Windows® Internals, Sixth Edition, Part 2

As stated earlier, BitLocker can be configured to use the TPM to perform system integrity checks on critical early boot components. At a high level, the TPM collects and stores measurements from multiple early boot components and boot configuration data to create a system identifier (much like a fingerprint) for that computer. It stores each part of this fingerprint as a hash in a 160-bit platform configuration register (PCR). BitLocker uses the hash of these functions to seal the VMK, which is the key that BitLocker uses to protect other keys, including the FVEKs used to encrypt volumes.

If the early boot components are changed or tampered with, such as by changing the BIOS or MBR, changing an operating system file, or moving the hard disk to a different computer, the TPM prevents BitLocker from unsealing the VMK, and Windows enters a key recovery mode (described later in the chapter). If the PCR values match those used to seal the key, the system is deemed to be tamper free, and it unseals the key, and BitLocker can decrypt the keys used to encrypt the volumes. Once the keys are unsealed, Windows starts and system protection becomes the responsibility of the user and the operating system.

Figure 9-21. The TPM MMC snap-in after initializing the TPM.

A platform validation profile supported by TPMs consists of at least 16, and as many as 24, PCRs that contain additional information and only reset after a TPM reset (implying a machine reboot). Each PCR is associated with components that run when an operating system starts, as shown in Table 9-2.

Table 9-2. Platform Configuration Registers

Index

Meaning

0

Core Root of Trust of Measurement (CRTM), BIOS, and platform extensions

1

Platform and motherboard configuration and data (BIOS data and CPU microcode)

2

Option ROM code

3

Option ROM configuration and data

4

Master Boot Record (MBR) code

5

Master Boot Record (MBR) partition table

6

Power-state transition and wake events

7

Computer manufacturer-specific

8

First NTFS boot sector (volume boot record)

9

Remaining NTFS boot sectors (volume boot record)

10

Boot Manager

11

BitLocker Access Control

12

Defined for use by the static operating system

13

Defined for use by the static operating system

14

Defined for use by the static operating system

15

Defined for use by the static operating system

16

Used for debugging

17

Dynamic CRTM

18

Platform defined

19

Used by a trusted operating system

20

Used by a trusted operating system

21

Used by a trusted operating system

22

Used by a trusted operating system

23

Application support

By default, BitLocker uses registers 0, 2, 4, 5, 8, 9, 10, and 11 to seal the VMK. The set of PCRs used by BitLocker is known as the Platform Validation Profile, which can be configured via Group Policy (Computer Configuration\Administrative Templates\Windows Components\BitLocker Drive Encryption\Operating System Drives\Configure TPM platform validation profile) and depends on the security requirements of your organization, as shown in Table 9-2. PCR 11 must be selected to enable BitLocker protection.

Note

If you change anything protected by the PCRs specified in your Platform Validation Profile, your system will not boot without either the recovery key or recovery password. For example, if you need to update the BIOS on your system, suspend BitLocker (using the BitLocker Drive Encryption Control Panel applet) before performing the update.

BitLocker Boot Process

The actual measurements stored in the TPM PCRs are generated by the TPM itself, the TPM BIOS, and Windows. When the system boots, the TPM does a self-test, following which the CRTM in the BIOS measures its own hashing and PCR loading code and writes the hash to the first PCR of the TPM. It then hashes the BIOS and stores that measurement in the first PCR as well. The BIOS in turn hashes the next component in the boot sequence, the MBR of the boot drive, and this process continues until the operating system loader is measured. Each subsequent piece of code that runs is responsible for measuring the code that it loads and for storing the measurement in the appropriate PCR in the TPM.