
Fourier-type analysis. Turing appended a proof of what he termed the “Fourier theorem,” evidence that these pages may have been a tutorial for Bayley.
The very appearance of these papers speaks to the character and challenging nature of the Delilah project. The normally top-secret Army red forms, the evidence of wartime shortages, the scribbled formulas, the complexity of the mathematics, the tutorials for Bayley—all contribute to the picture of the Prof and his young assistant working closely together at a secret military establishment on a device that pushed the engineering envelope.
The cover of the looseleaf ring binder is embossed in gilt letters “Queen Mary’s School, Walsall,” where Bayley had once been a pupil. It is crammed with handwritten notes taken by Bayley during a series of evening lectures that Turing gave at Hanslope Park. The size of Turing’s audience is unknown, but there were numerous young engineers like Bayley at Hanslope.
These notes can reasonably be given the title
Turing’s Lectures on Advanced Mathematics for Electrical Engineers. Running to 180 pages, they are the most extensive noncryptographic work by Turing currently known, vying in length with his 1940 write-up about Enigma and the Bombe, affectionately known at Bletchley Park as “Prof’s Book.”
Stepping back a little helps to put this important discovery into context. The traditional picture of Turing held him to be a mathematician’s mathematician, dwelling in a realm far removed from practical engineering. In 1966, for instance,
Scientific American ran an article by the legendary computer scientist and AI pioneer John McCarthy, in which he stated that Turing’s work did not play “any direct role in the labors of the men who made the computer a reality.” It was a common view at the time.
A binder filled with Bayley’s notes of Turing’s lectures is the jewel of the recently sold document collection.Bonhams
As we now know, though, after the war Turing himself designed an electronic computer, called the Automatic Computing Engine, or ACE. What’s more, he designed the programming system for the Manchester University “Baby” computer, as well as the hardware for its punched-tape input/output. Baby came to life in mid-1948. Although small, it was the first truly stored-program electronic computer. Two years later, the prototype of Turing’s ACE ran its first program. The prototype was later commercialized as the English Electric DEUCE (Digital Electronic Universal Computing Engine). Dozens of DEUCEs were purchased—big sales in those days—and so Turing’s computer became a major workhorse during the first decades of the Digital Age.
Yet the image has persisted of Turing as someone who made fundamental yet abstract contributions, rather than as someone whose endeavors sometimes fit onto the spectrum from bench electronics through to engineering theory. The Bayley papers bring a different Turing into focus: Turing the creative electrical engineer, with blobs of solder all over his shoes—even if his soldered joints did have a tendency to come apart, as Bayley loved to relate.
Turing’s lecture notes are in effect a textbook, terse and selective, on advanced math for circuit engineers, although now very out-of-date, of course.
There is little specifically about electronics in the lectures, aside from passing mentions, such as a reference to cathode followers. When talking about the Delilah project, Bayley liked to say that Turing had only recently taught himself elementary electronics, by studying an RCA vacuum tube manual while he crossed the Atlantic from New York to Liverpool in March 1943. This cannot be entirely accurate, however, because in 1940 Turing’s “Prof’s Book” described the use of some electronics. He detailed an arrangement of 26 thyratron tubes powered by a 26-phase supply, with each tube controlling a double-coil relay “which only trips if the thyratron fails to fire.”
Turing’s knowledge of practical electronics was probably inferior to his assistant’s, initially anyway, since Bayley had studied the subject at university and then was involved with radar before his transfer to Hanslope Park. When it came to the mathematical side of things, however, the situation was very different. The Bayley papers demonstrate the maturity of Turing’s knowledge of the mathematics of electrical circuit design—knowledge that was essential to the success of the Delilah project.
The unusual breadth of Turing’s intellectual talents—mathematician, logician, code breaker, philosopher, computer theoretician, AI pioneer, and computational biologist—is already part and parcel of his public persona. To these must now also be added an appreciation of his idiosyncratic prowess in electrical engineering.
Some of the content in this story originally appeared in Jack Copeland’s report for the Bonhams auction house.
From Your Site Articles
Related Articles Around the Web