BitLocker keys without PIN protection, where attackers could exploit stolen laptops, researchers now delve into PIN-secured setups, targeting insider threats seeking SYSTEM-level access.
This technique involves intercepting TPM communications via SPI bus analysis, revealing how even PIN-hardened BitLocker can yield to physical probing with known credentials.
While no true bypass occurs, the method unlocks drives efficiently, highlighting persistent hardware vulnerabilities in enterprise encryption.
Unraveling PIN-Protected BitLocker Mechanics
Unlike TPM-only configurations that auto-unseal keys during boot, PIN-protected BitLocker layers additional safeguards.
The Full Volume Encryption Key (FVEK) remains on the disk, encrypted by the Volume Master Key (VMK), but the VMK shifts to disk storage, protected by an Intermediate Key (IK).
This IK, in turn, is TPM-encrypted using a Stretched Key (SK) derived from the user’s PIN, ensuring dual authentication: unsealing the IK and deriving decryption keys.
This design thwarts brute-force attacks online via TPM lockouts, offline through randomized intermediates, but assumes secure hardware isolation.
Experiments by Guillaume Quéré on an HP ProBook 440 G1 revealed a discrete Nuvoton NPCT760HABYX TPM communicating over SPI, a shared bus easily tapped via nearby MX25U memory chip test points.
No soldering needed; just pins for clock, MOSI, and MISO lines, with CS optional for modern analyzers. Signal capture began pre-PIN entry using a DSLogic Plus analyzer, but quirks emerged: the clock idled high at intermediate voltages, distorting readings.
A simple 4.7kΩ pulldown resistor grounded it, stabilizing the 33MHz SPI bus. Yet, TIS protocol anomalies persisted double bytes per packet, likely from slow acknowledgments, crippling automated decoders.
Manual decoding proved essential. Filtering raw MOSI/MISO data with regexes stripped TIS wrappers (e.g., “00 D4 00 18 XX” for master requests), isolating TPM2.0 commands via headers like “80 01” (plain) or “80 02” (authenticated).
Captures, starting at PIN prompt, narrowed to key exchanges: ReadPublic for TPM keys, Load for objects, GetRandom for nonces, StartAuthSession, PolicyAuthValue/PCR for policies, and crucially, Unseal for the IK blob.
Interestingly, PINs never transmit to the TPM; they influence only the Unseal HMAC, an undocumented nuance verified across good/bad PIN trials.
The Unseal response holds the encrypted IK, differing from non-PIN blobs due to PIN-derived SK. Deriving SK involves UTF-16LE PIN hashing, doubled SHA-256, then 1,048,576 rounds with disk salt compute-intensive but feasible.
AES-CCM decryption with SK yields the IK, which unlocks the VMK from disk metadata via tools like dislocker.
For the ProBook, Python code stretched the PIN “67851922” against salt “c36496f98842c6fd9841de2ea743d5cf”, decrypting the 44-byte IK payload.
Dislocker then mounted the volume read-write, enabling backdoors like overwriting sethc.exe with cmd.exe for Shift+5 privilege escalation.
Automated scripts, such as SPITkey.py or tpm_sniffing_pin.py, streamline this, parsing volumes directly or leveraging dislocker outputs.
This attack underscores discrete TPMs’ false security; fTPM or PIN-plus-startup keys mitigate sniffing, though insiders remain risks. Enterprises should audit configurations beyond defaults.
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