Alan Turing’s Delilah: The Lost Portable Voice Encryption Device That Preceded Modern Secure Communications

Alan Turing, the mathematician who cracked the German Enigma code at Bletchley Park, also built a working portable voice encryption device called Delilah. Newly examined private papers held by his assistant Donald Bayley until 2020 reveal the device’s full design and operational history.

Delilah weighed 39 kg including its power pack, according to Jack Copeland’s article for IEEE Spectrum. Turing demonstrated the device by encrypting and decrypting a recording of Winston Churchill’s voice, proving the concept worked in real time.

What Delilah Did and How It Worked

Delilah used a combination of analog scrambling and a key tape to encrypt speech. The system relied on a shared secret key, which both sender and receiver needed to have in advance. Unlike modern digital encryption, Delilah’s approach was purely analog, mixing the voice signal with a noise-like pattern derived from the key.

• The device was designed for field use, not just lab experiments. Turing intended it for military radio communications.
• It used a paper tape key system, similar to early teletype encryption, but adapted for voice.
• The encryption was not perfect: Copeland noted that Delilah’s security was weaker than the later SIGSALY system used by the Allies.
• Bayley held the papers for decades after Turing’s death in 1954, only releasing them in 2020.
• The project was never officially adopted by the British government, which instead pursued other encryption methods.

Why Delilah Was Forgotten and What It Means Now

Turing’s work on Delilah was overshadowed by his Enigma breakthroughs and his later foundational work in computing. The device was never put into production, and the project remained classified or obscure for decades. The Bayley papers, now being studied by historians, show that Turing was thinking about portable voice encryption years before the Cold War made it a priority.

The rediscovery of Delilah highlights how much of Turing’s engineering work was lost or suppressed. Historians are now piecing together his full portfolio, which includes early work on speech synthesis and encryption. The papers are expected to be digitized and published in full by late 2025, giving researchers a clearer picture of Turing’s post-war projects.