Comment by tadfisher

Banks do this because they have made their own requirement that the mobile device is a trust root that can authenticate the user. There are better, limited-purpose devices that can do this, but they are not popular/ubiquitous like smartphones, so here we are.

The oppressive part of this scheme is that Google's integrity check only passes for _their_ keys, which form a chain of trust through the TEE/TPM, through the bootloader and finally through the system image. Crucially, the only part banks should care about should just be the TEE and some secure storage, but Google provides an easy attestation scheme only for the entire hardware/software environment and not just the secure hardware bit that already lives in your phone and can't be phished.

It would be freaking cool if someone could turn your TPM into a Yubikey and have it be useful for you and your bank without having to verify the entire system firmware, bootloader and operating system.