Visit Our UNIX and Linux User Community

Linux and UNIX Man Pages

Test Your Knowledge in Computers #549
Difficulty: Medium
a = 10; is an example of a unary operation.
True or False?
Linux & Unix Commands - Search Man Pages

cryptsetup(8) [bsd man page]

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.

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 volume.

       Character  encoding:  If you enter a passphrase with special symbols, the passphrase can change depending character encoding. Keyboard set-
       tings 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.

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, --read-
	      only, --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 kernel 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 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.

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  off-
       set.

       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 argu-
	      ment), --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, --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 encryp-
	      tion 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 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].

       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.

       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.

       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 fix-
	      able. 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 compatibility 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 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].

       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 luksFor-
	      mat, 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 regu-
	      lar 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 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 off-
	      set.  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.

	      For luksAddKey this allows adding a new passphrase without having 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 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 gen-
	      erator.

	      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 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 calcula-
	      tion.  Using --skip causes sector n to also be the first sector of the mapped device, but with its number for IV generation 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 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 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 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 luksU-
	      UID 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 dis-
	      carded 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.

       --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. 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 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. '
'.  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.

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 crypt-
       setup 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  --key-
       file-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 --key-
       file-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 pro-
       tection for low-entropy passphrases, but luksOpen 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.

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  sud-
       denly  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 pro-
       duced.

       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 Christophe Saout <christophe@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 (C) 2004 Christophe Saout
       Copyright (C) 2004-2006 Clemens Fruhwirth
       Copyright (C) 2009-2011 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 PARTICU-
       LAR 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)

Featured Tech Videos