Bypassing Bitlocker using a cheap logic analyzer on a Lenovo laptop

The BitLocker partition is encrypted using the Full Volume Encryption Key (FVEK). The FVEK itself is encrypted using the Volume Master Key (VMK) and stored on the disk, next to the encrypted data. This permits key rotations without re-encrypting the whole disk.

The VMK is stored in the TPM. Thus the disk can only be decrypted when booted from this computer (there is a recovery mechanism in Active Directory though).

In order to decrypt the disk, the CPU will ask that the TPM sends the VMK over the SPI bus.

The vulnerability should be obvious: at some point in the boot process, the VMK transits unencrypted between the TPM and the CPU. This means that it can be captured and used to decrypt the disk.

This seems like such an obvious design flaw, and yet, that’s exactly how it works – and yes, as this article notes, you can indeed capture the VMK in-transit and decrypt the disk.


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