Documentation/device-mapper/dm-crypt.txt

   1 dm-crypt
   2 =========
   3
   4 Device-Mapper's "crypt" target provides transparent encryption of block devices
   5 using the kernel crypto API.
   6
   7 For a more detailed description of supported parameters see:
   8 https://gitlab.com/cryptsetup/cryptsetup/wikis/DMCrypt
   9
  10 Parameters: <cipher> <key> <iv_offset> <device path> \
  11               <offset> [<#opt_params> <opt_params>]
  12
  13 <cipher>
  14     Encryption cipher and an optional IV generation mode.
  15     (In format cipher[:keycount]-chainmode-ivmode[:ivopts]).
  16     Examples:
  17        des
  18        aes-cbc-essiv:sha256
  19        twofish-ecb
  20
  21     /proc/crypto contains supported crypto modes
  22
  23 <key>
  24     Key used for encryption. It is encoded either as a hexadecimal number
  25     or it can be passed as <key_string> prefixed with single colon
  26     character (':') for keys residing in kernel keyring service.
  27     You can only use key sizes that are valid for the selected cipher
  28     in combination with the selected iv mode.
  29     Note that for some iv modes the key string can contain additional
  30     keys (for example IV seed) so the key contains more parts concatenated
  31     into a single string.
  32
  33 <key_string>
  34     The kernel keyring key is identified by string in following format:
  35     <key_size>:<key_type>:<key_description>.
  36
  37 <key_size>
  38     The encryption key size in bytes. The kernel key payload size must match
  39     the value passed in <key_size>.
  40
  41 <key_type>
  42     Either 'logon' or 'user' kernel key type.
  43
  44 <key_description>
  45     The kernel keyring key description crypt target should look for
  46     when loading key of <key_type>.
  47
  48 <keycount>
  49     Multi-key compatibility mode. You can define <keycount> keys and
  50     then sectors are encrypted according to their offsets (sector 0 uses key0;
  51     sector 1 uses key1 etc.).  <keycount> must be a power of two.
  52
  53 <iv_offset>
  54     The IV offset is a sector count that is added to the sector number
  55     before creating the IV.
  56
  57 <device path>
  58     This is the device that is going to be used as backend and contains the
  59     encrypted data.  You can specify it as a path like /dev/xxx or a device
  60     number <major>:<minor>.
  61
  62 <offset>
  63     Starting sector within the device where the encrypted data begins.
  64
  65 <#opt_params>
  66     Number of optional parameters. If there are no optional parameters,
  67     the optional paramaters section can be skipped or #opt_params can be zero.
  68     Otherwise #opt_params is the number of following arguments.
  69
  70     Example of optional parameters section:
  71         3 allow_discards same_cpu_crypt submit_from_crypt_cpus
  72
  73 allow_discards
  74     Block discard requests (a.k.a. TRIM) are passed through the crypt device.
  75     The default is to ignore discard requests.
  76
  77     WARNING: Assess the specific security risks carefully before enabling this
  78     option.  For example, allowing discards on encrypted devices may lead to
  79     the leak of information about the ciphertext device (filesystem type,
  80     used space etc.) if the discarded blocks can be located easily on the
  81     device later.
  82
  83 same_cpu_crypt
  84     Perform encryption using the same cpu that IO was submitted on.
  85     The default is to use an unbound workqueue so that encryption work
  86     is automatically balanced between available CPUs.
  87
  88 submit_from_crypt_cpus
  89     Disable offloading writes to a separate thread after encryption.
  90     There are some situations where offloading write bios from the
  91     encryption threads to a single thread degrades performance
  92     significantly.  The default is to offload write bios to the same
  93     thread because it benefits CFQ to have writes submitted using the
  94     same context.
  95
  96 Example scripts
  97 ===============
  98 LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
  99 encryption with dm-crypt using the 'cryptsetup' utility, see
 100 https://gitlab.com/cryptsetup/cryptsetup
 101
 102 [[
 103 #!/bin/sh
 104 # Create a crypt device using dmsetup
 105 dmsetup create crypt1 --table "0 `blockdev --getsz $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0"
 106 ]]
 107
 108 [[
 109 #!/bin/sh
 110 # Create a crypt device using dmsetup when encryption key is stored in keyring service
 111 dmsetup create crypt2 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 :32:logon:my_prefix:my_key 0 $1 0"
 112 ]]
 113
 114 [[
 115 #!/bin/sh
 116 # Create a crypt device using cryptsetup and LUKS header with default cipher
 117 cryptsetup luksFormat $1
 118 cryptsetup luksOpen $1 crypt1
 119 ]]