This story is a collaboration with Biography.com.

Bletchley Park, the center of espionage for Britain during the Second World War, was a house of secrets as the Nazi scourge made it way across Europe. But for some, the secrets didn’t stop when the war ended.

Much has been made of the tortured private life of Alan Turing during the early years of the 21st century, from his 2009 symbolic pardon by the British government (after he was prosecuted and chemically castrated in the early 1950s due to then-illegal homosexual acts) to the Oscar-winning 2014 biopic The Imitation Game, which depicted his greatest wartime achievement, deciphering the coded messages encrypted by the Nazis’ Enigma machine.

But as Turing’s life and legacy have been brought further into the light, additional mysteries have revealed themselves. Recently surfaced papers suggest that during the same period Turing was famously breaking codes, he was also pursuing a separate, previously unknown line of work that may have produced his most revolutionary innovation.

The papers in question are known as the Bayley Papers, so called for their previous owner, Donald Bayley, Turing’s assistant at Hanslope Park from 1944 through 1945, who held on to the documents until his death in 2020. A recent university graduate specializing in electrical engineering, it was Bayley, Jack Copeland wrote in an article for IEEE Spectrum, who taught Turing the “bench skills” necessary for turning electrical engineering ideas into practical circuitry. Bayley, who called Turing “Prof,” took copious notes on Turing’s thoughts and experiments during their electrical engineering work at Hanslope Park. After toiling there for hours, Turing would bike ten miles to Bletchley Park for his then-secretive code-breaking activities. Bayley’s notes make up a sizable portion of the aforementioned documents, and they offer never-before-seen insights into the seismic project the two worked on under the code name Delilah.

The Bayley Papers went up for auction in 2023, and before they were sold, Jack Copeland had been called in to look at them. Copeland, Director of the Turing Archive for the History of Computing and author of the authoritative biography Turing: Pioneer of the Information Age (2012), was asked for his “assistance in identifying some of the technical material.” What Copeland found within was “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.” It was a first-hand record of Turing and Bayley “working closely together at a secret military establishment on a device that pushed the engineering envelope”—specifically, a portable speech encryption device.

During the war, nearly everyone had an encryption device for written messages. Turing was instrumental in cracking the Germans’ written-message cipher machine Enigma, and the British had one of their own, Typex. But so much crucial wartime decision-making and debate was handled verbally, and those communiqués needed to be encrypted as well.

Before the U.S.’s direct involvement in World War II, they had relied on a rudimentary voice privacy system known as A-3 to communicate with London. But A-3, while indecipherable for an amateur, was vulnerable to “anyone with sophisticated unscrambling capability,” according to an NSA report. The Bell Telephone Company, fortunately, had been working on a device since the mid-1930s that could convert a voice into “digital data” and transmit it that way. The device was called a “vocoder,” and had even been exhibited as a novelty at the 1939 World’s Fair in New York City before the national call to arms turned a novelty into a necessity.

The Bell Telephone Company built upon their vocoder, adding layers of encryption capability and ultimately developing SIGSALY, a towering and powerful voice encryption system that was soon utilized by major figures of the Allied Forces from President Roosevelt and Winston Churchill to Dwight Eisenhower and Douglas MacArthur (who had a SIGSALY installed on a ship to follow him around). But SIGSALY had its drawbacks, mainly its massive size. “Each complete SIGSALY terminal consisted of about forty racks of equipment and was very heavy,” notes the NSA.

Which brings things back to Delilah, Alan Turing’s answer to the flaws of SIGSALY. The Bayley Papers document Turing’s efforts to conceptualize, and actualize, a voice encoder that wasn’t as cumbersome as SIGSALY; in fact, it would be portable. Naming his device after the Biblical deceiver Delilah, Turing’s design condensed SIGSALY’s forty racks of equipment into “three small units, each roughly the size of a shoebox.”

“Weighing just 39 kg, including its power pack,” Copeland summarizes, “Delilah would be at home in a truck, a trench, or a large backpack.”

Turing’s work at Bletchley Park actually informed the Delilah experimentation he was doing at Hanslope Park, and not just because he used Red Forms, the Army-issue sheets Hanslope staffers were meant to use to alert Bletchley staffers to enemy signals, as his personal scrap paper for Delilah experiments. He drew inspiration from one of the German cipher machines they had decoded at Bletchley; not the famed Enigma machine, but rather the SZ42. While the former relied on Morse Code, the latter utilized a 5-bit telegraph code, which Copeland notes “was a forerunner of ASCII and Unicode and is still used by some ham radio operators.”

The SZ42 produced an obscuring key of telegraph characters, with an identical key produced to both the sender and receiver. If it could be done for text, Turing reasoned it could be done for sound as well.

This is the part of the story where one might say “Well, I’ve never heard of Alan Turing’s voice encoder, so the experiments must have failed.” But remarkably, they didn’t. Turing and Bayley actually did create their Delilah, and even demonstrated it using a recording of a Winston Churchill speech, “successfully encrypting, transmitting, and decrypting it.”

Instead, the reason Delilah fell to the wayside of history isn’t because it was a failure, but rather because it simply wasn’t needed anymore. By the time Turing had built and demonstrated his device, the war was over. What good was a portable voice encryptor if you had no major enemies trying to intercept your calls, the government reasoned. So funding for the project stopped, and Turing’s two-year experiment ended with a whimper. Turing’s time as an electrical engineer at Hanslope Park became a footnote in his story, if even that.

But just because the timing of his invention didn’t quite align with the needs of the new peacetime Western powers doesn’t make the accomplishment any less remarkable, and that’s what the Bayley Papers illustrate. “The Bayley papers demonstrate the maturity of Turing’s knowledge of the mathematics of electrical circuit design,” Copeland concludes, “knowledge that was essential to the success of the Delilah project.” It offers a richer portrait of a man whose potential for innovation, not just in the theoretical but evidently the practical as well, was snuffed out far too early.