@defalt interesting info, good to know!

Latest android devices store the value of "VerifiedBootState" and "deviceLocked" in TEE. fastboot flashing unlock instructs bootloader to request TEE to change their values to "Unverified" and "false".

Nokia doesn't allow bootloader unlocking which I think is a software based restriction in bootloader of Nokia devices. A developer found that this restriction is not present in prototype bootloader which is also signed by Nokia. He flashed it through EDL mode and it accepted the unlock command but production one refuses.

There must be a direct way to do this like by sending the same request to TEE what bootloader sends for unlocking because anything outside of TEE can be impersonated.

I'm also exploring possible MITM between bootloader and TEE. On each boot, bootloader queries about device state from TEE which decides whether to verify vbmeta or not. I've been thinking that the response of TEE to the query can be manipulated during transit.

I might need a microchip for this which can MITM attack between Titan M chip and bootloader.

Google is very secretive about hardware root of trust.

@defalt please do share your findings should you manage to find success. :)