[linux-2.6-block.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 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:
        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 --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
]]
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>
	24	Key used for encryption. It is encoded as a hexadecimal number.
ed04d981 MB	25	You can only use key sizes that are valid for the selected cipher
	26	in combination with the selected iv mode.
	27	Note that for some iv modes the key string can contain additional
	28	keys (for example IV seed) so the key contains more parts concatenated
	29	into a single string.
e3dcc5a3	30
d1f96423 MB	31	<keycount>
	32	Multi-key compatibility mode. You can define <keycount> keys and
	33	then sectors are encrypted according to their offsets (sector 0 uses key0;
	34	sector 1 uses key1 etc.). <keycount> must be a power of two.
	35
e3dcc5a3 MB	36	<iv_offset>
	37	The IV offset is a sector count that is added to the sector number
	38	before creating the IV.
	39
	40	<device path>
	41	This is the device that is going to be used as backend and contains the
	42	encrypted data. You can specify it as a path like /dev/xxx or a device
	43	number <major>:<minor>.
	44
	45	<offset>
	46	Starting sector within the device where the encrypted data begins.
	47
772ae5f5 MB	48	<#opt_params>
	49	Number of optional parameters. If there are no optional parameters,
	50	the optional paramaters section can be skipped or #opt_params can be zero.
	51	Otherwise #opt_params is the number of following arguments.
	52
	53	Example of optional parameters section:
0f5d8e6e	54	3 allow_discards same_cpu_crypt submit_from_crypt_cpus
772ae5f5 MB	55
	56	allow_discards
	57	Block discard requests (a.k.a. TRIM) are passed through the crypt device.
	58	The default is to ignore discard requests.
	59
	60	WARNING: Assess the specific security risks carefully before enabling this
	61	option. For example, allowing discards on encrypted devices may lead to
	62	the leak of information about the ciphertext device (filesystem type,
	63	used space etc.) if the discarded blocks can be located easily on the
	64	device later.
	65
f3396c58 MP	66	same_cpu_crypt
	67	Perform encryption using the same cpu that IO was submitted on.
	68	The default is to use an unbound workqueue so that encryption work
	69	is automatically balanced between available CPUs.
	70
0f5d8e6e MP	71	submit_from_crypt_cpus
	72	Disable offloading writes to a separate thread after encryption.
	73	There are some situations where offloading write bios from the
	74	encryption threads to a single thread degrades performance
	75	significantly. The default is to offload write bios to the same
	76	thread because it benefits CFQ to have writes submitted using the
	77	same context.
	78
e3dcc5a3 MB	79	Example scripts
	80	===============
	81	LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
	82	encryption with dm-crypt using the 'cryptsetup' utility, see
e44f23b3	83	https://gitlab.com/cryptsetup/cryptsetup
e3dcc5a3 MB	84
	85	[[
	86	#!/bin/sh
	87	# Create a crypt device using dmsetup
	88	dmsetup create crypt1 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0"
	89	]]
	90
	91	[[
	92	#!/bin/sh
	93	# Create a crypt device using cryptsetup and LUKS header with default cipher
	94	cryptsetup luksFormat $1
	95	cryptsetup luksOpen $1 crypt1
	96	]]