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, encryption mode and Initial Vector (IV) generator.
  15
  16     The cipher specifications format is:
  17        cipher[:keycount]-chainmode-ivmode[:ivopts]
  18     Examples:
  19        aes-cbc-essiv:sha256
  20        aes-xts-plain64
  21        serpent-xts-plain64
  22
  23     Cipher format also supports direct specification with kernel crypt API
  24     format (selected by capi: prefix). The IV specification is the same
  25     as for the first format type.
  26     This format is mainly used for specification of authenticated modes.
  27
  28     The crypto API cipher specifications format is:
  29         capi:cipher_api_spec-ivmode[:ivopts]
  30     Examples:
  31         capi:cbc(aes)-essiv:sha256
  32         capi:xts(aes)-plain64
  33     Examples of authenticated modes:
  34         capi:gcm(aes)-random
  35         capi:authenc(hmac(sha256),xts(aes))-random
  36         capi:rfc7539(chacha20,poly1305)-random
  37
  38     The /proc/crypto contains a list of curently loaded crypto modes.
  39
  40 <key>
  41     Key used for encryption. It is encoded either as a hexadecimal number
  42     or it can be passed as <key_string> prefixed with single colon
  43     character (':') for keys residing in kernel keyring service.
  44     You can only use key sizes that are valid for the selected cipher
  45     in combination with the selected iv mode.
  46     Note that for some iv modes the key string can contain additional
  47     keys (for example IV seed) so the key contains more parts concatenated
  48     into a single string.
  49
  50 <key_string>
  51     The kernel keyring key is identified by string in following format:
  52     <key_size>:<key_type>:<key_description>.
  53
  54 <key_size>
  55     The encryption key size in bytes. The kernel key payload size must match
  56     the value passed in <key_size>.
  57
  58 <key_type>
  59     Either 'logon' or 'user' kernel key type.
  60
  61 <key_description>
  62     The kernel keyring key description crypt target should look for
  63     when loading key of <key_type>.
  64
  65 <keycount>
  66     Multi-key compatibility mode. You can define <keycount> keys and
  67     then sectors are encrypted according to their offsets (sector 0 uses key0;
  68     sector 1 uses key1 etc.).  <keycount> must be a power of two.
  69
  70 <iv_offset>
  71     The IV offset is a sector count that is added to the sector number
  72     before creating the IV.
  73
  74 <device path>
  75     This is the device that is going to be used as backend and contains the
  76     encrypted data.  You can specify it as a path like /dev/xxx or a device
  77     number <major>:<minor>.
  78
  79 <offset>
  80     Starting sector within the device where the encrypted data begins.
  81
  82 <#opt_params>
  83     Number of optional parameters. If there are no optional parameters,
  84     the optional paramaters section can be skipped or #opt_params can be zero.
  85     Otherwise #opt_params is the number of following arguments.
  86
  87     Example of optional parameters section:
  88         3 allow_discards same_cpu_crypt submit_from_crypt_cpus
  89
  90 allow_discards
  91     Block discard requests (a.k.a. TRIM) are passed through the crypt device.
  92     The default is to ignore discard requests.
  93
  94     WARNING: Assess the specific security risks carefully before enabling this
  95     option.  For example, allowing discards on encrypted devices may lead to
  96     the leak of information about the ciphertext device (filesystem type,
  97     used space etc.) if the discarded blocks can be located easily on the
  98     device later.
  99
 100 same_cpu_crypt
 101     Perform encryption using the same cpu that IO was submitted on.
 102     The default is to use an unbound workqueue so that encryption work
 103     is automatically balanced between available CPUs.
 104
 105 submit_from_crypt_cpus
 106     Disable offloading writes to a separate thread after encryption.
 107     There are some situations where offloading write bios from the
 108     encryption threads to a single thread degrades performance
 109     significantly.  The default is to offload write bios to the same
 110     thread because it benefits CFQ to have writes submitted using the
 111     same context.
 112
 113 integrity:<bytes>:<type>
 114     The device requires additional <bytes> metadata per-sector stored
 115     in per-bio integrity structure. This metadata must by provided
 116     by underlying dm-integrity target.
 117
 118     The <type> can be "none" if metadata is used only for persistent IV.
 119
 120     For Authenticated Encryption with Additional Data (AEAD)
 121     the <type> is "aead". An AEAD mode additionally calculates and verifies
 122     integrity for the encrypted device. The additional space is then
 123     used for storing authentication tag (and persistent IV if needed).
 124
 125 sector_size:<bytes>
 126     Use <bytes> as the encryption unit instead of 512 bytes sectors.
 127     This option can be in range 512 - 4096 bytes and must be power of two.
 128     Virtual device will announce this size as a minimal IO and logical sector.
 129
 130 iv_large_sectors
 131    IV generators will use sector number counted in <sector_size> units
 132    instead of default 512 bytes sectors.
 133
 134    For example, if <sector_size> is 4096 bytes, plain64 IV for the second
 135    sector will be 8 (without flag) and 1 if iv_large_sectors is present.
 136    The <iv_offset> must be multiple of <sector_size> (in 512 bytes units)
 137    if this flag is specified.
 138
 139 Example scripts
 140 ===============
 141 LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
 142 encryption with dm-crypt using the 'cryptsetup' utility, see
 143 https://gitlab.com/cryptsetup/cryptsetup
 144
 145 [[
 146 #!/bin/sh
 147 # Create a crypt device using dmsetup
 148 dmsetup create crypt1 --table "0 `blockdev --getsz $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0"
 149 ]]
 150
 151 [[
 152 #!/bin/sh
 153 # Create a crypt device using dmsetup when encryption key is stored in keyring service
 154 dmsetup create crypt2 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 :32:logon:my_prefix:my_key 0 $1 0"
 155 ]]
 156
 157 [[
 158 #!/bin/sh
 159 # Create a crypt device using cryptsetup and LUKS header with default cipher
 160 cryptsetup luksFormat $1
 161 cryptsetup luksOpen $1 crypt1
 162 ]]