[nuker]

If evil maid attack, and you see this prompt, you a) re-enable secure boot, if did not work b) throw away the device.

In any case data stays secure.

Edit: Hmm, you have a point, how do I know secure boot was disabled in the first place? Anyway, still works for servers and unattended reboots.

[KAMSPioneer]

No, GP is misinterpreting Windows's message. It prompts for a recovery key because the TPM is bound to, among other things, Secure Boot == enabled. When Secure Boot is disabled, the TPM notices that and refuses to release the key, that's how you know to reënable Secure Boot or throw away your device.

The fact that Windows is compromised does not make it capable of extracting secrets from the TPM, though maybe a naïve user can be convinced to enter the recovery key anyway...

[AnthonyMouse]

> When Secure Boot is disabled, the TPM notices that and refuses to release the key, that's how you know to reënable Secure Boot or throw away your device.

But the attacker isn't trying to get the key from the TPM right now, they're trying to get the credentials from the user. It's the same thing that happens with full disk encryption and no TPM. They can't read what's on the device without the secret but they can alter it.

So they alter it to boot a compromised Windows install -- not the original one -- and prompt for your credentials, which they then capture and use to unlock the original install.

They don't need secure boot to be turned on in order to do that, the original Windows install is never booted with it turned off and they can turn it back on later after they've captured your password. Or even leave it turned on but have it boot the second, compromised Windows install to capture your credentials with secure boot enabled.

How suspicious are you going to be if you enter your credentials and the next thing that happens is that Windows reboots "for updates" (into the original install instead of the compromised one)?