16.15 Encrypting Disk Partitions

Contributed by Lucky Green.

FreeBSD offers excellent online protections against unauthorized data access. File permissions and Mandatory Access Control (MAC) (see Chapter 15) help prevent unauthorized third-parties from accessing data while the operating system is active and the computer is powered up. However, the permissions enforced by the operating system are irrelevant if an attacker has physical access to a computer and can simply move the computer's hard drive to another system to copy and analyze the sensitive data.

Regardless of how an attacker may have come into possession of a hard drive or powered-down computer, GEOM Based Disk Encryption (gbde) can protect the data on the computer's file systems against even highly-motivated attackers with significant resources. Unlike cumbersome encryption methods that encrypt only individual files, gbde transparently encrypts entire file systems. No cleartext ever touches the hard drive's platter.

16.15.1 Enabling gbde in the Kernel

  1. Become root

    Configuring gbde requires super-user privileges.

    % su -
  2. Verify the Operating System Version

    gbde(4) requires FreeBSD 5.0 or higher.

    # uname -r
  3. Add gbde(4) Support to the Kernel Configuration File

    Using your favorite text editor, add the following line to your kernel configuration file:

    options GEOM_BDE

    Configure, recompile, and install the FreeBSD kernel. This process is described in Chapter 8.

    Reboot into the new kernel.

16.15.2 Preparing the Encrypted Hard Drive

The following example assumes that you are adding a new hard drive to your system that will hold a single encrypted partition. This partition will be mounted as /private. gbde can also be used to encrypt /home and /var/mail, but this requires more complex instructions which exceed the scope of this introduction.

  1. Add the New Hard Drive

    Install the new drive to the system as explained in Section 16.3. For the purposes of this example, a new hard drive partition has been added as /dev/ad4s1c. The /dev/ad0s1* devices represent existing standard FreeBSD partitions on the example system.

    # ls /dev/ad*
    /dev/ad0        /dev/ad0s1b     /dev/ad0s1e     /dev/ad4s1
    /dev/ad0s1      /dev/ad0s1c     /dev/ad0s1f     /dev/ad4s1c
    /dev/ad0s1a     /dev/ad0s1d     /dev/ad4
  2. Create a Directory to Hold gbde Lock Files

    # mkdir /etc/gbde

    The gbde lock file contains information that gbde requires to access encrypted partitions. Without access to the lock file, gbde will not be able to decrypt the data contained in the encrypted partition without significant manual intervention which is not supported by the software. Each encrypted partition uses a separate lock file.

  3. Initialize the gbde Partition

    A gbde partition must be initialized before it can be used. This initialization needs to be performed only once:

    # gbde init /dev/ad4s1c -i -L /etc/gbde/ad4s1c

    gbde(8) will open your editor, permitting you to set various configuration options in a template. For use with UFS1 or UFS2, set the sector_size to 2048:

    $FreeBSD: src/sbin/gbde/template.txt,v 1.1 2002/10/20 11:16:13 phk Exp $
    # Sector size is the smallest unit of data which can be read or written.
    # Making it too small decreases performance and decreases available space.
    # Making it too large may prevent filesystems from working.  512 is the
    # minimum and always safe.  For UFS, use the fragment size
    sector_size     =       2048

    gbde(8) will ask you twice to type the passphrase that should be used to secure the data. The passphrase must be the same both times. gbde's ability to protect your data depends entirely on the quality of the passphrase that you choose. [1]

    The gbde init command creates a lock file for your gbde partition that in this example is stored as /etc/gbde/ad4s1c.

    Caution: gbde lock files must be backed up together with the contents of any encrypted partitions. While deleting a lock file alone cannot prevent a determined attacker from decrypting a gbde partition, without the lock file, the legitimate owner will be unable to access the data on the encrypted partition without a significant amount of work that is totally unsupported by gbde(8) and its designer.

  4. Attach the Encrypted Partition to the Kernel

    # gbde attach /dev/ad4s1c -l /etc/gbde/ad4s1c

    You will be asked to provide the passphrase that you selected during the initialization of the encrypted partition. The new encrypted device will show up in /dev as /dev/device_name.bde:

    # ls /dev/ad*
    /dev/ad0        /dev/ad0s1b     /dev/ad0s1e     /dev/ad4s1
    /dev/ad0s1      /dev/ad0s1c     /dev/ad0s1f     /dev/ad4s1c
    /dev/ad0s1a     /dev/ad0s1d     /dev/ad4        /dev/ad4s1c.bde
  5. Create a File System on the Encrypted Device

    Once the encrypted device has been attached to the kernel, you can create a file system on the device. To create a file system on the encrypted device, use newfs(8). Since it is much faster to initialize a new UFS2 file system than it is to initialize the old UFS1 file system, using newfs(8) with the -O2 option is recommended.

    Note: The -O2 option is the default with FreeBSD 5.1-RELEASE and later.

    # newfs -U -O2 /dev/ad4s1c.bde

    Note: The newfs(8) command must be performed on an attached gbde partition which is identified by a *.bde extension to the device name.

  6. Mount the Encrypted Partition

    Create a mount point for the encrypted file system.

    # mkdir /private

    Mount the encrypted file system.

    # mount /dev/ad4s1c.bde /private
  7. Verify That the Encrypted File System is Available

    The encrypted file system should now be visible to df(1) and be available for use.

    % df -H
    Filesystem        Size   Used  Avail Capacity  Mounted on
    /dev/ad0s1a      1037M    72M   883M     8%    /
    /devfs            1.0K   1.0K     0B   100%    /dev
    /dev/ad0s1f       8.1G    55K   7.5G     0%    /home
    /dev/ad0s1e      1037M   1.1M   953M     0%    /tmp
    /dev/ad0s1d       6.1G   1.9G   3.7G    35%    /usr
    /dev/ad4s1c.bde   150G   4.1K   138G     0%    /private

16.15.3 Mounting Existing Encrypted File Systems

After each boot, any encrypted file systems must be re-attached to the kernel, checked for errors, and mounted, before the file systems can be used. The required commands must be executed as user root.

  1. Attach the gbde Partition to the Kernel

    # gbde attach /dev/ad4s1c -l /etc/gbde/ad4s1c

    You will be asked to provide the passphrase that you selected during initialization of the encrypted gbde partition.

  2. Check the File System for Errors

    Since encrypted file systems cannot yet be listed in /etc/fstab for automatic mounting, the file systems must be checked for errors by running fsck(8) manually before mounting.

    # fsck -p -t ffs /dev/ad4s1c.bde
  3. Mount the Encrypted File System

    # mount /dev/ad4s1c.bde /private

    The encrypted file system is now available for use. Automatically Mounting Encrypted Partitions

It is possible to create a script to automatically attach, check, and mount an encrypted partition, but for security reasons the script should not contain the gbde(8) password. Instead, it is recommended that such scripts be run manually while providing the password via the console or ssh(1).

16.15.4 Cryptographic Protections Employed by gbde

gbde(8) encrypts the sector payload using 128-bit AES in CBC mode. Each sector on the disk is encrypted with a different AES key. For more information on gbde's cryptographic design, including how the sector keys are derived from the user-supplied passphrase, see gbde(4).

16.15.5 Compatibility Issues

sysinstall(8) is incompatible with gbde-encrypted devices. All *.bde devices must be detached from the kernel before starting sysinstall(8) or it will crash during its initial probing for devices. To detach the encrypted device used in our example, use the following command:

# gbde detach /dev/ad4s1c

Also note that, as vinum(4) does not use the geom(4) subsystem, you cannot use gbde with vinum volumes.



For tips on how to select a secure passphrase that is easy to remember, see the Diceware Passphrase website.

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