cryptsetup is used to conveniently setup dm-crypt managed device-mapper mappings. These include plain dm-crypt volumes and LUKS volumes. The difference is that LUKS uses a metadata header and can hence offer more features than plain dm-crypt. On the other hand, the header is visible and vulnerable to damage.
Unless you understand the cryptographic background well, use LUKS. With plain dm-crypt there are a number of possible user errors that massively decrease security. While LUKS cannot fix them all, it can lessen the impact for many of them.
A lot of good information on the risks of using encrypted storage, on handling problems and on security aspects can be found in the Cryptsetup FAQ. Read it. Nonetheless, some risks deserve to be mentioned here.
Backup: Storage media die. Encryption has no influence on that. Backup is mandatory for encrypted data as well, if the data has any worth. See the Cryptsetup FAQ for advice on how to do backup of an encrypted volume.
Character encoding: If you enter a passphrase with special symbols, the passphrase can change depending character encoding. Keyboard settings can also change, which can make blind input hard or impossible. For example, switching from some ASCII 8-bit variant to UTF-8 can lead to a different binary encoding and hence different passphrase seen by cryptsetup, even if what you see on the terminal is exactly the same. It is therefore highly recommended to select passphrase characters only from 7-bit ASCII, as the encoding for 7-bit ASCII stays the same for all ASCII variants and UTF-8.
LUKS header: If the header of a LUKS volume gets damaged, all data is permanently lost unless you have a header-backup. If a key-slot is damaged, it can only be restored from a header-backup or if another active key-slot with known passphrase is undamaged. Damaging the LUKS header is something people manage to do with surprising frequency. This risk is the result of a trade-off between security and safety, as LUKS is designed for fast and secure wiping by just overwriting header and key-slot area.
Previously used partitions: If a partition was previously used, it is a very good idea to wipe filesystem signatures, data, etc. before creating a LUKS or plain dm-crypt container on it. For a quick removal of filesystem signatures, use "wipefs". Take care though that this may not remove everything. In particular md (RAID) signatures at the end of a device may survive. It also does not remove data. For a full wipe, overwrite the whole partition before container creation. If you do not know how to to that, the cryptsetup FAQ describes several options.
open <name> <device> --type <device_type>
Device type can be plain, luks (default), loopaes or tcrypt.
For backward compatibility there are open command aliases:
create: open --type plain <device> <name> switched arguments)
plainOpen: open --type plain
luksOpen: open --type luks
loopaesOpen: open --type loopaes
tcryptOpen: open --type tcrypt
<options> are type specific and are described below for individual device types.
close <name>
For backward compatibility there are close command aliases: remove, plainClose, luksClose, loopaesClose, tcryptClose (all behaves exactly the same, device type is determined automatically from active device).
status <name>
resize <name>
If --size (in sectors) is not specified, the size of the underlying block device is used. Note that this does not change the raw device geometry, it just changes how many sectors of the raw device are represented in the mapped device.
The following are valid plain device type actions:
open --type plain <device> <name>
create <name> <device> (OBSOLETE syntax)
<options> can be [--hash, --cipher, --verify-passphrase, --key-file, --keyfile-offset, --key-size, --offset, --skip, --size, --readonly, --shared, --allow-discards]
Example: 'cryptsetup open --type plain /dev/sda10 e1' maps the raw encrypted device /dev/sda10 to the mapped (decrypted) device /dev/mapper/e1, which can then be mounted, fsck-ed or have a filesystem created on it.
LUKS can manage multiple passphrases that can be individually revoked or changed and that can be securely scrubbed from persistent media due to the use of anti-forensic stripes. Passphrases are protected against brute-force and dictionary attacks by PBKDF2, which implements hash iteration and salting in one function.
Each passphrase, also called a key in this document, is associated with one of 8 key-slots. Key operations that do not specify a slot affect the first slot that matches the supplied passphrase or the first empty slot if a new passphrase is added.
The following are valid LUKS actions:
luksFormat <device> [<key file>]
You can only call luksFormat on a LUKS device that is not mapped.
<options> can be [--cipher, --verify-passphrase, --key-size, --key-slot, --key-file (takes precedence over optional second argument), --keyfile-offset, --keyfile-size, --use-random | --use-urandom, --uuid, --master-key-file].
WARNING: Doing a luksFormat on an existing LUKS container will make all data the old container permanently irretrievable, unless you have a header backup.
open --type luks <device> <name>
luksOpen <device> <name> (old syntax)
The <device> parameter can be also specified by LUKS UUID in the format UUID=<uuid>, which uses the symlinks in /dev/disk/by-uuid.
<options> can be [--key-file, --keyfile-offset, --keyfile-size, --readonly, --test-passphrase, --allow-discards, --header, --key-slot, --master-key-file].
luksSuspend <name>
After this operation you have to use luksResume to reinstate the encryption key and unblock the device or close to remove the mapped device.
WARNING: never suspend the device on which the cryptsetup binary resides.
<options> can be [--header].
luksResume <name>
<options> can be [--key-file, --keyfile-size, --header]
luksAddKey <device> [<key file with new key>]
<options> can be [--key-file, --keyfile-offset, --keyfile-size, --new-keyfile-offset, --new-keyfile-size, --key-slot, --master-key-file].
luksRemoveKey <device> [<key file with passphrase to be removed>]
<options> can be [--key-file, --keyfile-offset, --keyfile-size]
WARNING: If you read the passphrase from stdin (without further argument or with '-' as argument to --key-file), batch-mode (-q) will be implicitely switched on and no warning will be given when you remove the last remaining passphrase from a LUKS container. Removing the last passphrase makes the LUKS container permanently inaccessible.
luksChangeKey <device> [<new key file>]
If a key-slot is specified (via --key-slot), the passphrase for that key-slot must be given and the new passphrase will overwrite the specified key-slot. If no key-slot is specified and there is still a free key-slot, then the new passphrase will be put into a free key-slot before the key-slot containing the old passphrase is purged. If there is no free key-slot, then the key-slot with the old passphrase is overwritten directly.
WARNING: If a key-slot is overwritten, a media failure during this operation can cause the overwrite to fail after the old passphrase has been wiped and make the LUKS container inaccessible.
<options> can be [--key-file, --keyfile-offset, --keyfile-size, --new-keyfile-offset, --new-keyfile-size, --key-slot].
luksKillSlot <device> <key slot number>
<options> can be [--key-file, --keyfile-offset, --keyfile-size].
WARNING: If you read the passphrase from stdin (without further argument or with '-' as argument to --key-file), batch-mode (-q) will be implicitely switched on and no warning will be given when you remove the last remaining passphrase from a LUKS container. Removing the last passphrase makes the LUKS container permanently inaccessible.
luksUUID <device>
isLuks <device>
luksDump <device>
If the --dump-master-key option is used, the LUKS device master key is dumped instead of the keyslot info. Beware that the master key cannot be changed and can be used to decrypt the data stored in the LUKS container without a passphrase and even without the LUKS header. This means that if the master key is compromised, the whole device has to be erased to prevent further access. Use this option carefully.
In order to dump the master key, a passphrase has to be supplied, either interactively or via --key-file.
<options> can be [--dump-master-key, --key-file, --keyfile-offset, --keyfile-size].
WARNING: If --dump-master-key is used with --key-file and the argument to --key-file is '-', no validation question will be asked and no warning given.
luksHeaderBackup <device> --header-backup-file <file>
WARNING: This backup file and a passphrase valid at the time of backup allows decryption of the LUKS data area, even if the passphrase was later changed or removed from the LUKS device. Also note that with a header backup you lose the ability to securely wipe the LUKS device by just overwriting the header and key-slots. You either need to securely erase all header backups in addition or overwrite the encrypted data area as well. The second option is less secure, as some sectors can survive, e.g. due to defect management.
luksHeaderRestore <device> --header-backup-file <file>
WARNING: Header and keyslots will be replaced, only the passphrases from the backup will work afterwards.
This command requires that the master key size and data offset of the LUKS header already on the device and of the header backup match. Alternatively, if there is no LUKS header on the device, the backup will also be written to it.
open --type loopaes <device> <name> --key-file <keyfile>
loopaesOpen <device> <name> --key-file <keyfile> (old syntax)
If the key file is encrypted with GnuPG, then you have to use
--key-file=- and decrypt it before use, e.g. like this:
gpg --decrypt <keyfile> | cryptsetup loopaesOpen --key-file=- <device> <name>
Use --keyfile-size to specify the proper key length if needed.
Use --offset to specify device offset. Note that the units need to be specified in number of 512 byte sectors.
Use --skip to specify the IV offset. If the original device used an offset and but did not use it in IV sector calculations, you have to explicitly use --skip 0 in addition to the offset parameter.
Use --hash to override the default hash function for passphrase hashing (otherwise it is detected according to key size).
<options> can be [--key-file, --key-size, --offset, --skip, --hash, --readonly, --allow-discards].
See also section 7 of the FAQ and http://loop-aes.sourceforge.net for more information regarding loop-AES.
TCRYPT extension requires kernel userspace crypto API to be available (introduced in Linux kernel 2.6.38). If you are configuring kernel yourself, enable "User-space interface for symmetric key cipher algorithms" in "Cryptographic API" section (CRYPTO_USER_API_SKCIPHER .config option).
Because TCRYPT header is encrypted, you have to always provide valid passphrase and keyfiles.
Cryptsetup should recognize all header variants, except legacy cipher chains using LRW encryption mode with 64 bits encryption block (namely Blowfish in LRW mode is not recognized, this is limitation of kernel crypto API).
NOTE: Activation with tcryptOpen is supported only for cipher chains using LRW or XTS encryption modes.
The tcryptDump command should work for all recognized TCRYPT devices and doesn't require superuser privilege.
To map system device (device with boot loader where the whole encrypted system resides) use --tcrypt-system option. Use the whole device not the system partition as the device parameter.
To use hidden header (and map hidden device, if available), use --tcrypt-hidden option.
open --type tcrypt <device> <name>
tcryptOpen <device> <name> (old syntax)
<options> can be [--key-file, --tcrypt-hidden, --tcrypt-system, --readonly, --test-passphrase].
The keyfile parameter allows combination of file content with the passphrase and can be repeated. Note that using keyfiles is compatible with TCRYPT and is different from LUKS keyfile logic.
tcryptDump <device>
If the --dump-master-key option is used, the TCRYPT device master key is dumped instead of TCRYPT header info. Beware that the master key (or concatenated master keys if cipher chain is used) can be used to decrypt the data stored in the TCRYPT container without a passphrase. This means that if the master key is compromised, the whole device has to be erased to prevent further access. Use this option carefully.
<options> can be [--dump-master-key, --key-file, --tcrypt-hidden, --tcrypt-system].
The keyfile parameter allows combination of file content with the passphrase and can be repeated.
See also http://www.truecrypt.org for more information regarding TrueCrypt.
Please note that cryptsetup does not use TrueCrypt code, please report all problems related to this compatibility extension to cryptsetup project.
repair <device>
This command is useful to fix some known benign LUKS metadata header corruptions. Only basic corruptions of unused keyslot are fixable. This command will only change the LUKS header, not any key-slot data.
WARNING: Always create a binary backup of the original header before calling this command.
benchmark <options>
To benchmark other ciphers or modes, you need to specify --cipher and --key-size options or --hash for KDF test.
NOTE: This benchmark is using memory only and is only informative. You cannot directly predict real storage encryption speed from it.
For testing block ciphers, this benchmark requires kernel userspace crypto API to be available (introduced in Linux kernel 2.6.38). If you are configuring kernel yourself, enable "User-space interface for symmetric key cipher algorithms" in "Cryptographic API" section (CRYPTO_USER_API_SKCIPHER .config option).
<options> can be [--cipher, --key-size, --hash].
Specifies the hash used in the LUKS key setup scheme and volume key digest for luksFormat.
The specified hash name is passed to the compiled-in crypto backend. Different backends may support different hashes. For luksFormat, the hash algorithm must provide at least 160 bits of output, which excludes, e.g., MD5. Do not use a non-crypto hash like "crc32" as this breaks security.
Values compatible with old version of cryptsetup are "ripemd160" for open --type plain and "sha1" for luksFormat.
Use cryptsetup --help to show the defaults.
cryptsetup --help shows the compiled-in defaults. The current default in the distributed sources is "aes-cbc-essiv:sha256" for both plain dm-crypt and LUKS.
For XTS mode (a possible future default), use "aes-xts-plain" or better "aes-xts-plain64" as cipher specification and optionally set a key size of 512 bits with the -s option. Key size for XTS mode is twice that for other modes for the same security level.
XTS mode requires kernel 2.6.24 or later and plain64 requires kernel 2.6.33 or later. More information can be found in the FAQ.
If the name given is "-", then the passphrase will be read from stdin. In this case, reading will not stop at newline characters.
With LUKS, passphrases supplied via --key-file are always the existing passphrases requested by a command, except in the case of luksFormat where --key-file is equivalent to the positional key file argument.
If you want to set a new passphrase via key file, you have to use a positional argument to luksAddKey.
See section NOTES ON PASSPHRASE PROCESSING for more information.
This option is useful to cut trailing newlines, for example. If --keyfile-offset is also given, the size count starts after the offset. Works with all commands that accepts key files.
For luksFormat this allows creating a LUKS header with this specific master key. If the master key was taken from an existing LUKS header and all other parameters are the same, then the new header decrypts the data encrypted with the header the master key was taken from.
For luksAddKey this allows adding a new passphrase without having to know an exiting one.
For open this allows to open the LUKS device without giving a passphrase.
See NOTES ON RANDOM NUMBER GENERATORS for more information. Use cryptsetup --help to show the compiled-in default random number generator.
WARNING: In a low-entropy situation (e.g. in an embedded system), both selections are problematic. Using /dev/urandom can lead to weak keys. Using /dev/random can block a long time, potentially forever, if not enough entropy can be harvested by the kernel.
See /proc/crypto for more information. Note that key-size in /proc/crypto is stated in bytes.
This option can be used for open --type plain or luksFormat. All other LUKS actions will use the key-size specified in the LUKS header. Use cryptsetup --help to show the compiled-in defaults.
This is different from the --offset options with respect to the sector numbers used in IV calculation. Using --offset will shift the IV calculation by the same negative amount. Hence, if --offset n, sector n will get a sector number of 0 for the IV calculation. Using --skip causes sector n to also be the first sector of the mapped device, but with its number for IV generation is n.
If the -y option is not specified, this option also switches off the passphrase verification for luksFormat.
If not specified, cryptsetup tries to use the topology info provided by kernel for the underlying device to get optimal alignment. If not available (or the calculated value is a multiple of the default) data is by default aligned to a 1MiB boundary (i.e. 2048 512-byte sectors).
For a detached LUKS header this option specifies the offset on the data device. See also the --header option.
The UUID must be provided in the standard UUID format, e.g. 12345678-1234-1234-1234-123456789abc.
WARNING: This command can have a negative security impact because it can make filesystem-level operations visible on the physical device. For example, information leaking filesystem type, used space, etc. may be extractable from the physical device if the discarded blocks can be located later. If in doubt, do no use it.
A kernel version of 3.1 or later is needed. For earlier kernels this option is ignored.
This option is only relevant for LUKS devices and can be used with the luksFormat, open, luksSuspend, luksResume, status and resize commands.
For luksFormat with a file name as argument to --header, it has to exist and be large enough to contain the LUKS header. See the cryptsetup FAQ for header size calculation.
For other commands that change the LUKS header (e.g. luksAddKey), specify the device or file with the LUKS header directly as the LUKS device.
If used with luksFormat, the --align-payload option is taken as absolute sector alignment on ciphertext device and can be zero.
WARNING: There is no check whether the ciphertext device specified actually belongs to the header given. In fact you can specify an arbitrary device as the ciphertext device for open with the --header option. Use with care.
This option applies only to luksFormat, luksAddKey and luksChangeKey and is ignored if cryptsetup is built without password quality checking support.
For more info about password quality check, see manual page for pwquality.conf(5).
Error codes are: 1 wrong parameters, 2 no permission (bad passphrase), 3 out of memory, 4 wrong device specified, 5 device already exists or device is busy.
From a terminal: The passphrase is read until the first newline, i.e. '\n'. The input without the newline character is processed with the default hash or the hash specified with --hash. The has result will be truncated to the key size of the used cipher, or the size specified with -s.
From stdin: Reading will continue until a newline (or until the maximum input size is reached), with the trailing newline stripped. The maximum input size is defined by the same compiled-in default as for the maximum key file size and can be overwritten using --keyfile-size option.
The data read will be hashed with the default hash or the hash specified with --hash. The has result will be truncated to the key size of the used cipher, or the size specified with -s.
Note that if --key-file=- is used for reading the key from stdin, trailing newlines are not stripped from the input.
If "plain" is used as argument to --hash, the input data will not be hashed. Instead, it will be zero padded (if shorter than the key size) or truncated (if longer than the key size) and used directly as the binary key. This is useful for directly specifying a binary key. No warning will be given if the amount of data read from stdin is less than the key size.
From a key file: It will be truncated to the key size of the used cipher or the size given by -s and directly used as binary key. if the key file is shorter than the key, cryptsetup will quit with an error.
From a terminal: The passphrase is read until the first newline and then processed by PBKDF2 without the newline character.
From stdin: LUKS will read passphrases from stdin up to the first newline character or the compiled-in maximum key file length. If --keyfile-size is given, it is ignored.
From key file: The complete keyfile is read up to the compiled-in maximum size. Newline characters do not terminate the input. The --keyfile-size option can be used to limit what is read.
Passphrase processing: Whenever a passphrase is added to a LUKS header (luksAddKey, luksFormat), the user may specify how much the time the passphrase processing should consume. The time is used to determine the iteration count for PBKDF2 and higher times will offer better protection for low-entropy passphrases, but open will take longer to complete. For passphrases that have entropy higher than the used key length, higher iteration times will not increase security.
The default setting of one second is sufficient for most practical cases. The only exception is a low-entropy passphrase used on a slow device.
For the --hash option, if the crypto backend is libgcrypt, then all algorithms supported by the gcrypt library are available. For other crypto backends some algorithms may be missing.
There are two types of randomness cryptsetup/LUKS needs. One type (which always uses /dev/urandom) is used for salts, the AF splitter and for wiping deleted keyslots.
The second type is used for the volume (master) key. You can switch between using /dev/random and /dev/urandom here, see --use-random and --use-urandom options. Using /dev/random on a system without enough entropy sources can cause luksFormat to block until the requested amount of random data is gathered. In a low-entropy situation (embedded system), this can take a very long time and potentially forever. At the same time, using /dev/urandom in a low-entropy situation will produce low-quality keys. This is a serious problem, but solving it is out of scope for a mere man-page. See urandom(4) for more information.
When device mapping is active, you can see the loop backing file in the status command output. Also see losetup(8).
The reload action is no longer supported. Please use dmsetup(8) if you need to directly manipulate with the device mapping table.
The luksDelKey was replaced with luksKillSlot.
This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
The cryptsetup FAQ, contained in the distribution package and online at http://code.google.com/p/cryptsetup/wiki/FrequentlyAskedQuestions
The cryptsetup mailing list and list archive, see FAQ entry 1.6.
The LUKS on-disk format specification available at http://code.google.com/p/cryptsetup/wiki/Specification