Infosec Learning

The Core Process: System Integrity First
BitLocker Encryption and Key Sealing: When you enable BitLocker with a TPM, the BitLocker software generates a Volume Master Key (VMK) used to encrypt the entire drive. This VMK is then encrypted and "sealed" to the TPM chip. This sealing process binds the VMK to a specific, trusted state of the computer. This trusted state is defined by a set of cryptographic measurements stored in the TPM's Platform Configuration Registers (PCRs). These PCRs are hashes of critical components like the BIOS, bootloader, and other low-level software.

The Automated Boot Process (TPM-Only Mode): In the most common and user-transparent scenario (TPM-only mode), the user's role is minimal.
When the computer boots, the firmware and bootloader perform a series of checks.
During these checks, the TPM continuously measures the integrity of each component.
The TPM then compares the current PCR values to the "sealed" values from when BitLocker was enabled.
If the PCRs match, the TPM concludes that the system has not been tampered with. It then automatically "unseals" and releases the VMK to the operating system. The user sees a normal boot-up, and the process is completely transparent. The user's credential (e.g., their Windows login password) is not needed to unlock the drive itself; it's only needed to log into their Windows user account.

Where the User's Role Comes In: The "Additional Authentication" Methods
The TPM-only mode is highly secure against offline attacks and boot-level tampering, but it's not foolproof. An attacker could potentially bypass it if they get access to the unsealed key while the system is running. To add a layer of human-based authentication, BitLocker offers additional "protectors" that require user interaction. These are used in addition to the TPM's integrity checks.

TPM + PIN: This is a very popular and secure method. The BitLocker key is sealed to the TPM and is protected by a Personal Identification Number (PIN).

When the system boots, the user is prompted to enter a PIN before the operating system loads.

The TPM uses the PIN to authenticate the user.

Only if the PIN is correct, AND the TPM's integrity checks pass (the PCRs match), will the TPM release the decryption key. This adds a crucial "something you know" factor.

TPM + Startup Key: This option uses a physical device, typically a USB flash drive, as a key.

The BitLocker key is sealed to the TPM, and part of the key is stored on the USB drive.

During boot-up, the user must insert the USB drive.

The TPM's integrity checks must still pass.

The system then uses the data from the USB drive to unseal the key from the TPM. This adds a "something you have" factor.

TPM + PIN + Startup Key: This provides the highest level of security, combining all three factors: "something you are" (the trusted state of the hardware), "something you know" (the PIN), and "something you have" (the USB key). All three conditions must be met for the TPM to release the ke

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