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When a computer has a TPM module, BitLocker doesn't seem to require a password, which makes me really uncomfortable. Where's the key coming from? What's preventing someone from grabbing the key by booting that computer and using a bug in Windows log in system, USB, Thunderbolt, etc? Isn't asking for a password (that is used for key generation) at boot time much more secure?

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If you configure BitLocker with a TPM protector, the key is stored in the hardware-based TPM module, and furthermore, if the OS boot configuration changes the system would not be able to decrypt the drive. This helps protect against scenarios such as:

  1. The hard drive is taken out of your PC and someone tries to do a brute-force password guess to decrypt it.
  2. Someone tries to boot a Linux live USB on your PC (the OS boot configuration changed and therefore the disk won't be decrypted).

Someone would need to do some hardware tampering to get the encryption key out if it's protected with TPM.

HOWEVER - you are correct that if there is a bug/vulnerability in the Windows login system (e.g. this), the disk is already decrypted at that point and they can read/write anything. That's why you always want TPM+BitLocker PIN code (or other second factor of authentication). The PIN code is required at the very early boot phases during the BitLocker boot loader.

The combination of TPM+PIN is pretty strong as it doesn't allow easy brute-forcing of PIN codes (e.g. with an automated keyboard) - BitLocker will lock the decryption if there are too many failed attempts. It also doesn't allow the attacker to just grab your disk and go to brute-force somewhere else.

If instead you go with BitLocker and only a password protector (no TPM), an attacker can just grab your disk out of your PC, connect it to his PC/hacking kit and brute-force guess the password (he can try many combinations at a very fast rate as there is nothing forcing him to slow down - which TPM can).

So "BitLocker TPM+PIN" is better than "BitLocker password only".

There's also a nice article about this here.

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    Also worth reading dolosgroup.io/blog/2021/7/9/…. TPM + BitLocker will likely secure your data against casual theft, but not targeted attacks.
    – vidarlo
    Aug 22, 2021 at 16:52
  • "you always want TPM+BitLocker PIN code" - I disagree. There are good reasons not to use a PIN code (e.g., usability reasons). Whether or not you "want" a PIN depends on your use case.
    – hft
    Sep 21, 2021 at 19:12

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