The Auditor app (attestation.app/about) can also verify alternative operating systems on a Pixel 2, Pixel 2 XL, Pixel 3 or Pixel 3 XL as long as the OS preserves verified boot support. The app needs to be built with the verified boot key fingerprint added to the internal list.
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The SampleOS support in the sources uses the fingerprints of my AOSP verified boot keys: github.com/AndroidHardeni. It would be easy to expand this to other operating systems, but there aren't any left keeping the security model intact. It can support my future OS releases though.
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The Pixel 3 StrongBox implementation uses a different algorithm to calculate the verified boot key fingerprint, so depending on how StrongBox support is implemented there may end up being separate lists for TEE-based and StrongBox-based keys. I wish QR codes could fit more data.