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[SoloKeys](https://github.com/solokeys/solo)' hardware would work for this. They are also working on a [new board](https://solokeys.com/blogs/news/update-on-our-new-and-upcoming-security-keys) based on the LPC55S69.

I noticed after I wrote that that Tock OS has experimental support for an STM32 board -- SoloKeys currently uses an STM32L432. Anyway, it'd still likely be quite a bit...