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Dm-crypt

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dm-crypt is a disk encryption system using the kernels crypto API framework and device mapper subsystem. With dm-crypt, administrators can encrypt entire disks, logical volumes, partitions, but also single files.

The dm-crypt subsystem supports the Linux Unified Key Setup (LUKS) structure, which allows for multiple keys to access the encrypted data, as well as manipulate the keys (such as changing the keys, adding additional passphrases, etc.) Although dm-crypt supports non-LUKS setups as well, this article will focus on the LUKS functionality mostly due to its flexibility, manageability as well as broad support in the community.

Configuration

There are two prerequisites before one can start using dm-crypt:

  1. Configuration of the Linux kernel
  2. Installation of the sys-fs/cryptsetup package

Kernel Configuration

To use dm-crypt there are a number of configuration entries that are necessary.

First of all, support for the device mapper infrastructure as well as the crypt target must be included:

KERNEL Enabling device mapper and crypt target
[*] Enable loadable module support
Device Drivers --->
    [*] Multiple devices driver support (RAID and LVM) --->
        <*> Device mapper support
        <*>   Crypt target support

Next, the Linux kernel needs to support the set of cryptographic APIs that the administrator wants to use for encryption. These can be found under the Cryptographic API section:

KERNEL Enabling cryptographic API functions
[*] Cryptographic API --->
    <*> XTS support
    <*> SHA224 and SHA256 digest algorithm
    <*> AES cipher algorithms
    <*> AES cipher algorithms (x86_64)
    <*> User-space interface for symmetric key cipher algorithms

If the root file system will be encrypted as well, then an initial ram file system needs to be created in which the root filesystem is decrypted before it is mounted. Thus this requires initramfs support as well:

KERNEL Enabling initramfs support
General setup  --->
    [*] Initial RAM filesystem and RAM disk (initramfs/initrd) support

Cryptsetup installation

The sys-fs/cryptsetup package provides the cryptsetup command, which is used to open or close the encrypted storage as well as manage the passphrases or keys associated with it.

root #emerge --ask sys-fs/cryptsetup

Encrypted storage

Keyfile or passphrase

In order to start with encrypted storage, the administrator will need to decide which method to use for the encryption key. With cryptsetup the choice is either a passphrase or a keyfile. In case of a keyfile, this can be any file, but it is recommended to use a file with random data which is properly protected (considering that access to this keyfile will mean access to the encrypted data).

To create a keyfile, one can use the dd command:

root #dd if=/dev/urandom of=/etc/keys/enc.key bs=1 count=4096

In the next sections, we will show every command for both situations - passphrase and keyfile. Of course, only one method is necessary.

Creating an encrypted storage platform

In order to create an encrypted storage platform (which can be a disk, partition, file, ...) use the cryptsetup command with the luksFormat action.

For instance, to have /dev/vdb2 as the storage medium for the encrypted data:

root #cryptsetup -s 512 luksFormat /dev/vdb2
This will overwrite data on /dev/vdb2 irrevocably.
  
Are you sure? (Type uppercase yes): YES
Enter LUKS passphrase: ...
Verify passphrase: ...

To use a keyfile instead of a passphrase:

root #cryptsetup -s 512 luksFormat /dev/vdb2 /etc/keys/enc.key
This will overwrite data on /dev/vdb2 irrevocably.
  
Are you sure? (Type uppercase yes): YES

The -s 512 tells cryptsetup which keylength to use for the real encryption key (unlike the passphrase or keyfile, which are used to access this real encryption key).

Opening the encrypted storage

In order to open up the encrypted storage (i.e. make the real data accessible through transparent decryption), use the luksOpen action.

root #cryptsetup luksOpen /dev/vdb2 myname
Enter passphrase for /dev/vdb2: ...

If a keyfile is used, then the command would look like so:

root #cryptsetup luksOpen -d /etc/keys/enc.key /dev/vdb2 myname

When the command finishes successfully, then a new device file called /dev/mapper/myname will be made available.

If this is the first time this encrypted device is used, it needs to be formatted. The following example uses the Btrfs file system but of course any other file system will do:

root #mkfs.btrfs /dev/mapper/myname

Once the file system is formatted, or the formatting was already done in the past, then the device file can be mounted on the system:

root #mount /dev/mapper/myname /home

Closing the encrypted storage

In order to close the encrypted storage (i.e. ensure that the real data is no longer accessible through transparent decryption), use the luksClose action:

root #cryptsetup luksClose myname

Of course, make sure that the device is no longer in use.

Manipulating LUKS keys

LUKS keys are used to access the real encryption key. They are stored in slots in the header of the (encrypted) partition, disk or file.

Listing the slots

With the luksDump action, information about the encrypted partition, disk or file can be shown. This includes the slots:

root #cryptsetup luksDump /dev/vdb2
LUKS header information for /dev/vdb2
  
Version:        1
Cipher name:    aes
Cipher mode:    xts-plain64
Hash spec:      sha1
Payload offset: 4096
MK bits:        512
MK digest:      34 3b ec ac 10 af 19 e7 e2 d4 c8 90 eb a8 da 3c e4 4f 2e ce
MK salt:        ff 7c 7f 53 db 53 48 02 a4 32 dc e0 22 fc a3 51
                06 ba b3 48 b3 28 13 a8 7a 68 43 d6 46 79 14 fe
MK iterations:  59375
UUID:           2921a7c9-7ccb-4300-92f4-38160804e08c
  
Key Slot 0: ENABLED
        Iterations:             241053
        Salt:                   90 0f 0f db cf 66 ea a9 6c 7c 0c 0d b0 28 05 2f
                                8a 5c 14 54 98 62 1a 29 f3 08 25 0c ec c2 b1 68
        Key material offset:    8
        AF stripes:             4000
Key Slot 1: ENABLED
        Iterations:             273211
        Salt:                   01 4c 26 ed ff 18 75 31 b9 89 5d a6 e0 b5 f4 14
                                48 d0 23 47 a9 85 78 fb 76 c4 a9 d0 cd 63 fb d7
        Key material offset:    512
        AF stripes:             4000
Key Slot 2: DISABLED
Key Slot 3: DISABLED
Key Slot 4: DISABLED
Key Slot 5: DISABLED
Key Slot 6: DISABLED
Key Slot 7: DISABLED

In the above example, two slots are used. Note that luksDump does not give away anything sensitive - it is merely displaying the LUKS header content. No decryption key has to be provided in order to call luksDump.

Adding a keyfile or passphrase

In order to add an additional keyfile or passphrase to access the encrypted storage, use the luksAddKey action:

root #cryptsetup luksAddKey /dev/vdb2
Enter any passphrase: (Enter a valid, previously used passphrase to unlock the key)
Enter new passphrase for key slot: ... 
Verify passphrase: ...

To use a keyfile to unlock the key (but still add in a passphrase):

root #cryptsetup luksAddKey -d /etc/keys/enc.key /dev/vdb2
Enter new passphrase for key slot: ...
Verify passphrase: '''

If a keyfile is to be added (say /etc/keys/backup.key):

root #cryptsetup luksAddKey /dev/vdb2 /etc/keys/backup.key

Or, to use the first keyfile to unlock the main key:

root #cryptsetup luksAddKey -d /etc/keys/enc.key /dev/vdb2 /etc/keys/backup.key

Removing a keyfile or passphrase

With the luksRemoveKey action, a keyfile or passphrase can be removed (so they can no longer be used to decrypt the storage):

root #cryptsetup luksRemoveKey /dev/vdb2
Enter LUKS passphrase to be deleted: ...

Or to remove a keyfile:

root #cryptsetup luksRemoveKey -d /etc/keys/backup.key /dev/vdb2

Make sure that at least one method for accessing the data is still available. Once a passphrase or keyfile is removed for use, this cannot be recovered again.

Emptying a slot

Suppose the passphrase or keyfile is no longer known, then the slot can be freed. Of course, this does require prior knowledge of which slot that the passphrase or keyfile was stored in.

For instance, to empty out slot 2 (which is the third slot as slots are numbered starting from 0):

root #cryptsetup luksKillSlot /dev/vdb2 2

This command will ask for a valid passphrase before continuing. Or one can pass on the keyfile to use:

root #cryptsetup luksKillSlot -d /etc/keys/enc.key /dev/vdb2 2

Automate mounting encrypted file systems

Until now, the article focused on manual setup and mounting/unmounting of encrypted file systems. An init service dmcrypt exists which automates the decrypting and mounting of encrypted file systems.

Configuring dm-crypt

Edit the /etc/conf.d/dmcrypt file and add in entries for each file system. The supported entries are well documented in the file, the below example is just that - an example:

FILE /etc/conf.d/dmcryptAutomatically enabling two encrypted file systems
# Definition for /dev/mapper/home (for /home)
target=home
source=UUID="abcdef12-321a-a324-a88c-cac412befd98"
key=/etc/keys/home.key
 
# Definition for /dev/mapper/local (for /usr/local)
target=local
source=UUID="fedcba34-4823-b423-a94c-cadbefda2943"
key=/etc/keys/local.key

Configuring fstab

The next step is to configure /etc/fstab to automatically mount the (decrypted) file systems when they become available. It is recommended to first obtain the UUID of the decrypted (mounted) file system:

root #blkid /dev/mapper/home
/dev/mapper/home: UUID="4321421a-4321-a6c9-de52-ba6421efab76" TYPE="ext4"

Then, update the /etc/fstab file accordingly:

FILE /etc/fstabAutomounting the decrypted file systems
UUID="4321421a-4321-a6c9-de52-ba6421efab76"   /home        ext4   defaults   0   0
UUID="bdef2432-3bd1-4ab4-523d-badcf234a342"   /usr/local   ext4   defaults   0   0

Add initscript to bootlevel

Don't forget to have the dmcrypt init service launched at boot:

root #rc-update add dmcrypt boot

Additional resources

  • Dm-crypt full disk encryption on the Gentoo Wiki provides supplementary information on using encrypted file systems for Gentoo Linux installations
  • The cryptsetup FAQ hosted on GitLab covers a wide range of frequently asked questions.
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