[mirror_ubuntu-artful-kernel.git] / Documentation / device-mapper / dm-crypt.txt

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 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 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.

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
]]
Commit	Line	Data
e3dcc5a3 MB	1	dm-crypt
	2	=========
	3
	4	Device-Mapper's "crypt" target provides transparent encryption of block devices
	5	using the kernel crypto API.
	6
ed04d981	7	For a more detailed description of supported parameters see:
e44f23b3	8	https://gitlab.com/cryptsetup/cryptsetup/wikis/DMCrypt
ed04d981	9
772ae5f5 MB	10	Parameters: <cipher> <key> <iv_offset> <device path> \
772ae5f5 MB	11	<offset> [<#opt_params> <opt_params>]
e3dcc5a3 MB	12
	13	<cipher>
	14	Encryption cipher and an optional IV generation mode.
ed04d981	15	(In format cipher[:keycount]-chainmode-ivmode[:ivopts]).
e3dcc5a3 MB	16	Examples:
	17	des
	18	aes-cbc-essiv:sha256
	19	twofish-ecb
	20
	21	/proc/crypto contains supported crypto modes
	22
	23	<key>
c538f6ec OK	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.
ed04d981 MB	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.
e3dcc5a3	32
c538f6ec OK	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
d1f96423 MB	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
e3dcc5a3 MB	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
772ae5f5 MB	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:
0f5d8e6e	71	3 allow_discards same_cpu_crypt submit_from_crypt_cpus
772ae5f5 MB	72
	73	allow_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
f3396c58 MP	83	same_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
0f5d8e6e MP	88	submit_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
e3dcc5a3 MB	96	Example scripts
	97	===============
	98	LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
	99	encryption with dm-crypt using the 'cryptsetup' utility, see
e44f23b3	100	https://gitlab.com/cryptsetup/cryptsetup
e3dcc5a3 MB	101
	102	[[
	103	#!/bin/sh
	104	# Create a crypt device using dmsetup
95f21c5c	105	dmsetup create crypt1 --table "0 `blockdev --getsz $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0"
e3dcc5a3 MB	106	]]
e3dcc5a3 MB	107
c538f6ec OK	108	[[
	109	#!/bin/sh
	110	# Create a crypt device using dmsetup when encryption key is stored in keyring service
	111	dmsetup create crypt2 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 :32:logon:my_prefix:my_key 0 $1 0"
	112	]]
	113
e3dcc5a3 MB	114	[[
	115	#!/bin/sh
	116	# Create a crypt device using cryptsetup and LUKS header with default cipher
	117	cryptsetup luksFormat $1
	118	cryptsetup luksOpen $1 crypt1
	119	]]