Trusted Computing in Vista BitLocker
lcs Mixmaster Remailer
mix at anon.lcs.mit.edu
Thu Sep 7 13:20:02 PDT 2006
With its release of Vista, Microsoft is taking a step forward in
exploiting the controversial Trusted Computing technology. For the
first time outside of a research setting, the "sealed storage" feature
of Trusted Computing will be used in a way that can potentially restrict
user access to data.
Trusted Computing relies on the TPM chip to take a "fingerprint" of
the system configuration as it boots. During the PC boot process,
which consists of several stages, each stage hashes the next one,
storing the results into the TPM. At the end of this process the TPM's
Platform Configuration Registers (PCRs) contain hashes of the data that
was involved in booting.
Sealed storage is data encrypted to a TPM key, and locked to the
values in one or more PCRs. The TPM will only decrypt the data if the
current PCR values match what was specified when the data was encrypted.
This insures that sealed data can only be decrypted if the system is
booted into a specified configuration. Changes to the boot configuration,
either by altering the BIOS, changing the Master Boot Record, booting to
an external device like a floppy, CDROM or external disk, and similar
alterations, will result in different PCR values and prevent the TPM
from unsealing data.
Vista's new disk encryption software, called BitLocker, optionally uses
this feature of the TPM to strengthen its encryption. For example,
consider various attack models for disk encryption. A laptop is stolen
and the attacker now seeks to decrypt the disk and recover the data.
The first step often applied in this situation is to take an image of
the disk and run the attacks on that image, from a computer controlled by
the attacker. This prevents the laptop OS from performing self-destruct
operations or otherwise keeping the attacker from being able to reset
the disk to a pristine state. But with BitLocker, the disk decryption
key is sealed to a TPM key (a 2048 bit RSA key). No amount of brute
force password guessing will work to recover a key from a disk image;
the TPM chip itself has to be involved.
An alternative for an attacker, then, might be to use the laptop itself
but to boot into another OS, such as via a Linux "Live CD" or external
device. It can then mount the partitions with the encrypted data and
apply similar attacks. This will give access to the TPM hardware while
still preventing the BitLocker software from having control.
Again, the BitLocker design will thwart this attack, because the
sealed storage locks the encrypted disk key to the boot configuration.
Changing that configuration by booting into another OS will change PCR
values and prevent the TPM from unlocking the key, even if the correct
password is used.
The attacker is then forced to build a robot that can type on the
keyboard, or a special electronic circuit that can tap into the cable
connecting the keyboard to the computer and can mimic key presses.
Even then, TPM chips have features to limit the feasibility of password
guessing, and will shut down or slow down when too many wrong guesses
Overall, the BitLocker design appears to be a good application for TPM
based sealed storage, exploiting this technology to greatly improve
resistance to typical attacks against disk encryption. However there is
a side effect of this design which may have implications for Microsoft's
competitive strategy in the future.
Because BitLocker locks the disk key to the boot sequence, any dual-boot
system that uses this capability will not be able to decrypt the encrypted
data when the alternate OS is booted. Users who want to share their
PC data between Vista and Linux will not be able to use this advanced
For years, compatibility between Windows and Linux has been limited,
ever since Microsoft migrated to the undocumented NTFS file system.
Gradually this situation has improved as Linux developers have reverse
engineered NTFS, and now many Linux distributions ship with at least
read-only support for NTFS file systems. This is a benefit for Linux
and improves its usability and compatibility.
BitLocker changes this game once again and throws up an enormous
roadblock against allowing Linux and Vista operating systems to coexist
and cooperate on the same computer. Looking forward, we could imagine
a time when BitLocker encryption via the TPM is the default for new
systems shipped with Vista installed. This would provide the kind of
"security by default" often demanded by independent experts and would
indeed arguably be a boon for typical users. The problem would be that
any such user who wanted to experiment with Linux would find that his
Vista data was off limits unless he decrypts his disk.
BitLocker, especially if it becomes more widely fielded and used, could
play a significant part in slowing the deployment of Linux and similar
alternative operating systems. It is a small step towards the widely
discussed Trusted Computing tradeoff of users accepting limitations on
how their data can be accessed, in exchange for improved security and
protection of that data. Given the magnitude and importance of these
issues, it will be especially interesting to see how this element of
Vista technology is received as the new OS is rolled out.
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