Man page
cryptsetup
CRYPTSETUP(8) Maintenance Commands CRYPTSETUP(8)
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.
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 Crypt-
setup 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 vol-
ume.
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 crypt-
setup, 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.
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
(created), the usual device operations can be used on the mapped
device, including filesystem creation. Mapped devices usually reside
in /dev/mapper/<name>.
There are four operations:
create <name> <device>
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 create e1 /dev/sda10' 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.
remove <name>
Removes the existing mapping <name> and wipes the key from ker-
nel memory.
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 under-
lying 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.
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 con-
tainer 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 [--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.
luksOpen <device> <name>
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].
luksClose <name>
identical to remove.
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 luksClose 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].
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 con-
tainer. 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 over-
write 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 inaccessi-
ble.
<options> can be [--key-file, --keyfile-offset, --keyfile-size,
--new-keyfile-offset, --new-keyfile-size, --key-slot].
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 con-
tainer. Removing the last passphrase makes the LUKS container
permanently inaccessible.
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 success-
ful.' 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 mas-
ter key cannot be changed and can be used to decrypt the data
stored in the LUKS container without a passphrase and even with-
out the LUKS header. This means that if the master key is com-
promised, 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 sup-
plied, either interactively or via --key-file.
<options> can be [--dump-master-key, --key-file, --keyfile-off-
set, --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.
repair <device>
Tries to repair the LUKS device metadata if possible.
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.
loop-AES EXTENSION
cryptsetup supports mapping loop-AES encrypted partition using a com-
patibility mode.
loopaesOpen <device> <name> --key-file <keyfile>
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 --key-file-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 param-
eter.
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].
loopaesClose <name>
Identical to remove.
See also section 7 of the FAQ and http://loop-aes.sourceforge.net for
more information regarding loop-AES.
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 create and loopaesOpen.
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 create 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
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.
--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.
Ignores 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 charac-
ters.
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 spe-
cific 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 hav-
ing to know an exiting one.
For luksOpen this allows to open the LUKS device without giving
a passphrase.
--dump-master-key
For luksDump this option includes the master key in the dis-
played 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 sys-
tem), 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 luksOpen, 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 create 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 create 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 create and loopaesOpen actions.
--skip, -p <number of 512 byte sectors>
How many sectors of the encrypted data to skip at the beginning.
This option is only relevant for create and loopaesOpen action.
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 genera-
tion is n.
--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 create action. Use --offset, --size and --skip to spec-
ify the mapped area.
--iter-time, -i <number of milliseconds>
The number of milliseconds to spend with PBKDF2 passphrase pro-
cessing. 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 create, luksOpen, 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 create, luksOpen, 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 pro-
vided by kernel for the underlying device to get optimal align-
ment. 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 gen-
erating 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 create, luksOpen and loopaesOpen.
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 luksOpen.
--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 to store ciphertext
and LUKS header on different devices.
This option is only relevant for LUKS devices and can be used
with the luksFormat, luksOpen, 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. luksAdd-
Key), 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 speci-
fied actually belongs to the header given. In fact you can spec-
ify an arbitrary device as the ciphertext device for luksOpen
with the --header option. Use with care.
--version
Show the program version.
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 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 maxi-
mum key file size and can be overwritten using --keyfile-size option.
The data read will be hashed with the default hash or the hash speci-
fied 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 new-
line character or the compiled-in maximum key file length. If --key-
file-size is given, it is ignored.
From key file: The complete keyfile is read up to the compiled-in maxi-
mum 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 protec-
tion for low-entropy passphrases, but luksOpen will take longer to com-
plete. 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.
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 automati-
cally). 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 Christophe Saout <christophe@saout.de>
The LUKS extensions and original man page were written by Clemens Fruh-
wirth <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 (C) 2004 Christophe Saout
Copyright (C) 2004-2006 Clemens Fruhwirth
Copyright (C) 2009-2012 Red Hat, Inc.
Copyright (C) 2012 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
cryptsetup May 2012 CRYPTSETUP(8)