Comment by naikrovek

> A vulnerability can’t leak your key if the TPM doesn’t know the entire key and relies on the user to supply the missing parts of the key in the form of a PIN.

First off: I agree with your thesis, BitLocker with PIN is Just Fine, equivalent in all practical senses to most disk encryption strategies, and an enterprise standard.

I post this to reinforce what you're saying, because there are a ton of weird theories about how this works that make people think it's weaker than it is.

BitLocker with PIN works like this:

* BitLocker seals an encrypted key IK into the TPM using a policy on the TPM which requires the SHA-256 of the PIN to be sent to the TPM to unlock the record (and has anti-hammering at the TPM level).

* Encryption using another key called the SK. Once the OS acquires the e(IK) from the TPM, it needs to derive SK to decrypt the IK. The SK is generated by applying a salt and 0x100000 rounds SHA256 to the PIN.

* The IK can then decrypt the VMK, which can decrypt the FVEK which is the actual data encryption key used on the on-disk data.

Fully offline (no TPM access) decryption is hopeless; the IK is random and not attached to the PIN at all.

Fully online decryption relies on TPM anti-hammering; if you can manage to hammer the TPM and find a PIN where h(PIN) matches, you can then use the PIN to generate both h(PIN) (unseal TPM) and reverse e(IK).

Partially offline decryption (imagining TPM content is somehow dumped, for example by bus sniff while a user uses their PIN) relies on bruteforcing the key for e(IK), which is possible but difficult (IMO Microsoft should ratchet this up or use a more expensive algorithm than SHA-256 in order to deal with modern hardware capabilities, but they have issues like FIPS to deal with that make new algorithm selection difficult).