San Francisco / Leuven — 7 October, 2026. Belfort today released a hotword detection model that runs end to end on encrypted audio. Hotword models are the ones that listen for phrases like “OK Google” or “Hey Siri”. Built with Belfort’s upcoming eML platform, which combines Google's HEIR compiler with Belfort's GPU library, it is now available to try at hotword.belfortlabs.com.

Listening Without Hearing

In 2019, Time reported that thousands of Amazon employees were reviewing Echo recordings, including clips captured when the device mistakenly thought it had heard its hotword, “Alexa”. More recently, reporting on Meta's Ray-Ban glasses described footage reviewed by people working for Meta, some of it capturing deeply private moments.

The privacy risks behind personal assistants are growing quickly. Tools such as Wispr Flow have made dictation a default way of writing, and Granola sits in meetings that were never previously recorded at all. What flows through these systems is no longer "play some music" but salary conversations, client calls and product decisions, which is why privacy and security are becoming a major concern.

Encrypted computing removes the need to decrypt entirely. Audio is encrypted on the device, the hotword check runs on the encrypted signal, and only you can see the result. The server does the work but never hears a word.

Try It: Guess the Word

To show it working, we built a game. Players see a set of hints and have to guess three hidden words, speaking each guess aloud. The model listens under encryption and confirms when a player lands on one, but since the audio is encrypted before it leaves the browser, the server never learns what was said.

The game is simple, but it sets a higher bar than a benchmark. If the round trip doesn't feel instant, nobody plays. Hitting that bar was only possible because of our toolchain. Combining Google HEIR with Belfort's GPU acceleration library in a single platform let us build and iterate on this without hand-writing the cryptography.

Players who want full control can also generate their own keys in the browser. Thanks to state-of-the-art key compression, the public key material comes to about 1.2 GB, small enough to upload in a few minutes even on a slow connection.

Nothing in this stack is specific to audio. Encrypted computing works on the math underneath the data, not on what the data means, so the same approach applies to images, time series, text and tabular data.

"Hotword detection is the smallest piece of a voice pipeline, but it's the one that has to respond instantly. That's why we started there: if encryption can keep up with the hotword, it can keep up with everything that comes after. A year ago, this meant writing the cryptography by hand and hoping it would run fast enough. Today our platform handles that for us, so we can move on to the larger models next."

"If encryption can keep up with the hotword, it can keep up with everything that comes after."

— Michiel Van Beirendonck, CTO, Belfort

What's next

Hotword detection is one model in a broader effort to make encrypted computing practical rather than theoretical. Next is our full pipeline, which will let developers bring their own models and deploy them under encryption without cryptography expertise. Our first release supports convolutional neural networks, a widely used class of models, and we'll expand to larger and more complex models from there. Speed was never the only bottleneck. Using encrypted computing has always required a specialist, and that's what we want to change.

Belfort’s eML platform is currently in alpha with early partners. Organizations interested in running their own models and data under encryption can join the waitlist at eml.belfortlabs.com.

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About Belfort

At Belfort, we believe that in an AI-first world, trust is all you need and the future of computing is encrypted. Belfort enables that vision by accelerating Encrypted Compute to make it practical at scale, ensuring that sensitive data can be processed without ever being decrypted. A spin-off from KU Leuven’s world-renowned COSIC lab, Belfort combines breakthroughs in hardware and algorithms to build the next layer of secure computing. The company has offices in San Francisco, USA, and Leuven, Belgium. https://belfortlabs.com/

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