Bunny Huang has been doing a lot of work on this:

Open hardware you can buy now: https://www.crowdsupply.com/sutajio-kosagi/precursor

The open OS that runs on it: https://betrusted.io/xous-book/

A secret/credential manager built on top of the open hardware and open software: https://betrusted.io

His blog section about it: https://www.bunniestudios.com/blog/category/betrusted/precur...

"The principle of evidence-based trust was at work in our decision to implement Precursor’s brain as an SoC on an FPGA, which means you can compile your CPU from design source and verify for yourself that Precursor contains no hidden instructions or other backdoors. Accomplishing the equivalent level of inspection on a piece of hardwired silicon would be…a rather expensive proposition. Precursor’s mainboard was designed for easy inspection as well, and even its LCD and keyboard were chosen specifically because they facilitate verification of proper construction with minimal equipment."