Forum Discussion
Bitlocker TPM+PIN - Safety only when TPM is safe?
Hello,
i habe read the microsoft docs about Bitlocker on Windows 11. At the moment i understand, that the feature TPM+PIN ask for a PIN at startup. From the PIN Bitlocker creates a SHA-256 Hash and use only 160Bit for Authentication with the TPM. Are the PCR Register and the PIN is correct, the TPM give Bitlocker the Authenticationcode free and Bitlocker can unecrypt der Disk.
The safety of this require the safety from the TPM. Is the safety of the TPM compromised, than Bitlocker is unsafe, too. Knowledge of the PIN can be unnecessary.
Why Bitlocker don’t use the 96bit from the Hash, strechted the 96 Bit with a salt (saved on the unencrypted disk space), use a cryptographic safe hash function (e.g. Argon2id) and use this hash XOR with the TPM set free Authenticationcode as key for the master key?
Steps: PIN from user -> SHA-256 -> 160 bit for TPM S1 and 96Bit for the second half of the master key. -> Argon2id( 96bit ) S2 -> S1 XOR S2 -> Master key.
Pro: If someone can crack the TPM the attacker has only the half of the Masterkey.
Cons: little more calculation steps.
This make the safety better, because it need knowledge and hardware security.
What do you think about it?
Sincerely yours,
ElGuy
1 Reply
The SHA-256/160-bit description matches Microsoft's BitLocker FAQ, but the PIN is authorization data for sealing and unsealing the volume master key, rather than the disk-encryption key itself.
A TPM vulnerability needs a specific threat model: bypassing authorization, extracting sealed material and intercepting an already released key are different attacks. Microsoft's countermeasures still recommend TPM+PIN against attackers with extended physical access; they do not describe it as protection against every possible hardware compromise.
Your proposed extra derivation would require cryptographic design and implementation review. Both hash portions still originate from the same PIN, so they are not independent user secrets. The published BitLocker guidance does not establish Microsoft's rationale for omitting your particular construction.
For a supported additional factor, evaluate the combined TPM, startup-key and PIN protector. Verify that you configure the combined protector, rather than adding a separate alternative unlock method, and retain a protected recovery key.
