Windows® Internals, Sixth Edition, Part 2

When you configure BitLocker, you have a number of options for how the VMK will be protected, depending on the system’s hardware capabilities. If the system has a TPM, you can encrypt the VMK with the TPM, have the system encrypt the VMK using a key stored in the TPM and one stored on a USB flash device, encrypt the VMK using a TPM-stored key and a PIN you enter when the system boots, or encrypt the VMK with a combination of both a PIN and a USB flash device. For systems that don’t have a compatible TPM, BitLocker offers the option of encrypting the VMK using a key stored on an external USB flash device.

In any case you’ll need an unencrypted 100-MB NTFS system volume, the volume where the Boot Manager and BCD are stored, because the MBR and boot-sector code are legacy code, run in 16-bit real mode (as discussed in Chapter 13), and do not have the ability to perform any on-the-fly decryption of the same volume they’re running on. This means that these components must remain on an unencrypted volume so that the BIOS can access them and they can run and locate Bootmgr.

As covered earlier in this chapter, the system volume is created automatically when Windows is installed on a system, regardless of whether or not you are using BitLocker. This places the system volume at the beginning of the disk (the first partition), which keeps the rest of the disk contiguous.

Figure 9-20 and Table 9-1 summarize the various ways in which the VMK can be generated.

Table 9-1. VMK Sources

Source

Identifies

Security

User Impact

TPM only

What it is

Protects against software attacks, but vulnerable to hardware attacks.

None

TPM + PIN

What it is + What you know

Adds protection against most hardware attacks as well.

User must enter PIN each boot

TPM + USB key

What it is + What you have

Fully protects against hardware attacks, but vulnerable to stolen USB key.

User must insert USB key each boot

TPM + USB key + PIN

What it is + What you have + What you know

Maximum level of protection.

User must enter PIN and insert USB key each boot

USB key only

What you have

Minimum level of protection for systems without TPM, but vulnerable to stolen key.

User must insert USB key each boot

Finally, BitLocker also provides a simple encryption-based authentication scheme to ensure the integrity of the drive contents. Although AES encryption is currently considered uncrackable through brute-force attacks and is one of the most widely used algorithms in the industry today, it doesn’t provide a way to ensure that modified encrypted data can’t in some way be modified such that it is translated back to plaintext data that an attacker could make use of. For example, by precise manipulation of the encrypted data, a hacker might be able to cause a certain logon function to behave differently and allow all logons.

Figure 9-20. BitLocker key generation

To protect the system against this type of attack, BitLocker includes a diffuser algorithm called Elephant. The job of the diffuser is to make sure that even a single bit change in the ciphertext (encrypted data) will result in a totally random plaintext data output, ensuring that the modified executable code will most likely arbitrarily crash instead of performing a specific malicious function. Additionally, when combined with code integrity (see Chapter 3 in Part 1 for more information on code integrity), the diffuser will also cause core system files to fail their signature checks, rendering the system unbootable.

Trusted Platform Module (TPM)

A TPM is a tamper-resistant processor mounted on a motherboard that provides various cryptographic services such as key and random number generation and sealed storage. Support for TPM in Windows reaches beyond supporting BitLocker, however. Through the TPM Base Services (TBS), other applications on the system can also take advantage of compatible hardware TPM chips and use WMI to administer and script access to the TPM. For example, Windows uses a TPM as an additional seed into random number generation, which enhances the overall security of all applications on the system that depend on strong security or hashing algorithms (including mechanisms such as logons).

Although your computer may have a TPM, that does not necessarily mean that Windows will be able to support it. There are two requirements for Windows TPM support:

The computer must have a TPM version 1.2 or higher.

The computer must have a Trusted Computing Group (TCG)–compliant BIOS. The BIOS establishes a chain of trust for the preboot environment and must include support for TCG-specific Static Root of Trust Measurement (SRTM).

The easiest way to determine whether your machine contains a compatible TPM is to run the TPM MMC snap-in (%SystemRoot%\System32\Tpm.msc). If Windows detects a compatible TPM, you should see a window similar to the one shown in Figure 9-21. Otherwise, an error message will appear.