Now that the Auditor app will be generating StrongBox keys by default, I expanded the overview at attestation.app/about with links to information about the StrongBox Keymaster API and Titan M implementation on the Pixel 3 and 3 XL. It also has a list of supported devices there.
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There may be other Android 9 devices with StrongBox support. You can help with research and expanding device support by installing the app from Play (play.google.com/store/apps/det) or GitHub (github.com/GrapheneOS/Aud) and submitting a sample via the menu. See attestation.app/tutorial.
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If you have a device launched with Android 8 or later running the stock OS with the bootloader locked, expanding Auditor / AttestationServer support to it is really as simple as installing the app and pressing 'Submit sample data' in the menu. I regularly go through each sample.
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The only devices I have with attestation support are the Pixel 2, Pixel 2 XL, Pixel 3 and Pixel 3 XL. There are 39 supported models, so 35 of those are only supported because a contributor submitted a valid sample. Hundreds were submitted, but most from Pixels or unlocked phones.