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 as a hexadecimal number.
  25     You can only use key sizes that are valid for the selected cipher
  26     in combination with the selected iv mode.
  27     Note that for some iv modes the key string can contain additional
  28     keys (for example IV seed) so the key contains more parts concatenated
  29     into a single string.
  30
  31 <keycount>
  32     Multi-key compatibility mode. You can define <keycount> keys and
  33     then sectors are encrypted according to their offsets (sector 0 uses key0;
  34     sector 1 uses key1 etc.).  <keycount> must be a power of two.
  35
  36 <iv_offset>
  37     The IV offset is a sector count that is added to the sector number
  38     before creating the IV.
  39
  40 <device path>
  41     This is the device that is going to be used as backend and contains the
  42     encrypted data.  You can specify it as a path like /dev/xxx or a device
  43     number <major>:<minor>.
  44
  45 <offset>
  46     Starting sector within the device where the encrypted data begins.
  47
  48 <#opt_params>
  49     Number of optional parameters. If there are no optional parameters,
  50     the optional paramaters section can be skipped or #opt_params can be zero.
  51     Otherwise #opt_params is the number of following arguments.
  52
  53     Example of optional parameters section:
  54         3 allow_discards same_cpu_crypt submit_from_crypt_cpus
  55
  56 allow_discards
  57     Block discard requests (a.k.a. TRIM) are passed through the crypt device.
  58     The default is to ignore discard requests.
  59
  60     WARNING: Assess the specific security risks carefully before enabling this
  61     option.  For example, allowing discards on encrypted devices may lead to
  62     the leak of information about the ciphertext device (filesystem type,
  63     used space etc.) if the discarded blocks can be located easily on the
  64     device later.
  65
  66 same_cpu_crypt
  67     Perform encryption using the same cpu that IO was submitted on.
  68     The default is to use an unbound workqueue so that encryption work
  69     is automatically balanced between available CPUs.
  70
  71 submit_from_crypt_cpus
  72     Disable offloading writes to a separate thread after encryption.
  73     There are some situations where offloading write bios from the
  74     encryption threads to a single thread degrades performance
  75     significantly.  The default is to offload write bios to the same
  76     thread because it benefits CFQ to have writes submitted using the
  77     same context.
  78
  79 Example scripts
  80 ===============
  81 LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
  82 encryption with dm-crypt using the 'cryptsetup' utility, see
  83 https://gitlab.com/cryptsetup/cryptsetup
  84
  85 [[
  86 #!/bin/sh
  87 # Create a crypt device using dmsetup
  88 dmsetup create crypt1 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0"
  89 ]]
  90
  91 [[
  92 #!/bin/sh
  93 # Create a crypt device using cryptsetup and LUKS header with default cipher
  94 cryptsetup luksFormat $1
  95 cryptsetup luksOpen $1 crypt1
  96 ]]