src/kernel/linux/v4.19/Documentation/device-mapper/dm-crypt.txt - T800 - Gitiles

 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:
 https://gitlab.com/cryptsetup/cryptsetup/wikis/DMCrypt

 Parameters: <cipher> <key> <iv_offset> <device path> \
 	      <offset> [<#opt_params> <opt_params>]

 <cipher>
     Encryption cipher, encryption mode and Initial Vector (IV) generator.

     The cipher specifications format is:
        cipher[:keycount]-chainmode-ivmode[:ivopts]
     Examples:
        aes-cbc-essiv:sha256
        aes-xts-plain64
        serpent-xts-plain64

     Cipher format also supports direct specification with kernel crypt API
     format (selected by capi: prefix). The IV specification is the same
     as for the first format type.
     This format is mainly used for specification of authenticated modes.

     The crypto API cipher specifications format is:
         capi:cipher_api_spec-ivmode[:ivopts]
     Examples:
         capi:cbc(aes)-essiv:sha256
         capi:xts(aes)-plain64
     Examples of authenticated modes:
         capi:gcm(aes)-random
         capi:authenc(hmac(sha256),xts(aes))-random
         capi:rfc7539(chacha20,poly1305)-random

     The /proc/crypto contains a list of curently loaded crypto modes.

 <key>
     Key used for encryption. It is encoded either as a hexadecimal number
     or it can be passed as <key_string> prefixed with single colon
     character (':') for keys residing in kernel keyring service.
     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.

 <key_string>
     The kernel keyring key is identified by string in following format:
     <key_size>:<key_type>:<key_description>.

 <key_size>
     The encryption key size in bytes. The kernel key payload size must match
     the value passed in <key_size>.

 <key_type>
     Either 'logon' or 'user' kernel key type.

 <key_description>
     The kernel keyring key description crypt target should look for
     when loading key of <key_type>.

 <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:
         3 allow_discards same_cpu_crypt submit_from_crypt_cpus

 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.

 same_cpu_crypt
     Perform encryption using the same cpu that IO was submitted on.
     The default is to use an unbound workqueue so that encryption work
     is automatically balanced between available CPUs.

 submit_from_crypt_cpus
     Disable offloading writes to a separate thread after encryption.
     There are some situations where offloading write bios from the
     encryption threads to a single thread degrades performance
     significantly.  The default is to offload write bios to the same
     thread because it benefits CFQ to have writes submitted using the
     same context.

 integrity:<bytes>:<type>
     The device requires additional <bytes> metadata per-sector stored
     in per-bio integrity structure. This metadata must by provided
     by underlying dm-integrity target.

     The <type> can be "none" if metadata is used only for persistent IV.

     For Authenticated Encryption with Additional Data (AEAD)
     the <type> is "aead". An AEAD mode additionally calculates and verifies
     integrity for the encrypted device. The additional space is then
     used for storing authentication tag (and persistent IV if needed).

 sector_size:<bytes>
     Use <bytes> as the encryption unit instead of 512 bytes sectors.
     This option can be in range 512 - 4096 bytes and must be power of two.
     Virtual device will announce this size as a minimal IO and logical sector.

 iv_large_sectors
    IV generators will use sector number counted in <sector_size> units
    instead of default 512 bytes sectors.

    For example, if <sector_size> is 4096 bytes, plain64 IV for the second
    sector will be 8 (without flag) and 1 if iv_large_sectors is present.
    The <iv_offset> must be multiple of <sector_size> (in 512 bytes units)
    if this flag is specified.

 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
 https://gitlab.com/cryptsetup/cryptsetup

 [[
 #!/bin/sh
 # Create a crypt device using dmsetup
 dmsetup create crypt1 --table "0 `blockdev --getsz $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0"
 ]]

 [[
 #!/bin/sh
 # Create a crypt device using dmsetup when encryption key is stored in keyring service
 dmsetup create crypt2 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 :32:logon:my_prefix:my_key 0 $1 0"
 ]]

 [[
 #!/bin/sh
 # Create a crypt device using cryptsetup and LUKS header with default cipher
 cryptsetup luksFormat $1
 cryptsetup luksOpen $1 crypt1
 ]]
	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:
	https://gitlab.com/cryptsetup/cryptsetup/wikis/DMCrypt

	Parameters: <cipher> <key> <iv_offset> <device path> \
	<offset> [<#opt_params> <opt_params>]

	<cipher>
	Encryption cipher, encryption mode and Initial Vector (IV) generator.

	The cipher specifications format is:
	cipher[:keycount]-chainmode-ivmode[:ivopts]
	Examples:
	aes-cbc-essiv:sha256
	aes-xts-plain64
	serpent-xts-plain64

	Cipher format also supports direct specification with kernel crypt API
	format (selected by capi: prefix). The IV specification is the same
	as for the first format type.
	This format is mainly used for specification of authenticated modes.

	The crypto API cipher specifications format is:
	capi:cipher_api_spec-ivmode[:ivopts]
	Examples:
	capi:cbc(aes)-essiv:sha256
	capi:xts(aes)-plain64
	Examples of authenticated modes:
	capi:gcm(aes)-random
	capi:authenc(hmac(sha256),xts(aes))-random
	capi:rfc7539(chacha20,poly1305)-random

	The /proc/crypto contains a list of curently loaded crypto modes.

	<key>
	Key used for encryption. It is encoded either as a hexadecimal number
	or it can be passed as <key_string> prefixed with single colon
	character (':') for keys residing in kernel keyring service.
	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.

	<key_string>
	The kernel keyring key is identified by string in following format:
	<key_size>:<key_type>:<key_description>.

	<key_size>
	The encryption key size in bytes. The kernel key payload size must match
	the value passed in <key_size>.

	<key_type>
	Either 'logon' or 'user' kernel key type.

	<key_description>
	The kernel keyring key description crypt target should look for
	when loading key of <key_type>.

	<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:
	3 allow_discards same_cpu_crypt submit_from_crypt_cpus

	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.

	same_cpu_crypt
	Perform encryption using the same cpu that IO was submitted on.
	The default is to use an unbound workqueue so that encryption work
	is automatically balanced between available CPUs.

	submit_from_crypt_cpus
	Disable offloading writes to a separate thread after encryption.
	There are some situations where offloading write bios from the
	encryption threads to a single thread degrades performance
	significantly. The default is to offload write bios to the same
	thread because it benefits CFQ to have writes submitted using the
	same context.

	integrity:<bytes>:<type>
	The device requires additional <bytes> metadata per-sector stored
	in per-bio integrity structure. This metadata must by provided
	by underlying dm-integrity target.

	The <type> can be "none" if metadata is used only for persistent IV.

	For Authenticated Encryption with Additional Data (AEAD)
	the <type> is "aead". An AEAD mode additionally calculates and verifies
	integrity for the encrypted device. The additional space is then
	used for storing authentication tag (and persistent IV if needed).

	sector_size:<bytes>
	Use <bytes> as the encryption unit instead of 512 bytes sectors.
	This option can be in range 512 - 4096 bytes and must be power of two.
	Virtual device will announce this size as a minimal IO and logical sector.

	iv_large_sectors
	IV generators will use sector number counted in <sector_size> units
	instead of default 512 bytes sectors.

	For example, if <sector_size> is 4096 bytes, plain64 IV for the second
	sector will be 8 (without flag) and 1 if iv_large_sectors is present.
	The <iv_offset> must be multiple of <sector_size> (in 512 bytes units)
	if this flag is specified.

	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
	https://gitlab.com/cryptsetup/cryptsetup

	[[
	#!/bin/sh
	# Create a crypt device using dmsetup
	dmsetup create crypt1 --table "0 `blockdev --getsz $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0"
	]]

	[[
	#!/bin/sh
	# Create a crypt device using dmsetup when encryption key is stored in keyring service
	dmsetup create crypt2 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 :32:logon:my_prefix:my_key 0 $1 0"
	]]

	[[
	#!/bin/sh
	# Create a crypt device using cryptsetup and LUKS header with default cipher
	cryptsetup luksFormat $1
	cryptsetup luksOpen $1 crypt1
	]]