Alan Turing’s Secret “Delilah” Project

A collection of documents was recently sold at auction for almost half a million dollars. The documents detail a top-secret voice-encryption project led by Alan Turing, culminating in the creation of the Delilah machine. Turing: Archivio GBB/contrasto/Redux Pictures; Delilah: The National Archives, London; Notebook: Bonhams 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” system for 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.

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, LondonA 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.