Inside Project Delilah

The invention of the computer is a tricky thing to pinpoint. There were some early attempts that were not well known and some early attempts that were deliberately secret. [Alan Turing]’s efforts with Colossus were top secret for years, and while that work built on earlier efforts in Poland, [Turing] has as much claim to be the father of computers as anyone. But [Jack Copland] points out in a recent post that the famous computer scientist was also involved in another secret project: Delilah.

While [Turing] is best known for his work breaking ciphers at Bletchley Park, he also put time in on a second project about ten miles away in a secret electronics lab at Hanslope Park. There he worked with an assistant, [Donald Bayley] on Delilah — a portable system for encrypting voice transmissions.

The keyword is “portable.” In 1942, Bell Labs created SIGSALY for the U.S. Army to encrypt voice. It took up an entire room and weighed about 25 metric tons. [Turing] found a way to get the job done in a box that, including power, weighed in at 39 kilograms — not a cellphone, but portable in a truck. For comparison, an SCR-300 (the backpack radio used in the war, carried by “the lucky soldier”) weighed about 17 kilos with a full-sized battery.

The machine worked by generating a pseudo-random number sequence, synchronized with a similar unit on the other end of the transmission. Voice input was converted to digital, the numbers added on one end were transmitted, and the same numbers were subtracted from the other end. The result was not perfect for a number of reasons, but you could understand it, reportedly. But with the end of the war, interest in voice encryption wore off, and [Turing] and [Bayley] went on to other projects.

Luckily, [Bayley] saved his papers, which were auctioned off after his death for nearly half a million dollars. Without those papers, we wouldn’t know much about Delilah outside of a previously classified report (paywalled) and a few other notes.

The British National Museum of Computing rebuilt the device back in 2024, and you can see a video about it below. You can also see an interview in the video below with [Turing’s] nephew that mentions Delilah at the very end.


Title photo from The National Archives, London.

13 thoughts on “Inside Project Delilah

  1. Regarding Colossus, you seem to have confused Alan Turing with Tommy Flowers. Flowers designed and built Colossus entirely on his own initiative with no input from Turing, and in the face of extreme scepticism (as the vacuum tubes would be too unreliable, he was told).

    1. I agree. Flowers used Turing’s work on probability theory in cryptanalysis as the basis for his designs but Turing’s designed the British version of the Bombe, not Colossus.

    2. From my understanding, Flowers was, in fact, highly influenced by Turing’s Banburismus method (https://academic.oup.com/book/40646/chapter-abstract/348318398?redirectedFrom=fulltext) of cryptanalysis. From the Wikipedia page:

      Alan Turing invented a method of wheel-breaking that became known as Turingery. Turing’s technique was further developed into “Rectangling”, for which Colossus could produce tables for manual analysis. Colossi 2, 4, 6, 7 and 9 had a “gadget” to aid this process.

      ##

      The “public” registers Colossus better than Bombe (even though, granted, most don’t know either) and I did say work on Colossus — not that he designed it.

      Good Quora answer from Werner Herman: https://www.quora.com/Why-was-Alan-Turings-code-breaking-machine-named-Colossus-instead-of-The-Manchester-Mark-I

      1. Influenced by Turing’s theoretical work, yes of course, but the article makes it seem like Turing played a role in the electronic design and construction of the machine when he did not.

        I just think it’s a little unfair on Mr Flowers, especially considering how he was ignored and forgotten after the war to guard the state secrets of the code breakers (he had to burn the plans for colossus).

      2. The posted quora quote even corrects the question to state that Colossus “wasn’t Turing’s”.

        As a big fan of your articles on this site Mr Williams, I for one would enjoy a deep dive into how Flowers designed and built Colossus…

  2. Pretext for war author crypto project.

    1 RIGGING THE GAME Spy sting: Few at the Swiss factory …

    Baltimore Sun https://www.baltimoresun.com › 1995/12/10 › rigging-th…

    Dec 10, 1995 — For years, NSA secretly rigged Crypto AG machines
    so that US eavesdroppers could easily break their

    2 NSA, Crypto AG, and the Iraq-Iran Conflict hermetic.ch https://www.hermetic.ch › crypto › kalliste › speccoll [4] “For years, NSA secretly rigged Crypto AG machines so that U.S. eavesdroppers could easily break their codes, according to former company employees whose …

    3 Swiss Radio International Tehran Evin Prison audio.
    https://www.prosefights2.org/irp2014/p051315/swissradio.mp3

    February 4, 2025

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    MARTIN HEINRICH
    United States Senator

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