NAME¶
cryptsetup - manage plain dm-crypt and LUKS encrypted volumes
SYNOPSIS¶
cryptsetup <options> <action> <action args>
DESCRIPTION¶
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.
In addition, cryptsetup provides limited support for the use of historic loopaes
volumes and for TruerCrypt compatible volumes.
PLAIN DM-CRYPT OR LUKS?¶
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.
WARNINGS¶
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.
BASIC COMMANDS¶
The following are valid actions for all supported device types.
open <device> <name> --type <device_type>
- Opens (creates a mapping with) <name> backed by device
<device>.
Device type can be plain, luks (default), loopaes or
tcrypt.
For backward compatibility there are open command aliases:
create (argument-order <name> <device>): open --type
plain
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. For create, the order of the <name>
and <device> options is inverted for historical reasons, all other
aliases use the standard <device> <name> order.
close <name>
- Removes the existing mapping <name> and wipes the key from kernel
memory.
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>
- Reports the status for the mapping <name>.
resize <name>
- Resizes an active mapping <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.
PLAIN MODE¶
Plain dm-crypt encrypts the device sector-by-sector with a single, non-salted
hash of the passphrase. No checks are performed, no metadata is used. There is
no formatting operation. When the raw device is mapped (opened), the usual
device operations can be used on the mapped device, including filesystem
creation. Mapped devices usually reside in /dev/mapper/<name>.
The following are valid plain device type actions:
open --type plain <device> <name>
create <name> <device> (
OBSOLETE syntax)
- Opens (creates a mapping with) <name> backed by device
<device>.
<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 EXTENSION¶
LUKS, the Linux Unified Key Setup, is a standard for disk encryption. It adds a
standardized header at the start of the device, a key-slot area directly
behind the header and the bulk data area behind that. The whole set is called
a 'LUKS container'. The device that a LUKS container resides on is called a
'LUKS device'. For most purposes both terms can be used interchangeably. But
note that when the LUKS header is at a nonzero offset in a device, then the
device is not a LUKS device anymore, but has a LUKS container stored in it at
an offset.
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>]
- Initializes a LUKS partition and sets the initial passphrase (for key-slot
0), either via prompting or via <key file>. Note that if the second
argument is present, then the passphrase is taken from the file given
there, without the need to use the --key-file option. Also note that for
both forms of reading the passphrase from file you can give '-' as file
name, which results in the passphrase being read from stdin and the
safety-question being skipped.
You can only call luksFormat on a LUKS device that is not mapped.
<options> can be [--hash, --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, --iter-time, --header, --force-password].
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)
- Opens the LUKS device <device> and sets up a mapping <name>
after successful verification of the supplied passphrase. If the
passphrase is not supplied via --key-file, the command prompts for it
interactively.
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>
- Suspends an active device (all IO operations will blocked and accesses to
the device will wait indefinitely) and wipes the encryption key from
kernel memory. Needs kernel 2.6.19 or later.
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>
- Resumes a suspended device and reinstates the encryption key. Prompts
interactively for a passphrase if --key-file is not given.
<options> can be [--key-file, --keyfile-size, --header]
luksAddKey <device> [<key file with new key>]
- adds a new passphrase. An existing passphrase must be supplied
interactively or via --key-file. The new passphrase to be added can be
specified interactively or read from the file given as positional
argument.
<options> can be [--key-file, --keyfile-offset,
--keyfile-size, --new-keyfile-offset, --new-keyfile-size, --key-slot,
--master-key-file, --iter-time, --force-password].
luksRemoveKey <device> [<key file with passphrase to be
removed>]
- Removes the supplied passphrase from the LUKS device. The passphrase to be
removed can be specified interactively, as positional argument or via
--key-file.
<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>]
- Changes an existing passphrase. The passphrase to be changed must be
supplied interactively or via --key-file. The new passphrase can be
supplied interactively or in a file given as positional argument.
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,
--force-password].
luksKillSlot <device> <key slot number>
- Wipe the key-slot number <key slot> from the LUKS device. A
remaining passphrase must be supplied, either interactively or via
--key-file. This command can remove the last remaining key-slot, but
requires an interactive confirmation when doing so. Removing the last
passphrase makes a LUKS container permanently inaccessible.
<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.
erase <device>
luksErase <device>
- Erase all keyslots and make the LUKS container permanently inaccessible.
You do not need to provide any password for this operation.
WARNING: This operation is irreversible.
luksUUID <device>
- Print the UUID of a LUKS device.
Set new UUID if --uuid option is specified.
isLuks <device>
- Returns true, if <device> is a LUKS device, false otherwise. Use
option -v to get human-readable feedback. 'Command successful.' means the
device is a LUKS device.
luksDump <device>
- Dump the header information of a LUKS 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>
- Stores a binary backup of the LUKS header and keyslot area.
Note: Using '-' as filename writes the header backup to a file named '-'.
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>
- Restores a binary backup of the LUKS header and keyslot area from the
specified file.
Note: Using '-' as filename reads the header backup from a file named '-'.
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.
loop-AES EXTENSION¶
cryptsetup supports mapping loop-AES encrypted partition using a compatibility
mode.
open --type loopaes <device> <name> --key-file
<keyfile>
loopaesOpen <device> <name> --key-file <keyfile>
(
old syntax)
- Opens the loop-AES <device> and sets up a mapping <name>.
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 (TrueCrypt-compatible) EXTENSION¶
cryptsetup supports mapping of TrueCrypt or tcplay encrypted partition using a
native Linux kernel API. Header formatting and TCRYPT header change is not
supported, cryptsetup never changes TCRYPT header on-device.
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. You can use partition device as
the parameter (parameter must be real partition device, not image in file),
then only this partition is mapped.
If you have whole TCRYPT device as a file image and you want to map multiple
partition encrypted with system encryption, please create loopback mapping
with partitions first (
losetup -P, see
losetup(8) man page for
more info), and use loop partition as the device parameter.
If you use whole base device as parameter, one device for the whole system
encryption is mapped. This mode is available only for backward compatibility
with older cryptsetup versions which mapped TCRYPT system encryption using
whole device.
To use hidden header (and map hidden device, if available), use
--tcrypt-hidden option.
To explicitly use backup (secondary) header, use
--tcrypt-backup option.
NOTE: There is no protection for a hidden volume if the outer volume is
mounted. The reason is that if there were any protection, it would require
some metadata describing what to protect in the outer volume and the hidden
volume would become detectable.
open --type tcrypt <device> <name>
tcryptOpen <device> <name> (
old syntax)
- Opens the TCRYPT (a TrueCrypt-compatible) <device> and sets up a
mapping <name>.
<options> can be [--key-file, --tcrypt-hidden,
--tcrypt-system, --tcrypt-backup, --readonly, --test-passphrase,
--allow-discards].
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.
WARNING: Option --allow-discards cannot be combined with
option --tcrypt-hidden. For normal mapping it can cause
destruction of hidden volume (hidden volume appears as unused space
for outer volume so this space can be discarded).
tcryptDump <device>
- Dump the header information of a TCRYPT 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, --tcrypt-backup].
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.
MISCELLANEOUS¶
repair <device>
- Tries to repair the device metadata if possible. Currently supported only
for LUKS device type.
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>
- Benchmarks ciphers and KDF (key derivation function). Without parameters
it tries to measure few common configurations.
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].
OPTIONS¶
- --verbose, -v
- Print more information on command execution.
- --debug
- Run in debug mode with full diagnostic logs. Debug output lines are always
prefixed by '#'.
- --hash, -h <hash-spec>
- Specifies the passphrase hash for open (for plain and loopaes
device types).
Specifies the hash used in the LUKS key setup scheme and volume key digest
for luksFormat. The specified hash is used as hash-parameter for
PBKDF2 and for the AF splitter.
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.
- --cipher, -c <cipher-spec>
- Set the cipher specification string.
cryptsetup --help shows the compiled-in defaults. The current
default in the distributed sources is "aes-cbc-essiv:sha256" for
plain dm-crypt and "aes-xts-plain64" for LUKS.
If a hash is part of the cipher spefification, then it is used as part of
the IV generation. For example, ESSIV needs a hash function, while
"plain64" does not and hence none is specified.
For XTS mode you can 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.
- --verify-passphrase, -y
- When interactively asking for a passphrase, ask for it twice and complain
if both inputs do not match. Advised when creating a regular mapping for
the first time, or when running luksFormat. Ignored on input from
file or stdin.
- --key-file, -d name
- Read the passphrase from file.
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.
- --keyfile-offset value
- Skip value bytes at the beginning of the key file. Works with all
commands that accepts key files.
- --keyfile-size, -l value
- Read a maximum of value bytes from the key file. Default is to read
the whole file up to the compiled-in maximum that can be queried with
--help. Supplying more data than the compiled-in maximum aborts the
operation.
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.
- --new-keyfile-offset value
- Skip value bytes at the start when adding a new passphrase from key
file with luksAddKey.
- --new-keyfile-size value
- Read a maximum of value bytes when adding a new passphrase from key
file with luksAddKey. Default is to read the whole file up to the
compiled-in maximum length that can be queried with --help. Supplying more
than the compiled in maximum aborts the operation. When
--new-keyfile-offset is also given, reading starts after the offset.
- --master-key-file
- Use a master key stored in a file.
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.
WARNING: If you create your own master key, you need to make sure to
do it right. Otherwise you can end up with a low-entropy or otherwise
partially predictable master key which will compromise security.
For luksAddKey this allows adding a new passphrase without having to
know an exiting one.
For open this allows one to open the LUKS device without giving a
passphrase.
- --dump-master-key
- For luksDump this option includes the master key in the displayed
information. Use with care, as the master key can be used to bypass the
passphrases, see also option --master-key-file.
- --use-random
- --use-urandom
- For luksFormat these options define which kernel random number
generator will be used to create the master key (which is a long-term
key).
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.
- --key-slot, -S <0-7>
- For LUKS operations that add key material, this options allows you to
specify which key slot is selected for the new key. This option can be
used for luksFormat, and luksAddKey.
In addition, for open, this option selects a specific key-slot to
compare the passphrase against. If the given passphrase would only match a
different key-slot, the operation fails.
- --key-size, -s <bits>
- Sets key size in bits. The argument has to be a multiple of 8. The
possible key-sizes are limited by the cipher and mode used.
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.
- --size, -b <number of 512 byte sectors>
- Force the size of the underlying device in sectors of 512 bytes. This
option is only relevant for the open and resize
actions.
- --offset, -o <number of 512 byte sectors>
- Start offset in the backend device in 512-byte sectors. This option is
only relevant for the open action with plain or loopaes device
types.
- --skip, -p <number of 512 byte sectors>
- Start offset used in IV calculation in 512-byte sectors (how many sectors
of the encrypted data to skip at the beginning). This option is only
relevant for the open action with plain or loopaes device types.
Hence, if --offset n, and --skip s, sector n (the first
sector of encrypted device) will get a sector number of s for the
IV calculation.
- --readonly, -r
- set up a read-only mapping.
- --shared
- Creates an additional mapping for one common ciphertext device. Arbitrary
mappings are supported. This option is only relevant for the open
--type plain action. Use --offset, --size and --skip to specify the
mapped area.
- --iter-time, -i <number of milliseconds>
- The number of milliseconds to spend with PBKDF2 passphrase processing.
This option is only relevant for LUKS operations that set or change
passphrases, such as luksFormat or luksAddKey. Specifying 0
as parameter selects the compiled-in default.
- --batch-mode, -q
- Suppresses all confirmation questions. Use with care!
If the -y option is not specified, this option also switches off the
passphrase verification for luksFormat.
- --timeout, -t <number of seconds>
- The number of seconds to wait before timeout on passphrase input via
terminal. It is relevant every time a passphrase is asked, for example for
open, luksFormat or luksAddKey. It has no effect if
used in conjunction with --key-file.
This option is useful when the system should not stall if the user does not
input a passphrase, e.g. during boot. The default is a value of 0 seconds,
which means to wait forever.
- --tries, -T
- How often the input of the passphrase shall be retried. This option is
relevant every time a passphrase is asked, for example for open,
luksFormat or luksAddKey. The default is 3 tries.
- --align-payload <number of 512 byte sectors>
- Align payload at a boundary of value 512-byte sectors. This option
is relevant 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.
- --uuid=UUID
- Use the provided UUID for the luksFormat command instead of
generating new one. Changes the existing UUID when used with the
luksUUID command.
The UUID must be provided in the standard UUID format, e.g.
12345678-1234-1234-1234-123456789abc.
- --allow-discards
- Allow the use of discard (TRIM) requests for device. This option is only
relevant for open action.
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.
- --test-passphrase
- Do not activate device, just verify passphrase. This option is only
relevant for open action (the device mapping name is not mandatory
if this option is used).
- --header <device or file storing the LUKS header>
- Use a detached (separated) metadata device or file where the LUKS header
is stored. This options allows one to store ciphertext and LUKS header on
different devices.
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.
- --force-password
- Do not use password quality checking for new LUKS passwords.
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).
- --version
- Show the program version.
- --usage
- Show short option help.
- --help, -?
- Show help text and default parameters.
RETURN CODES¶
Cryptsetup returns 0 on success and a non-zero value on error.
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.
NOTES ON PASSPHRASE PROCESSING FOR PLAIN MODE¶
Note that no iterated hashing or salting is done in plain mode. If hashing is
done, it is a single direct hash. This means that low-entropy passphrases are
easy to attack in plain mode.
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 hash 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.
NOTES ON PASSPHRASE PROCESSING FOR LUKS¶
LUKS uses PBKDF2 to protect against dictionary attacks and to give some
protection to low-entropy passphrases (see RFC 2898 and the cryptsetup FAQ).
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 device with a slow CPU,
as this will result in a low iteration count. On a slow device it may be
advisable to increase the iteration time using the --iter-time option in order
to obtain a higher iteration count. This does slow down all later luksOpen
operations accordingly.
INCOHERENT BEHAVIOR FOR INVALID PASSPHRASES/KEYS¶
LUKS checks for a valid passphrase when an encrypted partition is unlocked. The
behavior of plain dm-crypt is different. It will always decrypt with the
passphrase given. If the given passphrase is wrong, the device mapped by plain
dm-crypt will essentially still contain encrypted data and will be unreadable.
NOTES ON SUPPORTED CIPHERS, MODES, HASHES AND KEY SIZES¶
The available combinations of ciphers, modes, hashes and key sizes depend on
kernel support. See /proc/crypto for a list of available options. You might
need to load additional kernel crypto modules in order to get more options.
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.
NOTES ON PASSPHRASES¶
Mathematics can't be bribed. Make sure you keep your passphrases safe. There are
a few nice tricks for constructing a fallback, when suddenly out of the blue,
your brain refuses to cooperate. These fallbacks need LUKS, as it's only
possible with LUKS to have multiple passphrases. Still, if your attacker model
does not prevent it, storing your passphrase in a sealed envelope somewhere
may be a good idea as well.
NOTES ON RANDOM NUMBER GENERATORS¶
Random Number Generators (RNG) used in cryptsetup are always the kernel RNGs
without any modifications or additions to data stream produced.
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.
NOTES ON LOOPBACK DEVICE USE¶
Cryptsetup is usually used directly on a block device (disk partition or LVM
volume). However, if the device argument is a file, cryptsetup tries to
allocate a loopback device and map it into this file. This mode requires Linux
kernel 2.6.25 or more recent which supports the loop autoclear flag (loop
device is cleared on last close automatically). Of course, you can always map
a file to a loop-device manually. See the cryptsetup FAQ for an example.
When device mapping is active, you can see the loop backing file in the status
command output. Also see
losetup(8).
DEPRECATED ACTIONS¶
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.
REPORTING BUGS¶
Report bugs, including ones in the documentation, on the cryptsetup mailing list
at <dm-crypt@saout.de> or in the 'Issues' section on LUKS website.
Please attach the output of the failed command with the --debug option added.
AUTHORS¶
cryptsetup originally written by Jana Saout <jana@saout.de>
The LUKS extensions and original man page were written by Clemens Fruhwirth
<clemens@endorphin.org>.
Man page extensions by Milan Broz <gmazyland@gmail.com>.
Man page rewrite and extension by Arno Wagner <arno@wagner.name>.
COPYRIGHT¶
Copyright © 2004 Jana Saout
Copyright © 2004-2006 Clemens Fruhwirth
Copyright © 2009-2012 Red Hat, Inc.
Copyright © 2009-2014 Milan Broz
Copyright © 2012-2014 Arno Wagner
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
SEE ALSO¶
The LUKS website at
http://code.google.com/p/cryptsetup/
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