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kernel/linux-imx6_3.14.28/Documentation/device-mapper/dm-crypt.txt 2.8 KB
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  dm-crypt
  =========
  
  Device-Mapper's "crypt" target provides transparent encryption of block devices
  using the kernel crypto API.
  
  For a more detailed description of supported parameters see:
  http://code.google.com/p/cryptsetup/wiki/DMCrypt
  
  Parameters: <cipher> <key> <iv_offset> <device path> \
  	      <offset> [<#opt_params> <opt_params>]
  
  <cipher>
      Encryption cipher and an optional IV generation mode.
      (In format cipher[:keycount]-chainmode-ivmode[:ivopts]).
      Examples:
         des
         aes-cbc-essiv:sha256
         twofish-ecb
  
      /proc/crypto contains supported crypto modes
  
  <key>
      Key used for encryption. It is encoded as a hexadecimal number.
      You can only use key sizes that are valid for the selected cipher
      in combination with the selected iv mode.
      Note that for some iv modes the key string can contain additional
      keys (for example IV seed) so the key contains more parts concatenated
      into a single string.
  
  <keycount>
      Multi-key compatibility mode. You can define <keycount> keys and
      then sectors are encrypted according to their offsets (sector 0 uses key0;
      sector 1 uses key1 etc.).  <keycount> must be a power of two.
  
  <iv_offset>
      The IV offset is a sector count that is added to the sector number
      before creating the IV.
  
  <device path>
      This is the device that is going to be used as backend and contains the
      encrypted data.  You can specify it as a path like /dev/xxx or a device
      number <major>:<minor>.
  
  <offset>
      Starting sector within the device where the encrypted data begins.
  
  <#opt_params>
      Number of optional parameters. If there are no optional parameters,
      the optional paramaters section can be skipped or #opt_params can be zero.
      Otherwise #opt_params is the number of following arguments.
  
      Example of optional parameters section:
          1 allow_discards
  
  allow_discards
      Block discard requests (a.k.a. TRIM) are passed through the crypt device.
      The default is to ignore discard requests.
  
      WARNING: Assess the specific security risks carefully before enabling this
      option.  For example, allowing discards on encrypted devices may lead to
      the leak of information about the ciphertext device (filesystem type,
      used space etc.) if the discarded blocks can be located easily on the
      device later.
  
  Example scripts
  ===============
  LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
  encryption with dm-crypt using the 'cryptsetup' utility, see
  http://code.google.com/p/cryptsetup/
  
  [[
  #!/bin/sh
  # Create a crypt device using dmsetup
  dmsetup create crypt1 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0"
  ]]
  
  [[
  #!/bin/sh
  # Create a crypt device using cryptsetup and LUKS header with default cipher
  cryptsetup luksFormat $1
  cryptsetup luksOpen $1 crypt1
  ]]