1dm-crypt
  2=========
  3
  4Device-Mapper's "crypt" target provides transparent encryption of block devices
  5using the kernel crypto API.
  6
  7For a more detailed description of supported parameters see:
  8https://gitlab.com/cryptsetup/cryptsetup/wikis/DMCrypt
  9
 10Parameters: <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
 73allow_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
 83same_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
 88submit_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
 96Example scripts
 97===============
 98LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
 99encryption with dm-crypt using the 'cryptsetup' utility, see
100https://gitlab.com/cryptsetup/cryptsetup
101
102[[
103#!/bin/sh
104# Create a crypt device using dmsetup
105dmsetup 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
111dmsetup 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
117cryptsetup luksFormat $1
118cryptsetup luksOpen $1 crypt1
119]]