linux/Documentation/device-mapper/dm-crypt.txt
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   1dm-crypt
   2=========
   3
   4Device-Mapper's "crypt" target provides transparent encryption of block devices
   5using the kernel crypto API.
   6
   7Parameters: <cipher> <key> <iv_offset> <device path> \
   8              <offset> [<#opt_params> <opt_params>]
   9
  10<cipher>
  11    Encryption cipher and an optional IV generation mode.
  12    (In format cipher[:keycount]-chainmode-ivopts:ivmode).
  13    Examples:
  14       des
  15       aes-cbc-essiv:sha256
  16       twofish-ecb
  17
  18    /proc/crypto contains supported crypto modes
  19
  20<key>
  21    Key used for encryption. It is encoded as a hexadecimal number.
  22    You can only use key sizes that are valid for the selected cipher.
  23
  24<keycount>
  25    Multi-key compatibility mode. You can define <keycount> keys and
  26    then sectors are encrypted according to their offsets (sector 0 uses key0;
  27    sector 1 uses key1 etc.).  <keycount> must be a power of two.
  28
  29<iv_offset>
  30    The IV offset is a sector count that is added to the sector number
  31    before creating the IV.
  32
  33<device path>
  34    This is the device that is going to be used as backend and contains the
  35    encrypted data.  You can specify it as a path like /dev/xxx or a device
  36    number <major>:<minor>.
  37
  38<offset>
  39    Starting sector within the device where the encrypted data begins.
  40
  41<#opt_params>
  42    Number of optional parameters. If there are no optional parameters,
  43    the optional paramaters section can be skipped or #opt_params can be zero.
  44    Otherwise #opt_params is the number of following arguments.
  45
  46    Example of optional parameters section:
  47        1 allow_discards
  48
  49allow_discards
  50    Block discard requests (a.k.a. TRIM) are passed through the crypt device.
  51    The default is to ignore discard requests.
  52
  53    WARNING: Assess the specific security risks carefully before enabling this
  54    option.  For example, allowing discards on encrypted devices may lead to
  55    the leak of information about the ciphertext device (filesystem type,
  56    used space etc.) if the discarded blocks can be located easily on the
  57    device later.
  58
  59Example scripts
  60===============
  61LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
  62encryption with dm-crypt using the 'cryptsetup' utility, see
  63http://code.google.com/p/cryptsetup/
  64
  65[[
  66#!/bin/sh
  67# Create a crypt device using dmsetup
  68dmsetup create crypt1 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0"
  69]]
  70
  71[[
  72#!/bin/sh
  73# Create a crypt device using cryptsetup and LUKS header with default cipher
  74cryptsetup luksFormat $1
  75cryptsetup luksOpen $1 crypt1
  76]]
  77
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