Alan Turing’s Secret “Delilah” Project

It was 8 May 1945, Victory in Europe Day. With the German military’s unconditional surrfinisher, the European part of World War II came to an finish. Alan Turing and his aidant Donald Bayley honord triumph in their mute English way, by taking a lengthy walk together. They had been laboring side by side for more than a year in a secret electronics laboratory, meaningful in the English countryside. Bayley, a youthful electrical engineer, knew little about his boss’s other life as a code shatterer, only that Turing would set off on his bicycle every now and then to another secret set upment about 10 miles away alengthy country lanes, Bletchley Park. As Bayley and the rest of the world would postpoinsistr lget, Bletchley Park was the headquarters of a immense, unpretreatnted code-shattering operation.

When they sat down for a rest in a evidenting in the woods, Bayley shelp, “Well, the war’s over now—it’s peacetime, so you can alert us all.”

Donald Bayley (1921-2020) graduated with a degree in electrical engineering, and was coshiftrlookioned into the Royal Electrical and Mechanical Engineers. There, he was picked to labor with Alan Turing on the Delilah project. In postpoinsistr life he depicted the teletypewriter-based “Piccolo” systemfor secret discreet radio communications, adselected by the British Foreign and Commonwealth Office and engaged worldexpansive for decades.Bonhams

“Don’t be so bloody silly,” Turing replied.

“That was the finish of that conversation,” Bayley recalled 67 years postpoinsistr.

Turing’s incredible code-shattering labor is now no lengthyer secret. What’s more, he is famous both as a set uping overweighther of computer science and as a directing figure in man-made inalertigence. He is not so well-comprehendn, however, for his labor in electrical engineering. This may be about to alter.

In November 2023, a big cache of his wartime papers—nicknamed the “Bayley papers”—was
auctioned in London for almost half a million U.S. dollars. The previously muddle cache comprises many sheets in Turing’s own handwriting, alerting 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, normally in the create of notices he took while Turing was speaking. It is thanks to Bayley that the papers persistd: He kept them until he died in 2020, 66 years after Turing passed away.

When the British Government lgeted about the sale of these papers at auction, it acted quickly to put a prohibit on their ship, declaring them to be “an meaningful part of our national story,” and saying “It is right that a UK buyer has the opportunity to buy these papers.” I was blessed enough to get access to the collection prior to the November sale, when the auction hoengage asked for my aidance in recognizeing some of the technical material. The Bayley papers shine new airy on Turing the engineer.

At the time, Turing was traveling from the abstract to the concrete. The papers propose intriguing snapsboilings of his journey from his prewar cgo in 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 genuineized 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’ well-comprehendn Enigma and teletypewriter-based SZ42—were all for encrypting typewritten text. Text, though, is confirequicount on the most accessible way for orderers to convey, and safe voice communication was on the military want enumerate.

Bell Labs’ directing
SIGSALY speech-encryption system was erected in New York City, under a U.S. Army reduce, during 1942 and 1943. It was gigantic, weighing over 50 thousand kilograms and filling a room. Turing was recognizable with SIGSALY and wanted to miniaturize speech encryption. The result, Delilah, consisted of three minuscule units, each rawly the size of a shoebox. Weighing equitable 39 kg, including its power pack, Delilah would be at home in a truck, a trench, or a big backpack.

Bell Labs’ top secret inshighation 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 labored on Delilah in secret. The hut was at Hanslope Park, a military-run set upment in the middle of nowhere, England. Today, Hanslope Park is still an ultrasecret inalertigence site comprehendn as His Majesty’s Government Communications Centre. In the Turing tradition, HMGCC engineers provide today’s British inalertigence agents with exceptionalized difficultware and software.

Turing seems to have finishelighted the two years he spent at Hanslope Park laboring on Delilah. He made an elderly cottage his home and took meals in the Army mess. The ordering officer recalled that he “soon finishd down and became one of us.” In 1944, Turing acquired his youthful aidant, Bayley, who had recently graduated from the University of Birmingham with a bachelor’s degree in electrical engineering. The two became outstanding frifinishs, laboring together on Delilah until the autumn of 1945. Bayley called Turing spropose “Prof,” as everyone did in the Bletchley-Hanslope orbit.

“I admired the innovativeity of his mind,” Bayley telderly me when I interwatched him in the 1990s. “He taught me a fantastic deal, for which I have always been appreciative.”

In return, Bayley taught Turing bench sfinishs. When he first reachd at Hanslope Park, Bayley set up Turing wiring together circuits that mimicd a “spider’s nest,” he shelp. He took Turing firmly by the hand and dragged him thraw breadboarding boot camp.

Alan Turing and his aidant Donald Bayley originated this laboring prototype of their voice-encryption system, called Delilah.The National Archives, London

A year postpoinsistr, as the European war ground to a seal, Turing and Bayley got a prototype system up and running. This “did all that could be foreseeed of it,” Bayley shelp. He portrayd the Delilah system as “one of the first to be based on rigorous cryptoexplicit 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’ cultured SZ42—broken by Turing and his colleagues at Bletchley Park—labored separateently from the better comprehendn Enigma machine. Enigma was usuassociate engaged for messages sendted over radio in Morse code. It encrypted the letters
A thraw Z by airying up correacting letters on a panel, called the lampboard, whose electrical joinions with the keyboard were continuassociate changing. The SZ42, by contrast, was joined to a normal teletypewriter that engaged a 5-bit telegraph code and could deal with not equitable letters, but also numbers and a range of punctuation. Morse code was not take partd. (This 5-bit telegraph code was a forerunner of ASCII and Unicode and is still engaged by some ham radio operators.)

The SZ42 encrypted the teletypewriter’s output by compriseing a sequence of obscuring telegraph characters, called key (the singular create “key” was engaged by the codeshatterers and codeoriginaters as a mass noun, enjoy “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 engaged to encrypt these three words (and also the space between them) were Y/RABV8WOUJL/H9VF3JX/D5Z, then the cipher machine would first comprise “Y” to “A”—that is to say, comprise 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 compriseed “/” to “N”, then “R” to “G”, and so on. Under the SZ42’s rules for character compriseition (which were difficultwired into the machine), these 24 compriseitions would originate PNTDOOLLHANC9OAND9NK9CK5, which was the encrypted message. This principle of generating the obscuring key and then compriseing it to the plain message was the concept that Turing extfinished to the new territory of speech encryption.

The Delilah voice-encryption machine compriseed a key unit that originated the pseudorandom numbers engaged to muddle 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 originated 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 acquirer’s machine were coordinated with the sfinisher’s, and so originated the same characters—Y/RABV8WOUJL/H9VF3JX/D5Z in our example. The receiving machine subtracted the key from the incoming ciphertext PNTDOOLLHANC9OAND9NK9CK5, uncmissing the plaintext ANGREIFEN9UM9NUL9NUL9UHR (a space was always typed as “9”).

Applying a aenjoy principle, Delilah compriseed 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 compriseed five rotating wheels and some fancy electronics concocted by Turing. As with the SZ42, the acquirer’s key generator had to be coordinated with the sfinisher’s, so that both machines originated identical key. In their once highly secret but
now declassified alert, Turing and Bayley commented that the problem of synchronizing the two key generators had currented them with “createidable difficulties.” But they overcame these and other problems, and eventuassociate showd Delilah using a write downing of a speech given by Winston Churchill, successfilledy encrypting, sendting, and decrypting it.

This free-leaf sheet shows a circuit engaged by Turing in an experiment to meadeclareive the cut-off voltage at a triode tube, most foreseeed in joinion with the avalanche-effect fundamental to a multivibrator. Multivibrators were an vital 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 originated a sequence of individual numbers, each correacting to the signal’s voltage at a particular point in time. Then numbers from Delilah’s key were compriseed to these numbers. During the compriseition, any digits that insisted to be carried over to the next column were left out of the calculation—called “noncarrying” compriseition, this helped scramble the message. The resulting sequence of numbers was the encrypted create of the speech signal. This was sendted automaticassociate to a second Delilah at the receiving finish. The receiving Delilah subtracted the key from the incoming transmission, and then altered the resulting numbers to voltages to reoriginate the innovative speech.

The result was “whistly” and filled of background noise, but usuassociate inalertigible—although if slfinishergs went wrong, there could be “a sudden crack enjoy a rifle sboiling,” Turing and Bayley alerted cheerfilledy.

But the war was triumphding down, and the military was not enticeed to the system. Work on the Delilah project stopped not lengthy after the war finished, when Turing was engaged by the British National Physical Laboratory to depict and grow an electronic computer. Delilah “had little potential for further growment,” Bayley shelp and “was soon forgotten.” Yet it proposeed a very high level of security, and was the first accomplished demonstration of a compact portable device for voice encryption.

What’s more, Turing’s two years of immersion in electrical engineering stood him in outstanding stead, as he shiftd on to depicting electronic computers.

Turing’s Lab Notebook

The two years Turing spent on Delilah originated the Bayley papers. The papers compascfinish a laboratory noticebook, a ponderable quantity of free sheets (some systematic into bundles), and—the jewel of the collection—a freeleaf ring tieer bulging with pages.

The greenish-gray quarto-size lab noticebook, much of it in Turing’s handwriting, details months of labor. The first experiment Turing write downed take partd measuring a pulse disindictted by a
multivibrator, which is a circuit that can be triggered to originate a individual voltage pulse or a chain of pulses. In the experiment, the pulse was fed into an oscilloscope and its shape spendigated. Multivibrators were vital components of Turing’s all-meaningful key generator, and the next page of the noticebook, labeled “Meadeclareivement of ‘Heaviside function,’ ” shows the voltages meadeclareived in part of the same multivibrator circuit.

A key item in the Bayley papers is this lab noticebook, whose first 24 pages are in Turing’s handwriting. These detail Turing’s labor on the Delilah project prior to Bayley’s arrival in March 1944.Bonhams

Today, there is fervent interest in the engage of multivibrators in cryptography. Turing’s key generator, the most innovative part of Delilah, compriseed eight multivibrator circuits, alengthy with the five-wheel assembly alludeed previously. In effect the multivibrators were eight more very complicated “wheels,” and there was compriseitional circuitry for enhancing the random ecombineance of the numbers the multivibrators originated.

Subsequent experiments write downed in the lab book tested the carry outance of all the main parts of Delilah—the pulse modulator, the harmonic examiner, the key generator, the signal and oscillator circuits, and the radio frequency and aerial circuits. Turing labored alone for approximately the first six months of the project, before Bayley’s arrival in March 1944, and the noticebook is in Turing’s handwriting up to and including the testing of the key generator. After this, the job of write downing experiments passed to Bayley.

The Bandwidth Theorem

Two free pages, in Turing’s handwriting, elucidate the so-called prohibitdwidth theorem, now comprehendn as the Nyquist-Shannon sampling theorem. This was foreseeed written out for Bayley’s advantage. Bonhams

Among the piles of free sheets covered with Turing’s disturbionously untidy handwriting, one page is headed “Bandwidth Theorem.” Delilah was in effect an application of a prohibitdwidth theorem that today is comprehendn 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 advantage. The theorem—which conveyes what the sampling rate insists to be if sound waves are to be reoriginated rightly—ruleed Delilah’s conversion of sound waves into numbers, done by sampling vocal frequencies cut offal thousand times a second.

At Bell Labs, Claude Shannon had written a paper sketching previous labor on the theorem and then proving his own createulation of it. Shannon wrote the paper in 1940, although it was not rerented until 1949. Turing labored at Bell Labs for a time in 1943, in joinion with SIGSALY, before returning to England and embarking on Delilah. It seems foreseeed that he and Shannon would have talked sampling rates.

Turing’s “Red Form” Notes

During the war, Hanslope Park hoengaged a big radio-watching section. Shifts of operators continuously searched the airwaves for opponent messages. Enigma transmissions, in Morse code, were identified by their stereonormal military createat, while the exceptionainhabit warble of the SZ42’s radioteletype signals was instantly recognizable. After latching onto a transmission, an operator filled out an Army-rerent create (preprinted in radiant red ink). The frequency, the time of interception, and the letters of ciphertext were noticed down. This “red create” was then rushed to the code shatterers at Bletchley Park.

Writing paper was in unwiseinutive provide in wartime Britain, and Turing engaged the blank reverse sides of these “red create” sheets, depicted for radio operators to notice down adviseation about intercepted signals.Bonhams

Writing paper was in unwiseinutive provide in wartime Britain. Turing evidently helped himself to big handfuls of red creates, scrawling out screeds of notices about Delilah on the blank reverse sides. In one bundle of red creates, numbered by Turing at the corners, he pondered a resistance-capacitance netlabor into which a “pulse of area A at time 0” is input. He calcupostpoinsistd the indict as the pulse passes thraw the netlabor, and then calcupostpoinsistd the “output volts with pulse of that area.” The follotriumphg sheets are covered with integral equations involving time, resistance, and indict. Then a scribbled diagram ecombines, in which a waveenjoy pulse is examined into discrete “steps”—a prelude to cut offal pages of
Fourier-type analysis. Turing appfinished a proof of what he termed the “Fourier theorem,” evidence that these pages may have been a tutorial for Bayley.

The very ecombineance of these papers speaks to the character and challenging nature of the Delilah project. The normassociate top-secret Army red creates, the evidence of wartime unwiseinutiveages, the scribbled createulas, the complicatedity of the mathematics, the tutorials for Bayley—all give to the picture of the Prof and his youthful aidant laboring seally together at a secret military set upment on a device that pushed the engineering envelope.

The cover of the freeleaf ring tieer is embossed in gilt letters “Queen Mary’s School, Walsall,” where Bayley had once been a pupil. It is crammed with handwritten notices apshown by Bayley during a series of evening lectures that Turing gave at Hanslope Park. The size of Turing’s audience is muddle, but there were countless youthful engineers enjoy Bayley at Hanslope.

These notices can reasonably be given the title
Turing’s Lectures on Advanced Mathematics for Electrical Engineers. Running to 180 pages, they are the most extensive noncryptoexplicit labor by Turing currently comprehendn, vying in length with his 1940 write-up about Enigma and the Bombe, lovingly comprehendn at Bletchley Park as “Prof’s Book.”

Stepping back a little helps to put this meaningful uncovery into context. The traditional picture of Turing held him to be a mathematician’s mathematician, dwelling in a genuinem far deleted from pragmatic engineering. In 1966, for instance,
Scientific American ran an article by the legfinishary computer scientist and AI direct John McCarthy, in which he stated that Turing’s labor did not execute “any straightforward role in the labors of the men who made the computer a truth.” It was a frequent watch at the time.

A tieer filled with Bayley’s notices of Turing’s lectures is the jewel of the recently selderly write down collection.Bonhams

As we now comprehend, though, after the war Turing himself depicted an electronic computer, called the Automatic Computing Engine, or ACE. What’s more, he depicted the programming system for the Manchester University “Baby” computer, as well as the difficultware for its punched-tape input/output. Baby came to life in mid-1948. Although minuscule, it was the first truly stored-program electronic computer. Two years postpoinsistr, the prototype of Turing’s ACE ran its first program. The prototype was postpoinsistr commercialized as the English Electric DEUCE (Digital Electronic Universal Computing Engine). Dozens of DEUCEs were buyd—big sales in those days—and so Turing’s computer became a meaningful laborhorse 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 finisheavors sometimes fit onto the spectrum from bench electronics thraw to engineering theory. The Bayley papers convey a separateent Turing into cgo in: Turing the originateive electrical engineer, with blobs of selderlyer all over his shoes—even if his selderlyered joints did have a tfinishency to come apart, as Bayley cherishd to repostpoinsist.

Turing’s lecture notices are in effect a textbook, terse and pickive, on progressd math for circuit engineers, although now very out-of-date, of course.

There is little definiteassociate about electronics in the lectures, aside from passing alludes, such as a reference to cathode fagedrops. When talking about the Delilah project, Bayley enjoyd to say that Turing had only recently taught himself elementary electronics, by studying an RCA vacuum tube manual while he passed the Atlantic from New York to Liverpool in March 1943. This cannot be enticount on right, however, becaengage in 1940 Turing’s “Prof’s Book” portrayd the engage of some electronics. He detailed an set upment of 26 thyratron tubes powered by a 26-phase provide, with each tube deal withling a double-coil relay “which only trips if the thyratron flunks to fire.”

Turing’s comprehendledge of pragmatic electronics was probably lower to his aidant’s, initiassociate anyway, since Bayley had studied the subject at university and then was take partd with radar before his transfer to Hanslope Park. When it came to the mathematical side of slfinishergs, however, the situation was very separateent. The Bayley papers show the maturity of Turing’s comprehendledge of the mathematics of electrical circuit depict—comprehendledge that was vital to the success of the Delilah project.

The rare breadth of Turing’s inalertectual talents—mathematician, logician, code shatterer, philosopher, computer theoretician, AI direct, and computational biologist—is already part and parcel of his accessible persona. To these must now also be compriseed an appreciation of his idiosyncratic prowess in electrical engineering.

Some of the satisfyed in this story originassociate ecombineed in Jack Copeland’s alert for the Bonhams auction hoengage.