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This page is a translated version of the page Dm-crypt and the translation is 42% complete.


dm-crypt ist ein Verschlüsselungssystem welches das crypto-API-Framework und das device-mapper Subsystem des Kernels nutzt. Mit dm-crypt können Administratoren sowohl ganze Laufwerke, logische Datenträger oder Partitionen als auch einzelne Dateien verschlüsseln.

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.


Folgende zwei Voraussetzungen müssen erfüllt sein, bevor man dm-crypt benutzen kann:

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

Konfiguration des Kernels

Um dm-crypt zu verwenden, müssen einige Konfigurationsoptionen zwingend gesetzt sein.

Zu allererst muss die Unterstützung für die "device mapper" Infrastruktur und für das "crypt target" aktiviert werden.

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 Aktivieren von kryptografischen API-Funktionen
[*] Cryptographic API --->
    <*> XTS support
    <*> SHA224 and SHA256 digest algorithm
    <*> AES cipher algorithms
    <*> AES cipher algorithms (x86_64)
    <*> User-space interface for hash algorithms
    <*> 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 Aktivieren der initramfs-Unterstützung
General setup  --->
    [*] Initial RAM filesystem and RAM disk (initramfs/initrd) support

If using the tcrypt encryption option (TrueCrypt/tcplay/VeraCrypt compatibility mode), then the following items will also need to be added to the kernel. Otherwise, cryptsetup will return the following errors: "device-mapper: reload ioctl failed: Invalid argument" and "Kernel doesn't support TCRYPT compatible mapping".

KERNEL Aktivieren der tcrypt-Unterstützung (TrueCrypt/tcplay/VeraCrypt-Kompatibilitätsmodus)
Device Drivers --->
    [*] Block Devices ---> 
        <*> Loopback device support 
File systems ---> 
     <*> FUSE (Filesystem in Userspace) support 
[*] Cryptographic API ---> 
     <*> RIPEMD-160 digest algorithm 
     <*> SHA384 and SHA512 digest algorithms 
     <*> Whirlpool digest algorithms 
     <*> LRW support 
     <*> Serpent cipher algorithm 
     <*> Twofish cipher algorithm

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


cryptsetup provides a benchmarking tool which will help to decide which setup to choose. The output depends on kernel settings as well as USE flags and destination (HDD, SSD etc.).

root #cryptsetup benchmark
# Tests are approximate using memory only (no storage IO).
PBKDF2-sha1      1707778 iterations per second for 256-bit key
PBKDF2-sha256    2131252 iterations per second for 256-bit key
PBKDF2-sha512    1630755 iterations per second for 256-bit key
PBKDF2-ripemd160  882639 iterations per second for 256-bit key
PBKDF2-whirlpool  664496 iterations per second for 256-bit key
argon2i       9 iterations, 1048576 memory, 4 parallel threads (CPUs) for 256-bit key (requested 2000 ms time)
argon2id      9 iterations, 1048576 memory, 4 parallel threads (CPUs) for 256-bit key (requested 2000 ms time)
#     Algorithm |       Key |      Encryption |      Decryption
        aes-cbc        128b      1197.2 MiB/s      3788.1 MiB/s
    serpent-cbc        128b               N/A               N/A
    twofish-cbc        128b               N/A               N/A
        aes-cbc        256b       888.2 MiB/s      3011.1 MiB/s
    serpent-cbc        256b               N/A               N/A
    twofish-cbc        256b               N/A               N/A
        aes-xts        256b      3670.7 MiB/s      3708.4 MiB/s
    serpent-xts        256b               N/A               N/A
    twofish-xts        256b               N/A               N/A
        aes-xts        512b      2929.2 MiB/s      2974.0 MiB/s
    serpent-xts        512b               N/A               N/A
    twofish-xts        512b               N/A               N/A

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 -c aes-xts-plain64 -s 512 -y 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 -c aes-xts-plain64 -s 512 -y luksFormat /dev/vdb2 /etc/keys/enc.key
This will overwrite data on /dev/vdb2 irrevocably.
Are you sure? (Type uppercase yes): YES

The -c aes-xts-plain64 tells cryptsetup the cipher used to encrypt the disk (cat /proc/crypto will show you all possibilities). -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). Finally -y forces you to type your password twice.

XTS splits the key into two halves, only one being used for the actual encryption. That means "aes-xts" with a 512-bit key actually uses 256 bits for the AES part.
If the LUKS header gets damaged, your encrypted data will be lost forever, even if you have a backup of the GPG key and passphrase. Therefore, you may wish to consider backing up this header to a separate device, and storing it securely. See the LUKS FAQ for more details on how to do this.
root #cryptsetup luksHeaderBackup /dev/sdXn --header-backup-file /tmp/efiboot/luks-header.img

Be aware, that if you do keep a LUKS header backup in this fashion and subsequently revoke any of the keyslots, the old keys will still be usable to unlock the LUKS partition for those with an access to that header backup file.

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:

DATEI /etc/conf.d/dmcryptAutomatically enabling two encrypted file systems
# Definition for /dev/mapper/home (for /home)
# Definition for /dev/mapper/local (for /usr/local)
# Using an encrypted partition as key source.
key=/keys/other.key                                # Relative to the root of the encrypted partition.
remdev=/dev/mapper/home                            # The recently decrypted partition.
# An empty line is important at the end of the file

Fstab konfigurieren

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:

DATEI /etc/fstabAutomatisches Einhängen der entschlüsselten Dateisysteme
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

Make decrypted device nodes visible

If you have decrypted/unlocked a device before the services were started for example your root disk in an with an initramfs then it's possible that the mapped device is not visible. In this case you can run the following to recreate it.

root #dmsetup mknodes

Mounting TrueCrypt/tcplay/VeraCrypt volumes

root #cryptsetup --type tcrypt open container-to-mount container-name

Replace container-to-mount with the device file under /dev or the path to the file you wish to open. Upon successful opening, the plaintext device will appear as /dev/mapper/container-name, which you can mount like any normal device.

If you are using key files, supply them using the --key-file option, to open a hidden volume, supply the --tcrypt-hidden option and for a partition or whole drive that is encrypted in system mode use the --tcrypt-system option.

When done, unmount the volume, and close the container using the following command:

root #cryptsetup close container-name

Siehe auch

Externe Ressourcen

  • Die cryptsetup FAQ, die auf GitLab gehostet wird, deckt ein breites Spektrum häufig gestellter Fragen ab.