Didier Stevens

TrueCrypt allows for full disk encryption of a system disk. I use it on my Windows machines.

You probably know that the TrueCrypt password you type is not the key. But it is, simply put, used to decrypt the master key that is in the volume header.

On a system drive, the volume header is stored in the last sector of the first track of the encrypted system drive (TrueCrypt 7.0 or later). Usually, a track is 63 sectors long and a sector is 512 bytes long. So the volume header is in sector 62.

When this header is corrupted or modified, you can no longer decrypt the disk, even with the correct password. You need to use the TrueCrypt Rescue Disk to restore the volume header. This rescue disk was created when you encrypted the disk.

I’m using Tiny Hexer on the Universal Boot CD For Windows to erase the volume header (you can’t modify the volume header easily when you booted from the TrueCrypt system disk; using a live CD like UBCD4WIN is one possible workaround).

First I’m checking the geometry of the system drive with MBRWizard:

Take a look at the CHS (Cylinders Heads Sectors) value: S = 63 confirms that a track is 63 sectors long.

Then I open the system drive with Tiny Hexer (notice that the sector size is 512 bytes or 0x200 bytes):

I go to sector 62, the last sector of the first track:

It contains the volume header (an encrypted volume header has no recognizable patterns, it looks like random bytes):

Then I erase the volume header by filling the sector with zeroes and writing it back to disk:

And if you absolutely want to prevent recovery of this erased sector, write several times to it with random data.

Booting is no longer possible, even with the correct password. The TrueCrypt bootloader will tell you the password is incorrect:

One can say that I’ve created a TrueCrypt disk that requires 2-factor authentication. To decrypt this disk, you need 2 factors: the password and the corresponding TrueCrypt Rescue Disk.

First you need to boot from the TrueCrypt Rescue Disk, and select Repair Options (F8):

And then you write the volume header back to the system disk. Remark that the TrueCrypt Rescue Disk requires you to enter the password before it writes the volume header to the disk:

And now you can boot from the system disk with your password.

Use this method if you need to travel with or mail an encrypted system disk and want to be 100% sure there is no way to decrypt the drive while in transit. But don’t travel with the 2 factors on you, send the TrueCrypt Rescue Disk via another channel.

Remark: MBRWizard allows you to wipe sectors, but for whatever reason, it couldn’t successfully wipe sector 62 on my test machine.

Oh yeah, don’t forget to make a full backup before you attempt this technique 😉

Quickpost info

A couple of people asked me how to get self-signed certificates recognized by Windows.

For example, when you check the digital signature of one of my programs (like ariad.exe), you’ll see this:

The digital signature is valid, but the root certificate used in the signature is not trusted. This is because this root certificate is not installed in the repository of trusted root certificates. I’ll show you how to achieve this, but understand that by installing a new root certificate, you automatically trust all signatures and subordinate certificates issued by this root certificate authority.

The first 2 methods I’ll present add the new root certificate to your own certificate repository (i.e. the one associated with your account). This means that under other user accounts, the new root certificate will not be trusted. The third method explains how to add the new root certificate to the computer’s repository, so that it is trusted by all users.

Say you’ve a root certificate, like one created using this method. Here’s how to install it in your account’s “Trusted Root Certificate Authorities” certificate store:

And from now on, all executables signed by this root certificate authority (or it’s subordinate authorities) are trusted:

As the root certificate we used in this example is good for all purposes, and because your certificate store also integrates with Internet Explorer, SSL certificates issued by this certificate authority will also be trusted by Internet Explorer.

If you don’t have the root certificate to install, you can also get it installed from the AuthentiCode signature like this:

And from here on, you follow the same steps as in the first method;

If you want to install certificates for all users, you’ll need to follow another method. But because this other method requires a certificate file, I’ll show you how to extract a certificate file from an AuthentiCode signature:

Follow the second method to view the root certificate, but instead of installing the certificate, look at the Details tab and export the certificate:

To install a root certificate for all users, you’ll need to start the Microsoft Management Console (mmc.exe) as an administrator:

And now you can import the root certificate following the same steps as in the first method: