dm-crypt is a transparent disk encryption subsystem. dm-crypt maps a physical block device to a virtual block device and does encryption/decryption transparently between the two block devices.

Modes[edit | edit source]

There are two common modes when using dm-crypt: Plain-mode and with LUKS.

Plain mode encrypts sector by sector without any headers or metadata. The entire volume contains just the raw encrypted cyphertext and nothing else. LUKS on the other hand has a standardized format facilitating compatibility across different distributions in addition to supporting multi-user passwords.

Disadvantages with plain mode:

  • dm-crypt won't know whether your password is correct or not.
  • dm-crypt won't know what encryption settings were used and any changes to this from software upgrades will break
  • passphrase cannot be changed without re-encrypting the volume

Advantages with using LUKS:

  • passphrase is used to decrypt a master key stored in the header and therefore can be changed without re-encrypting a volume
  • passphrase can be keystretched to strengthen against brute-forcee attacks
  • encryption settings are saved unencrypted in a header making it compatible with different systems

An attacker can easily recognize the type of volume by inspecting the unencrypted header.

Usage[edit | edit source]

On a Red Hat based system, install the cryptsetup package.

Plain Mode[edit | edit source]

# cryptsetup --verify-passphrase open --type plain /dev/sdX sdX-plain
## Use /dev/mapper/sdX-plain as any other block device
# cryptsetup close sdX-plain

LUKS[edit | edit source]

Basic usage:

# cryptsetup luksFormat /dev/sdX
# cryptsetup luksOpen /dev/sdX sdX-luks
## Use /dev/mapper/sdX-luks 
# cryptsetup luksClose sdX-luks

The LUKS header can be backed up and restored:

# cryptsetup luksHeaderBackup /dev/sdX --header-backup-file sdX-luks-header
# cryptsetup luksHeaderRestore /dev/sdX --header-backup-file sdX-luks-header

Options[edit | edit source]

There are some options that can be passed to cryptsetup:

Option Defaults Description
--hash, -h sha256 Hash algorithm used for key derivation. Eg: sha256, sha512, ripemd160
--cipher, -c aes-xts-plain64 Cipher to use for encryption. Eg: aes-xts-plain64, aes-cbc-essiv:sha256
--key-size, -s 256 (512 for XTS) Key size used for encryption
--verify-passphrase, -y yes Verify passphrase for sanity check

When using luksFormat or luksAddKey, you may want to specify --iter-time or -i to specify the number of milliseconds to spend with PBKDF2.

Keys[edit | edit source]

There are in total of 8 key slots that can be use. The first key slot is used by the passphrase in the initial creation step.

To change your passphrase:

# cryptsetup luksChangeKey /dev/sdX

To set up an additional passphrase that can unlock the container.

# cryptsetup luksAddKey /dev/sdX

To remove a passphrase from a key slot:

# cryptsetup luksRemoveKey /dev/sdX

To see which key slots are in use:

# cryptsetup luksDump /dev/sdX

See Also[edit | edit source]