18.12. Encrypting Disk Partitions

Contributed by Lucky Green.

FreeBSD offers excellent online protections against unauthorized data access. File permissions and Mandatory Access Control (MAC) help prevent unauthorized users 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 move the computer's hard drive to another system to copy and analyze the data.

Regardless of how an attacker may have come into possession of a hard drive or powered-down computer, the GEOM-based cryptographic subsystems built into FreeBSD are able to protect the data on the computer's file systems against even highly-motivated attackers with significant resources. Unlike encryption methods that encrypt individual files, the built-in gbde and geli utilities can be used to transparently encrypt entire file systems. No cleartext ever touches the hard drive's platter.

This chapter demonstrates how to create an encrypted file system on FreeBSD. It first demonstrates the process using gbde and then demonstrates the same example using geli.

18.12.1. Disk Encryption with gbde

The objective of the gbde(4) facility is to provide a formidable challenge for an attacker to gain access to the contents of a cold storage device. However, if the computer is compromised while up and running and the storage device is actively attached, or the attacker has access to a valid passphrase, it offers no protection to the contents of the storage device. Thus, it is important to provide physical security while the system is running and to protect the passphrase used by the encryption mechanism.

This facility provides several barriers to protect the data stored in each disk sector. It encrypts the contents of a disk sector using 128-bit AES in CBC mode. Each sector on the disk is encrypted with a different AES key. For more information on the cryptographic design, including how the sector keys are derived from the user-supplied passphrase, refer to gbde(4).

FreeBSD provides a kernel module for gbde which can be loaded with this command:

# kldload geom_bde

If using a custom kernel configuration file, ensure it contains this line:

options GEOM_BDE

The following example demonstrates adding a new hard drive to a system that will hold a single encrypted partition that will be mounted as /private.

Procedure 18.3. Encrypting a Partition with gbde
  1. Add the New Hard Drive

    Install the new drive to the system as explained in Section 18.2, “Adding Disks”. For the purposes of this example, a new hard drive partition has been added as /dev/ad4s1c and /dev/ad0s1* represents the existing standard FreeBSD partitions.

    # 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. This command will open the default editor, in order to set various configuration options in a template. For use with the UFS file system, set the sector_size to 2048:

    # gbde init /dev/ad4s1c -i -L /etc/gbde/ad4s1c.lock# $FreeBSD: src/sbin/gbde/template.txt,v 1.1.36.1 2009/08/03 08:13:06 kensmith 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
    [...]

    Once the edit is saved, the user will be asked twice to type the passphrase used to secure the data. The passphrase must be the same both times. The ability of gbde to protect data depends entirely on the quality of the passphrase. For tips on how to select a secure passphrase that is easy to remember, see http://world.std.com/~reinhold/diceware.htm.

    This initialization creates a lock file for the gbde partition. In this example, it is stored as /etc/gbde/ad4s1c.lock. Lock files must end in .lock in order to be correctly detected by the /etc/rc.d/gbde start up script.

    Caution:

    Lock files must be backed up together with the contents of any encrypted partitions. Without the lock file, the legitimate owner will be unable to access the data on the encrypted partition.

  4. Attach the Encrypted Partition to the Kernel

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

    This command will prompt to input the passphrase that was selected during the initialization of the encrypted partition. The new encrypted device will appear 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, a file system can be created on the device. This example creates a UFS file system with soft updates enabled. Be sure to specify the partition which has a *.bde extension:

    # newfs -U /dev/ad4s1c.bde
  6. Mount the Encrypted Partition

    Create a mount point and mount the encrypted file system:

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

    The encrypted file system should now be visible and 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

After each boot, any encrypted file systems must be manually re-attached to the kernel, checked for errors, and mounted, before the file systems can be used. To configure these steps, add the following lines to /etc/rc.conf:

gbde_autoattach_all="YES"
gbde_devices="ad4s1c"
gbde_lockdir="/etc/gbde"

This requires that the passphrase be entered at the console at boot time. After typing the correct passphrase, the encrypted partition will be mounted automatically. Additional gbde boot options are available and listed in rc.conf(5).

Note:

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

# gbde detach /dev/ad4s1c

18.12.2. Disk Encryption with geli

Contributed by Daniel Gerzo.

An alternative cryptographic GEOM class is available using geli. This control utility adds some features and uses a different scheme for doing cryptographic work. It provides the following features:

  • Utilizes the crypto(9) framework and automatically uses cryptographic hardware when it is available.

  • Supports multiple cryptographic algorithms such as AES, Blowfish, and 3DES.

  • Allows the root partition to be encrypted. The passphrase used to access the encrypted root partition will be requested during system boot.

  • Allows the use of two independent keys.

  • It is fast as it performs simple sector-to-sector encryption.

  • Allows backup and restore of master keys. If a user destroys their keys, it is still possible to get access to the data by restoring keys from the backup.

  • Allows a disk to attach with a random, one-time key which is useful for swap partitions and temporary file systems.

More features and usage examples can be found in geli(8).

The following example describes how to generate a key file which will be used as part of the master key for the encrypted provider mounted under /private. The key file will provide some random data used to encrypt the master key. The master key will also be protected by a passphrase. The provider's sector size will be 4kB. The example describes how to attach to the geli provider, create a file system on it, mount it, work with it, and finally, how to detach it.

Procedure 18.4. Encrypting a Partition with geli
  1. Load geli Support

    Support for geli is available as a loadable kernel module. To configure the system to automatically load the module at boot time, add the following line to /boot/loader.conf:

    geom_eli_load="YES"

    To load the kernel module now:

    # kldload geom_eli

    For a custom kernel, ensure the kernel configuration file contains these lines:

    options GEOM_ELI
    device crypto
  2. Generate the Master Key

    The following commands generate a master key (/root/da2.key) that is protected with a passphrase. The data source for the key file is /dev/random and the sector size of the provider (/dev/da2.eli) is 4kB as a bigger sector size provides better performance:

    # dd if=/dev/random of=/root/da2.key bs=64 count=1
    # geli init -s 4096 -K /root/da2.key /dev/da2
    Enter new passphrase:
    Reenter new passphrase:

    It is not mandatory to use both a passphrase and a key file as either method of securing the master key can be used in isolation.

    If the key file is given as -, standard input will be used. For example, this command generates three key files:

    # cat keyfile1 keyfile2 keyfile3 | geli init -K - /dev/da2
  3. Attach the Provider with the Generated Key

    To attach the provider, specify the key file, the name of the disk, and the passphrase:

    # geli attach -k /root/da2.key /dev/da2
    Enter passphrase:

    This creates a new device with an .eli extension:

    # ls /dev/da2*
    /dev/da2  /dev/da2.eli
  4. Create the New File System

    Next, format the device with the UFS file system and mount it on an existing mount point:

    # dd if=/dev/random of=/dev/da2.eli bs=1m
    # newfs /dev/da2.eli
    # mount /dev/da2.eli /private

    The encrypted file system should now be available for use:

    # df -H
    Filesystem     Size   Used  Avail Capacity  Mounted on
    /dev/ad0s1a    248M    89M   139M    38%    /
    /devfs         1.0K   1.0K     0B   100%    /dev
    /dev/ad0s1f    7.7G   2.3G   4.9G    32%    /usr
    /dev/ad0s1d    989M   1.5M   909M     0%    /tmp
    /dev/ad0s1e    3.9G   1.3G   2.3G    35%    /var
    /dev/da2.eli   150G   4.1K   138G     0%    /private

Once the work on the encrypted partition is done, and the /private partition is no longer needed, it is prudent to put the device into cold storage by unmounting and detaching the geli encrypted partition from the kernel:

# umount /private
# geli detach da2.eli

A rc.d script is provided to simplify the mounting of geli-encrypted devices at boot time. For this example, add these lines to /etc/rc.conf:

geli_devices="da2"
geli_da2_flags="-p -k /root/da2.key"

This configures /dev/da2 as a geli provider with a master key of /root/da2.key. The system will automatically detach the provider from the kernel before the system shuts down. During the startup process, the script will prompt for the passphrase before attaching the provider. Other kernel messages might be shown before and after the password prompt. If the boot process seems to stall, look carefully for the password prompt among the other messages. Once the correct passphrase is entered, the provider is attached. The file system is then mounted, typically by an entry in /etc/fstab. Refer to Section 4.7, “Mounting and Unmounting File Systems” for instructions on how to configure a file system to mount at boot time.

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